farming marine shrimp in recirculating freshwater systems

Farming Marine Shrimp in Recirculating Freshwater Systems

Florida Department of Agriculture and Consumer Services BOB CRAWFORD, COMMISSIONER

Contract No. 4520 1999

Prepared by Harbor Branch Oceanographic Institution

Peter Van Wyk, Megan Davis-Hodgkins, Rolland Laramore,

Kevan L. Main, Joe Mountain, John Scarpa

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Farming Marine Shrimp in Recirculating Freshwater Systems

Prepared by Harbor Branch Oceanographic Institution

Table of Contents

Farming Marine Shrimp in Freshwater Systems: An Economic Development Strategy for Florida – Final Report – Peter Van Wyk Introduction 1 System Descriptions 3 Production Trials 10 Results 12 Discussion 19 Recommendations 22 Literature Cited 23 Figures 24 Chapter 1 Introduction - Kevan L. Main and Peter Van Wyk 33 An Overview of the Development of Shrimp Farming 33

New Approaches and Considerations for Shrimp Farming 35 Freshwater Culture of Marine Shrimp 36 Literature Cited 37

Chapter 2 Getting Started - Megan Davis-Hodgkins, John Scarpa and

Joe Mountain 39 Introduction 39

Planning Your Aquaculture Business 39 Expectations 40

Research and Training 40 Production Planning 41 Market Feasibility 44 Marketing Farm-Raised Shrimp 44 Business Plan 45 Executive Summary 46 Business Description 46 Market Analysis 46 Management Team 46 Financial Information 47 Milestone Schedule 47 Appendix 47 Demonstration or Pilot-Scale Operation 47 Commercial-Scale Operation 47

Water Selection Criteria 48 Permitting 49

Literature Cited 50

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Chapter 3 Greenhouse Construction – Megan Davis-Hodgkins 51 Introduction 51

Zoning and Permitting 51 General Construction 52 Site Preparation 52 Anchor Layout 52 Assembling Arches 53 Gable Ends 53 Covering the Greenhouse 54 Flooring 56 Raceway Installation 56 Systems and Filtration Equipment 58 Chapter 4 Principles of Recirculating System Design - Peter Van Wyk 59

Introduction 59 The Culture Tank 59 Circular Tanks 60 Raceways 60 Racetrack Configuration 61 Water Depth 63 Artificial Substrates 64 Standpipes and Drain Structures 65 Solids Filtration 66 Sources and Types of Solid Wastes 66 Consequences of Excessive Solid Wastes 66 Solid Waste Filters 67 Sedimentation Tanks 68 Hydrocyclones 69 Tube Settlers 69 Microscreen Filters 70 Bead Filters 72 Sand Filters 73 Foam Fractionators 74 Ozone 76 Biofiltration 77 Sources of Ammonia and Nitrite 77 Ammonia and Nitrite Toxicity 78 Mechanisms for Controlling Ammonia 79 Water Exchange 79 Plant Uptake 79 Nitrification 80

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Chapter 4 - Continued Types of Biofilters 81

Submerged Biofilters 81 Trickling Biofilters 83 Rotating Biological Contactors 84 Bead Filters 85 Sand Filters 86 Fluidized Bed Biofilter 87 Sizing a Biofilter 90 Pumps 91 Required Flow Rates 93 Calculation of Friction Losses and Total Head 93 Pump Sizing Procedure 95 Pump Performance Curves 95 Trimmed Impellers 95 Literature Cited 96 Chapter 5 Harbor Branch Shrimp Production Systems - Peter Van Wyk 99 Design Objectives 99 System A 100 Greenhouses 100 Culture Tanks 101 Drains 103 Pumps 104 Sand Filters 104 Aeration 105 System B 105 Greenhouses 106 Culture Tanks 106 Single-Phase and Three-Phase Production Systems 107 Water Treatment Systems 108 Pumps 110 Aeration 112 Water Supply 112 Retention Ponds 113 Literature Cited 113

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Chapter 6 Receiving and Acclimation of Postlarvae - Peter Van Wyk 115 Purchasing Postlarvae 115 Preparations for Receiving Postlarval Shrimp 116 Acclimation Systems 116 Acclimation Equipment Requirements 117 Acclimation Stations 118 Receiving the Postlarvae 119 Acclimation Procedures 120 Acclimation in Shipping Bags 120 Acclimation in AcclimationTanks 120 Acclimation Schedules 121 Calculating Water Exchange Requirements 121 Literature Cited 124 Chapter 7 Nutrition and Feeding of Litopenaeus vannamei in Intensive Culture

Systems - Peter Van Wyk 125 Elements of a Good Feeding Program 125 Nutritional Requirements 125 Protein Requirements 125 Lipids 127 Carbohydrates 128 Vitamins 128 Minerals 129 Shrimp Feeds 129 Formulated Diets 129 Feed Processing 130 Pellet Stability 131 Pellet Diameter 131 Feed Application 132 Feeding Rates 132 Feed Tables 133 Demand-Based Feeding 134 Feeding Frequency 135 Feed Distribution 135 Feed Conversion Ratios 136 Feed Storage 136 Sources of Shrimp Feeds 137 Literature Cited 139

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Chapter 8 Water Quality Requirements and Management – Peter Van Wyk and John Scarpa 141

Introduction 141 Water Quality Testing During Site Selection 142 Salinity 144

Temperature 146 Dissolved Oxygen 148 pH 150

Dissolved Carbon Dioxide 152 Ammonia 153 Nitrite 158 Nitrate 159 Hardness 159 Alkalinity 160 Hydrogen Sulfide 160 Iron 161 Chlorine 161 Selected Literature 161

Chapter 9 Shrimp Health Management: Issues and Strategies

- Kevan L. Main and Rolland Laramore 163 Introduction 163 Variables to Consider in Determining the

Health of Your Shrimp 164 Survival Rates 164 Mortality Rates 164 Growth Rates 164 Size Variation 164 Feed Conversion Ratio 164 Appearance of Shrimp 165 Effect of the Environment on Shrimp Health 165 Health Evaluation Tests 165 Stress Tests 165 Gill Examination 165 Gut Content Examination 165 Detecting Diseases and Diagnostic Techniques 166 Factors Leading to Losses and Disease Outbreaks

During Growout 166

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Chapter 9 - Continued Poor Quality Postlarvae 166 Postlarval Acclimation Procedures 166 Management Strategies that Lead to

Disease Problems 166 Dietary Issues 167 Human Factors 167 Environmental Factors 167 Factors to Consider in Disease Prevention 167 Site Selection and Environmental Conditions 168 Feed Quality 168 Biosecurity 168 Probiotics 168 Transfers and Handling 169 Record Keeping 169 Personnel 169

Practical Approaches to Disease Control 169 Eradication of Viral Diseases 170 Important Shrimp Pathogens 170

Overview 170 Common Disease Concerns During Growout 171

Infectious Hypodermal and Hematopoietic Necrosis Virus 171 Runt-Deformity Syndrome 172 Taura Syndrome Virus 172 White Spot Syndrome Virus 172 Yellowhead Virus 173 Vibriosis 173 Necrotizing hepatopancreatitis 174 Mycobacteriosis 174 Epicommensal fouling disease 174 Black spot disease 175

Gas Bubble Disease 175 Dissolved Oxygen Crisis 176

Literature Cited 177

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Chapter 10 Economics of Shrimp Culture in Recirculating Aquaculture Systems

- Peter VanWyk 179 Introduction 179 Baseline Assumptions 179 Facility Description 179 Investment Requirements 182 Land 182 Buildings and Improvements 182 Tanks and Sumps 183 Machinery and Equipment 184 Office Equipment 186 Total Investment Requirements 186 Process Description and Production Assumptions 188 Production Schedule 189 Expected Production 191 Production Inputs and Operating Costs 191 Seed 191 Feed 193 Labor 194 Energy 194

Maintenance 195 Marketing Assumptions 195 Revenues 196 Cash Flow 196 Income Statement 198 Breakeven Analysis 200 Investment Analysis 200 Sensitivity Analysis 201 Survival 201 Growth Rates 202 Seed Costs 204 Market Prices 206 Conclusion 207 Literature Cited 208 Appendix A Ammonia Mass Balance - Peter VanWyk 209 Ammonia Mass Balance Analysis 210 System Ammonia Mass Balance 211 Estimating Required Recycle Flow Rates Based On Ammonia Mass Balance 213 Example 215 Literature Cited 216 Appendix B Friction Loss Tables 217 Flow Velocity & Friction Loss-Schedule 40 Pipe 218

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Friction Losses Through Pipe Fittings In Terms of Equivalent Lengths Of Standard Pipe 219 Friction Loss Nomograph 220

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Florida Department of Agriculture and Consumer Services

BOB CRAWFORD, Commissioner

For additional production, technical or regulatory information contact:

Division of Aquaculture 1203 Governor’s Square Boulevard, Fifth Floor

Tallahassee, Florida 32301 Tel: 850-488-4033 Fax: 850-410-0893

The information, procedures and conclusions described by this report are the sole responsibility of the authors. The Florida Department of Agriculture and Consumer

Services does not approve, recommend nor endorse any proprietary product, material or process mentioned in this publication. 01/00

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“Farming Marine Shrimp in Freshwater Systems: An Economic

Development Strategy for Florida: Final Report”

FDACS Contract #4520

Principle: Investigator: Peter M. Van Wyk Harbor Branch Oceanographic Institution

5600 Highway U.S. 1 North Ft. Pierce, Florida 34946

Introduction: In recent years there has been renewed interest in shrimp culture here in Florida due to technological developments that now make it possible to culture Litopenaeus vannamei indoors in near-freshwater recirculating aquaculture systems. Harbor Branch and others have demonstrated in recent years that L. vannamei can be successfully produced in water with chloride concentrations as low as 300 ppm. Water with chloride levels this low is generally classified as freshwater and can be used to irrigate most crops. The significance of this is that shrimp production can now be practiced on cheaper, non-coastal agricultural land. New advances in the technology for producing L. vannamei indoors in high-density recirculating aquaculture systems now allows for year-round production of this species even in temperate climates with relatively cold winters. Year-round production improves the economic potential of an enterprise in several ways. The annual revenues of the operation are increased because year-round production increases annual productivity. Continuous harvesting facilitates direct marketing to retail markets, which may allow for a higher price to be received for the product. Producing shrimp indoors in recirculating systems benefits the producer by significantly reducing the risk of exposing the shrimp to the viral diseases that have wreaked havoc in open coastal ponds throughout the world. In the wake of devastating epidemics of Taura Syndrome Virus (TSV) and White Spot Syndrome Virus (WSSV), some shrimp farm managers in Latin America are considering switching to intenstive tank-based production systems because of the additional biosecurity these systems can provide. Indoor production systems provide the additional benefit of reducing crop loss due to predation. In addition, these systems significantly reduce the risk of accidental release of non-native shrimp into Florida’s coastal waters. The objective of the current study was to demonstrate the production technology required to successfully cultivate the marine shrimp, Litopenaeus vannamei, in freshwater recirculating aquaculture systems, and to evaluate the economic potential of this approach to shrimp culture.

Final Report – Farming Marine Shrimp in Freshwater Recirculating Systems

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Production Systems: Two different production systems were evaluated in this study: 1) a single-phase (direct stock) production system, and 2) a three-phase, partitioned production system. In a single-phase production system shrimp postlarvae are stocked into a culture tank and remain in that same tank until final harvest. The stocking density in the culture tank is based on the desired final harvest density plus overstock to compensate for expected mortalities. Initially, the system biomass is extremely low relative to the carrying capacity of the system, which is only reached at the end of the production cycle. In a three-phase production system, the production process is divided into three distinct phases, each carried out in a different culture tank, or in different sections of a partitioned culture tank. The shrimp typically spend one-third of the total culture period in each of the three sections of the tank. Postlarval shrimp are initially stocked into a small nursery tank, representing 10-13% of the total culture area of the complete three-phase system. At the end of the nursery period (after 50-60 days) the juvenile shrimp are transferred to the second section of the tank, called the intermediate growout section. This section is larger than the nursery section, representing about 27-30% of the total culture area. The shrimp remain in the intermediate growout section for another 50-60 days before being transferred to the final growout section, which occupies 60% of the total culture area. After another 50-60 day period the shrimp are harvested for market. The objective of a three-phase production system is to utilize the available production area more efficiently by operating closer to the carrying capacity of the system for a greater percentage of the culture period. In a single-phase system the biomass is very low relative to the carrying capacity of the tank for the first two-thirds of the culture cycle. The number of postlarvae stocked into the nursery section is determined by the projected harvest density of the final growout section, with overstocking to account for expected mortalities. The amount of area devoted to each phase is calculated to allow the shrimp to continue to grow until they reach the end of that phase. When the shrimp are ready to be transferred to the next section they should be approaching the carrying capacity for the section they are in. The three-phase system permits higher production levels than can be achieved in a single-phase system. Each section of a three-phase system is stocked at a density which will grow to the carrying capacity for the alloted area in one-third the amount of time it takes the shrimp in a single-phase system to reach the carrying capacity. Tank space is used more efficiently than in a single-phase system in which culture tanks are maintained at low densities throughout the early part of the growout cycle. The production of a three-phase system should, theoretically, be 1.8 times greater than the production in a single-phase system, assuming survival and growth rates are equivalent between the two systems. Although the area harvested for each crop is only 60% of the area harvested in a single-phase system, the final growout section of the 3-phase system is harvested three times for every harvest of the single-phase system. Increasing the harvest frequency has obvious advantages from a marketing standpoint, and may also smooth out the cash flow for the business. The potential disadvantages of a three-phase production system are the increased risk of mortality during the transfer process and potential density-dependent reduction in shrimp growth rates and survivals.


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Greenhouse Recirculating Aquaculture Systems: The objective of the Harbor Branch Oceanographic Institution (HBOI) shrimp culture program has been to develop a cost-effective indoor, freshwater production system based on a recirculating water treatment system. The principle that guided HBOI in the development of new system designs is: “Keep It Simple”. The ideal system should be simple to build using inexpensive, readily available materials, and should be operable by individuals with limited training specific to systems operation. With this in mind, HBOI focused its efforts on designing an inexpensive system capable of growing shrimp at moderately high densities (up to 150 shrimp/m2). These densities are significantly lower than the highest densities (>600 shrimp/m2) that have been reported for L. vannamei in more sophisticated recirculating systems (Davis and Arnold, 1998). The shrimp production systems utilized in this project represent two generations of system design. First generation systems at HBOI (System A) feature above-ground raceways and sand filters. Given their simplicity, these systems perform surprisingly well, supporting loading rates of up to 2.25 kg shrimp/m3. However, sand filters are expensive to operate because they require inefficient, high-head pumps to push the water through the compacted sand filter media and because sand filter maintenance is very labor intensive. The cost of operating the pumps on these systems can be quite high because they operate on a continuous basis. The second generation systems at HBOI (System B) feature in-ground raceways and low-head water treatment systems. The in-ground raceway should be less prone to heat-loss, reducing heating costs in the winter. Pumping costs in System B are lower because the low-head system design cuts the horsepower requirements by more than half. A second key objective of this study was to compare the productivity and economics of these two types of recirculation systems.

System Descriptions:

System A System A is housed in a 30’ x 152’ Quonset-style greenhouse. The greenhouse consists of a series of arches or bows made of 2” diameter galvanized steel pipe. The bows are anchored in concrete at their bases. The arches are supported by purlins running the length of the greenhouse connected by clamps to each rib. Cross-struts span every second arch providing additional support. A double layer of 6-mil clear UV-resistant polyethylene plastic material covers the greenhouse. The space between the two layers of plastic is inflated by means of a small blower. The two layers of plastic are highly efficient at collecting and retaining solar heat. The dead air space between the layers of plastic functions as an insulating layer. During the night, greenhouses with a single layer covering lose much of the heat collected during the daytime. Nighttime heat loss is greatly reduced when a double layer of plastic is used to cover a greenhouse. The improved heat retention justifies the added expense of the second layer of plastic and the inflation system. During the summer months an 80% shade cloth covers the outside of the greenhouse. The shade cloth minimizes algal growth within the raceways. The greenhouse is ventilated by two 1.5 hp extractor fans and by one 0.5-hp


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extractor fan mounted on one end of the greenhouse. The ventilating air enters at the greenhouse at the opposite end through two mechanical louver windows. The fans and the louvers are thermostatically activated, providing a measure of automated temperature control to the greenhouse. The System A greenhouse contains four culture tanks, each operating on separate filter systems (Figure 1). Two of the culture tanks (H5-NE and H5-SE) are set up as single-phase culture systems and two tanks (H5-NW and H5-SW) are set up as three phase systems . H5-NE (single-phase), and H5-NW (three-phase) both measure 13.5’ x 56’., while (H5-SE and H5-SE (single-phase) and H5-NW (three-phase) both measure 13.5’ x 64’. The three-phase systems are subdivided into three sections. The nursery section of H5-NW (H5-NW1) measures 13.5’ x 6.5’, with the long axis of the raceway perpendicular to the long axis of the overall growout area. The intermediate growout section (H5-NW2) measures 13.5’ x 13.5’. The final growout section (H5-NW3) measures 13.5’ x 36’. The nursery section of H5-SW (H5-SW1) measures 13.5’ x 7’. The intermediate growout section (H5-SW2) measures 13.5’ x 18.5’. The final growout section (H5-NW3) measures 13.5’ x 38.5’. Four-inch diameter bulkhead fittings positioned at the bottom of the walls dividing the three sections all shrimp to be transferred from one section to the next without being handled. The culture tanks consist of a wooden frame supporting a black 30-mil high-density polyethylene liner. The wooden frame is two board widths high and is built using 2”x12” boards of pressure-treated lumber supported by galvanized pipe set vertically in a concrete anchor. The vertical pipe supports are set on 4-ft centers. The arches forming the frame of the greenhouse support the outside walls of the culture tanks. The culture tanks are rectangular in shape and have been set up with a "racetrack" configuration. The racetrack configuration is essentially a hybrid between a circular tank and rectangular tank. Each culture tank is set up with two drain outlets at either end of the tank, centered between the end wall and the sides of the tank. A center divider baffle has been positioned between the two drain outlets, and functions to separate water flowing down one side of the "racetrack" from the water flowing down the opposite side. The water in the tank flows in an elongated oval pattern, travelling down one side of the tank, circling around the drain outlet at one end, then travelling up the other side of the raceway and circling around the opposite drain outlet. Baffles have been placed in the corners of the tank to prevent

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Figure 1: Culture tank layout in System A. Upper two culture tanks are single phase systems. Lower tanks are three-phase systems.


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eddies from developing in the corners. The baffles help create a semi-circular flow pattern at each end of the tank so that the water pivots about the drain outlets. This flow pattern generates centrifugal forces as the water circles the drain, concentrating the suspended solid wastes in the area around the drains. Water is introduced into the tank at the head of the straight runs. The incoming water mixes with the water circling the racetrack, creating relatively uniform water quality throughout the tank. The water enters the tank through spray bars spanning the width of the straight run of the tank. All drain outlets are 4 inches in diameter, with the exception of those in the nursery tank, which are 2 inches in diameter. All drain outlets consist of bulkhead fittings which pass through the liner and feed into a common 4-inch central drainage pipe. A PVC standpipe is set in each drain outlet. The height of the standpipe sets the minimum water level in the tank. The top of the standpipe is fitted with a cylindrical screen extending to 6 inches above the maximum water level in the tank. The purpose of this screen is to exclude shrimp from the drain outlets. An outer sleeve is placed over the standpipe to allow the water flowing out of the drain outlet to be drawn from the bottom of the tank. The outer sleeve consists of a PVC pipe with a slightly larger diameter than that used for the standpipe. The pipe is scalloped or screened at the bottom to allow bottom water to pass through it. Water passing through the drain outlets empties into a 4-inch diameter central drainage pipe. The central drainage pipe discharges into a 4’ x 3’ x 4’ polyethylene sump located on the outside of the tank at one end of the raceway. The sump serves as a settling basin and pump well. A 2-hp centrifugal pool pump circulates water through the system. The intake for the pump is located near the bottom of the sump and is fitted with a check valve to prevent the pump from losing its prime when it is turned off. A 36-inch diameter high-rate downflow sand filter serves as both the solids filter and the biofilter for the system. The sand filters are loaded with 500 lbs of Number 20 silica sand. A 2.5-hp regenerative blower supplies air to the system. Each culture tank is provided with forty 1” x 3” medium pore diffusers. Each diffuser supplies approximately 0.3 standard cubic feet per minute (scfm) of air to the raceway, providing each culture tank with a total of 12.0 scfm of air. The airstones are distributed at 3-foot intervals along the sidewalls of the culture tanks. A beltdrive blower powered by a 9-hp diesel motor serves as an emergency backup. The backup blower has a pressure-actuated switch that starts the blower motor whenever the pressure in the air system drops to zero. System B: System B represents a second generation in HBOI shrimp production system design. The design objectives for this system were to:

1) reduce the cost of the greenhouse structure 2) reduce the construction costs for the culture tanks 3) make the systems more energy efficient 4) reduce the labor required to maintain the systems 5) increase the carrying capacity of the systems, while keeping system costs down 6) provide for consistent circulation of water throughout the system.


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The System B culture tanks are housed in two 30’ x 96’ Quonset-style greenhouses . These greenhouses are similar to the System A greenhouses described above, but are less expensive. The System A greenhouses are rated to be able to withstand winds of up to 120 mph, while the System B greenhouses are only rated for winds of up to 80 mph. During the summer months a 95% shade cloth was placed on the outside of the greenhouse. This provided significantly more shade than the System A shade cloth, which provided only 80% shading. The greenhouse ventilation system consists of two 42-inch x 3/4-hp exhaust fans and two 51-inch shuttered windows. A single thermostat controls both the windows and the exhaust fans. An 8’ x 8’ sliding door is located at one end of the greenhouse. This door allows large pieces of equipment or harvest boxes to be easily moved into the greenhouse. The culture tanks in System B are similar to those in System A, except that they are partially excavated below ground level. Instead of having the floor of the culture at ground level and the tank depth determined by the height of the wooden frame, the floor of the System B culture tanks is excavated to a depth of 18-inches below grade. A wooden frame surrounds the perimeter of the excavated area, adding an additional 12-inches to the depth of the raceway. The wooden frame is similar to the frame used to create the System A culture tanks. A berm with a 1:1 slope extends from the bottom of the wooden frame down to the floor of the tank. The overall tank depth, when filled with water is 24-inches, or 6-inches deeper than the System A tanks. The culture tank is lined with the same 30-mil high density polyethylene liner material as is used in System A. There are several advantages to this approach to raceway construction. Because much of the volume of the raceway is below ground level there should be less heat loss from the raceways during the winter. The raceways can be made slightly deeper without appreciably increasing the cost of construction. A deeper tank will sustain a higher biomass, and will also have more stable temperature and water quality characteristics. Each of the System B greenhouses is occupied by two culture tanks, each with its own water treatment system. The culture tanks lie side by side in the greenhouse, sharing a common central wall. The 3-foot wide walkway between the tanks has been replaced with a 1-foot wide catwalk above the tanks. In this configuration approximately 90% of the available area in the greenhouse is under cultivation, compared to about 80% in System A. Reducing the number of systems per greenhouse from four to two reduces the labor requirement by half,

Figure 2: System B Single-Phase Raceways with Axial Flow Pump


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without sacrificing any production. One greenhouse in System B (Figure 2) contains two single-phase shrimp production systems (J1 and J2). Each single-phase culture tank measures 14.5’ x 88’. Except for the fact that they are in-ground culture tanks, most of the details of their construction are essentially the same as in System A. The tracks are configured in “racetrack” configuration with a central baffle and corner baffles. The second greenhouse in System B (Figure 3) contains two three-phase shrimp production systems (J3 and J4). The culture tanks in the three-phase system are layed out like the single-phase culture tanks, except that they are separated into three discrete sections by divider walls. The nursery sections measures 10’ x 14.5’ (11% of the culture area). The intermediate growout section measures 14.5’ x 27’ (31% of the culture area) and the final growout section measures 14.5’ x 51’ (58% of the culture area). Four-inch diameter bulkhead fittings positioned at the bottom of the walls dividing the three sections all shrimp to be transferred from one section to the next without being handled. One of the design objectives was to build a system that was more energy efficient than our sand filter-based systems. Towards this end it was decided to incorporate into the design a low-head filtration system that flowed by gravity through the solids filter and biofilter. An upflow bead filter is used in System B to filter out solid wastes as well as for biofiltration. The upflow bead filter consists of cylindro-conical sump (4’ diameter x 4’ deep, 1,200-liter capacity ), filled with 16 ft3 of biofilter beads. The beads are polyethylene cylinders 7 mm long by 10 mm in diameter with radiating fins that provide additional surface area. These beads are positively buoyant. The tank is plumbed so that the raw water from the culture tank enters the filter tank through a 4-inch bulkhead fitting cut into the conical portion of the tank. A second bulkhead fitting is cut into the sidewall about 12-inches below the culture tank water level and connects the solids filter to the biofilter tank. A 4-inch pipe with 5-mm slots cut into its upper surface is inserted into upper bulkhead fitting on the inside of the tank. This pipe collects filtered water from near the surface of the water and allows it to pass into

Figure 3: System B 3-Phase Production Systems, powered by a 3/4 hp centrifugal pump.


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the biofilter tank. The water must take a tortuous pass through the filter bed before it reaches the collecting pipe at the top of the water column. In the process settleable solids and larger suspended solids are trapped on the sticky surfaces of the beads. The solids are flushed from the system once a day by plugging the raw water inlet and the filtered water and removing a 2-inch standpipe from the drain outlet in the bottom of the cone. A 6-inch diameter outer standpipe keeps the beads from beads from draining out of the system. Windows covered by a 1/4” mesh screen are located near the bottom of the outer standpipe and allow water and solid wastes to pass through the outer standpipe and out of the tank when the central standpipe is removed. An aerated, submerged biofilter receives water after it flows out of the low-head bead filter. The biofilter uses the same beads as are used in the solids filter, but in this application the beads are tumbled by air bubbles introduced into the bottom of the filter bed through a grid of 10 medium pore airstones. The beads are contained within a 3.5’ x 5’ x 4’ cage made of 1” Schedule 40 PVC pipe and 1/4” square-mesh polyethylene mesh screen. The cage serves to contain the beads so that they do not get sucked into the pump or go out down the drain. The cage sits in a rectangular, polyethylene sump (4’ x 6’ x 4’), two inches above the bottom of the sump. The biofilter sump doubles as a pump reservoir for the main system pump. The amount of head required to return the water to the culture tanks is minimal since there are no filter components between the pump and the culture tanks, and the elevation head that must be overcome is less than 12-inches. Two different types of low-head pumps are being used in System B. A 1/4-hp axial flow pump (designed and built by Harbor Branch personnel) performs the pumping duties in the single-phase culture systems. This pump utilizes a plastic propeller as an impeller. The pump column is made of 4-inch PVC pipe. A tee halfway up the column directs the flow out of the pump and into the culture tank. The pump inserts into a bulkhead fitting that passes through the wall of the sump and the culture tank just below the tank water surface. There is essentially zero head pressure. These axial flow pumps are capable of moving large volumes of water with very little energy expenditure. The pump discharges 160 gpm of water in this application. Despite the high discharge volume of these axial flow pumps, the water is discharged with very little pressure or velocity. As a result, the return flow does not generate a great deal of circulation within the culture tank. Nor does the return flow provide any additional aeration or degassing. The axial flow pump could not be used in the three-phase systems because the return flow had to be piped to the opposite end of the greenhouse to the nursery and intermediate growout sections. The discharge out of these axial flow pumps drops off rapidly as head pressure increases. At only 18 inches of head the pump discharge is less than one-quarter of the discharge at zero feet of head. For this reason a centrifugal pump was used with the three-phase production systems. The pump selected was a low-head, high-efficiency 3/4-hp centrifugal pump. This pump will push 150 gpm of water against 10 feet of head and is much more efficient than the high-head 2-hp pool pump used in System A. The System A pump will only pump 100 gpm at this head and requires more than twice the horsepower.


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Using a centrifugal pump in the three-phase systems permits the return flow to be introduced through spray bars, which span the raceways. Each spray bar consists of a 2” diameter PVC pipe drilled with 1/4” diameter orifices. The momentum of the water passing at high velocity out of these orifices is transferred to the mass of water circulating in the tank. We observed that, although the discharge of the centrifugal pumps is slightly less than that of the axial flow pumps, the velocity of water flow rate in the three-phase culture tanks is much higher. This is because the water enters the culture tank at a high velocity and its momentum sets the entire mass of water in the tank moving. This is very important because it prevents the solid wastes from settling out and accumulating on the floor of the culture tank. Another important benefit derived from spray bars is that the water is aerated as it enters the tank and excess carbon dioxide is de-gassed as the water passes out of the spray bar. Whichever pumping system is used, configuring the system so that there are no filter components between the pump and the culture tank guarantees that the flow rates through the system are constant over time. This is in sharp contrast to System A, where flow rates declined by 50-75% between sand filter backwashes. A single 2.5-h.p regenerative blower supplies the air supply for the four culture tanks in System B. This blower supplies approximately 100 scfm of air against a head pressure of 50-inches of water. Each system is supplied with 25 scfm of air, which is delivered through submerged 3”x1” medium pore diffusers. A total of 44 diffusers are positioned in the culture tanks at 4-ft intervals on either side of the central baffle. Ten additional airstones are set into an air manifold at the bottom of the biofilter cage and serve to aerate and tumble the biofilter media. A belt-drive blower powered by a 9-hp diesel engine provides emergency backup aeration. This blower is twice as large as it needs to be, delivering 200 scfm of air at 50-inches of water, but was sized to accommodate future expansions. A pressure switch turns the diesel engine on whenever it detects a loss of air pressure in the air system. Freshwater and seawater are both supplied by wells. Wellwater is a desirable water source because it virtually free from bacterial, viral, or parasitic pathogens. The wellwater does, however, have some undesirable chemical characteristics. Like a lot of wellwater, HBOI’s wellwater is high in hydrogen sulfide, carbon dioxide, and ammonia, and is low in oxygen. Before it can be used the water must pass through a series of pretreatments. The first step in the pretreatment process is to remove the supersaturated gases such as hydrogen sulfide and carbon dioxide by passing the water through a degassing tower. The degassing tower consists of an eight-foot tall polyethylene tank with a six-foot diameter. Inside the tank is a screened plate spanning the entire cross-sectional area of the tank. This plate supports plastic coiled packing media, which fills the volume of the tank above the plate. The water distributed over the packing media at the top of the tank trickles down through the media in thin sheets and small droplets. A 1/4-hp blower pumps air into the bottom of the column. The column is open at the top, allowing the air to escape. By increasing the area of the air-water interface, gas-exchange between the air and the water occurs at an accelerated rate. Supersaturated gases in the well water are transferred from the


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water to the air, and gases that are under-saturated in the water, such as oxygen, are transferred from the air to the water. The water passing out of the degassing tower should be close to the saturation level for all of these gases. The next step in the treatment process is to remove the majority of the ammonia that is in the water. This is accomplished by passing the water through a 12,000 liter biofilter tank. The biofilter tank contains numberous barrels of oyster shell. Each of the barrels is provided with an airlift, which functions to circulate water through the oyster shell biofilter media. Oyster shell is lifted from the bottom of the bed by the airlift and deposited again at the top of bed. This circulation of the oyster bed through the airlift serves to slough off biofloc from the surface of the oyster shell. The flow rate through the biofilter tank is approximately 200 liters per hour. The total residence time in the biofilter is approximately one hour. During this time the ammonia is reduced from nearly 1 ppm to about 0.05 ppm. The nitrite concentration of the water leaving the biofilter is typically less than 0.01 ppm. The treated water flows by gravity into one of two 20,000-liter water storage reservoirs. The water storage reservoirs are enclosed polyethylene chemical storage tanks that have been given a double coat of paint to keep them dark inside to prevent algal growth. A 2-hp centrifugal pool pump draws water from the reservoirs and pumps it through a sand filter and out to the culture systems. The water delivery pump operates continuously so that water is available on a demand basis. A return to the reservoir tank is provided to protect the pump when there is little or no demand. The effluent from the shrimp production tanks discharges into a sump containing chlorine tablets to kill any escaping shrimp. A submersible trash pump with a mercury float switch pumps water from the chlorination sump to a series of retention ponds. The retention ponds for the Harbor Branch aquaculture park consist of three one-quarter acre ponds connected in series. All effluent from the facility discharge into one corner of the first pond in the series. Overflow pipes pass through the levees separating each of the three ponds. The first retention pond is the primary solids settling pond, and typically has the densest growths of algae and aquatic plants. The aquatic plants absorb nitrogenous wastes from the water. Evaporation and seepage account for virtually all of the losses of water from the retention ponds. The second and third ponds in the series provide extended residence time for the water to guarantee that the water has enough time to evaporate, or seep out of the ponds. Every few years it will be necessary to pump out the sludge that collects in the bottom of the ponds. One of the advantages associated with producing shrimp in near-freshwater systems is that this sludge can be used as fertilizer for certain vegetable or row crops. Retention ponds similar to the ones used by Harbor Branch are likely to be required of all feed-based aquaculture operations in the state of Florida.

Production Trials

Methods Three sets of paired production trials were conducted during the project. In each of the paired trials a single-phase and a three-phase culture system were stocked at the same time with postlarvae from the same cohort to permit comparisons to be made. One of the three-


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phase culture tanks, H5-NW, was stocked on 4/12/99 in a non-paired trial. No single-phase culture tanks were available for stocking at that time. System H5-NE (single-phase) and system H5-NW (three-phase) were paired in the first trial (System A Winter Trial). These tanks were stocked November 27, 1998 and harvested on May 25, 1999, after 180 days. Systems H5-SE (single-phase) and H5-SW (three-phase) were paired in the second trial (System A Spring Trial). These tanks were stocked on February 23, 1999 and harvested on August 21, 1999, after 179 days. A third production trial paired single-phase and three-phase culture tanks in System B (System B Spring Trial). This study was to have been initiated at the same time as the System A Spring Trial, but was delayed by two months by unforeseen problems in obtaining building permits to construct the System B greenhouses. Post-larvae were stocked into System B during the last week of April. The System B trials were terminated on September 14, after only 135-140 days, in anticipation of possible landfall of Hurricane Floyd. These tanks were harvested early to prevent accidental escape of the shrimp in the event of flooding. In each of the trials stocking densities were calculated to achieve a harvest density of 150 shrimp/m2, with overstock to compensate for the expected 35% mortality from PL to harvest. The target stocking density in the single-phase systems was 230 shrimp/m2. The target stocking density in the in the nursery section of the three-phase systems was 1,250 shrimp/m2. Table 1 summarizes both stocking and harvest data for each of the three trials. The System B culture systems were stocked at densities that were approximately 30% less than the targeted initial densities. This was because of heavy pre-stocking mortality among postlarval shrimp held in the hatchery from the originally scheduled stocking date in late February until the actual stocking date in April. We wanted to give the postlarvae a head start in the hatchery to make up for the delayed startup date. Cannibalism in the hatchery tanks resulted in heavy losses, so not enough of the large postlarvae were available to permit stocking at the targeted densities. All systems were stocked with Specific Pathogen Free (SPF) postlarvae produced for the study at Harbor Branch. SPF postlarvae are guaranteed to be free from the known viral diseases, including Baculovirus, Infectious Hypodermal Hematopoetic Necrosis Virus (IHHN), Taura Syndrome Virus (TSV), and WSSV. The postlarvae were acclimated to near-freshwater conditions in the hatchery prior to stocking. Acclimation to near-freshwater conditions is possible only after the shrimp have sufficient gill development to permit osmoregulation, usually after they reach PL12. All crops were reared in near-freshwater conditions. Salinities in the growout tanks averaged 0.7 pppt, chloride concentrations averaged 400 mg Cl-/L , total hardness averaged 400 mg/L as CaCO3, and alkalinity averaged 150 mg/L as CaCO3. The feeds used in this study were specially formulated with elevated levels of calcium, phosphorus, potassium, Vitamin C, and other vitamins and minerals. The elevated levels of these ingredients are necessary for normal growth and development of shrimp in high density freshwater recirculating systems. A variety of feeds are required to raise the shrimp from postlarvae to harvest size. During the nursery phase we fed postlarval and juvenile diets manufactured at HBOI by Rolland Laramore. For the first five days the shrimp were fed


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J400, a 50% protein 400µm postlarval diet. The shrimp were fed J1000, a 50% protein juvenile diet (particle size 850-1200 µm) until they reached a size of 0.2 g/shrimp. When they reached that size they were weaned onto a 1.6 mm pellet (J1600). J1600 has a protein content of 45%. The shrimp remained on J1600 until they reached a size of about 0.8 grams/shrimp, when they were switched to a 3/32" 45% protein juvenile pellet. The shrimp remained on this diet until the end of the nursery phase. In the intermediate and final phases of the growout the shrimp were fed a 3/32” grower pellet. Until April we were feeding a diet manufactured by Burris Mill and Feed that contained 38% protein . This diet was not particularly attractive to the shrimp, and we were dissatisfied with the growth of the shrimp on this diet. In April we switched to a 35% protein Rangen diet formulated for intensive culture. This diet was more palatable to the shrimp and produced better growth rates. The shrimp were fed four times per day by hand at 8:00 A.M., 11:00 A.M., 2:00 P.M., and 5:00 P.M. The shrimp were fed according to their appetite. Feeders were instructed to monitor feed consumption and adjust feeding rates upward by 10% if all of the feed was consumed in a 3-hour period, and downwards by 10% if significant quantities of feed remained from the previous feeding. Throughout the project records were kept on feed consumption, temperature, salinity, dissolved oxygen, total ammonia (TAN), unionized ammonia, nitrites, alkalinity, and hardness. Shrimp from each culture system were weighed on a biweekly basis to monitor growth. The daily maintenance routine included twice daily jetting and backwashing of sand filters and upflow bead filters. Dead shrimp were removed and counted whenever they were observed.

Results The production results from all trials are summarized in Table 1.

System A Winter Trial Survival in the single-phase system (H5-NE) stocked on 11/27/98 was a surprising 88%, with nearly 13,800 shrimp surviving out of 15,750 shrimp stocked. The harvest density was 201 shrimp/m2. A total of 142 kg of shrimp were harvested. The biomass loading in the system was 2.25 kg/m2, which is very close to what we had hoped to produce from the system. However, the average size of the shrimp at harvest was only 10.3 grams after 180 days (Figure 4). With such a high survival, the carrying capacity for the system was reached before the shrimp reached an acceptable harvest size. The average growth rate for this crop was a very disappointing 0.4 grams/week. System breakdowns may also have contributed to the small size of the shrimp in the single-phase tank (H5-NE). Beginning in March we encountered problems with the H5-NE sand filter. One of the laterals in the bottom of the sand filter broke. The broken lateral caused all of the sand to be lost from the sand filter and a complete loss of biofiltration. Feed rates were sharply reduced until new sand became biologically active. The following month two valves on the sand filter broke off on different occasions. The resulting downtime forced


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additional reductions in feed rates. Throughout April and May ammonia levels were often high (Figures 6) and oxygen levels (Figure 7) were routinely less than 5.0 mg/l. The 47% survival rate of shrimp in the three-phase system, H5-NW, was almost half that of the single-phase system. The relatively low survival resulted from high rates of cannibalism during the nursery and intermediate phases, and from Vibrio infection. Cold weather in December and February was followed by relatively warm period (Figure 5). These temperature fluctuations may have stressed the shrimp, precipitating the outbreak of Vibrio. With less crowding and generally good water quality conditions, the shrimp in H5-NW grew much better than those in H5-NE, reaching the size of 15.1 grams in 180 days. This corresponds to a growth rate of 0.58 grams/week. System H5-NW did not experience the same water quality problems late in the study that were experienced in H5-NE. Ammonia (Figure 6) levels remained low throughout the trial, and dissolved oxygen levels (Figure 7) remained high. Nevertheless, the growth rates were slower than expected. Previous experience led us to expect the shrimp would reach a harvest size of 18 grams in 180 days. A major factor contributing to the slower than expected growth rates in both systems was the cold weather in December and February. Neither the tanks nor the greenhouses were heated, so passive solar heating of the greenhouses was the only means of maintaining temperatures. In the first month of the trial a shade cloth covered the outside of the greenhouse, cutting down on the warming effect of solar radiation during the day, but doing little to prevent heat loss at night. Cold weather in December and in February (Figure 5) resulted in water temperatures of 22ºC or less for extended periods of time. Temperatures rarely rose above 26ºC for the first 60 days of the culture period. When the water temperature is less than 22º C, the shrimp do not grow. Growth rates are significantly reduced when temperatures drop below 26ºC.

System A Spring Trial: As was the case in the Fall production trials, the survival in the single-phase production system, H5-SE (76%) was significantly higher than the survivals achieved in either of the two three-phase systems stocked at the same time (Table 1). The survival in the two three-phase studies stocked in February were 61% (H5-SW) and 40% (H5-NW). The survival in H5-NW was estimated at 65% until July 7, when the air supply to the raceway was interrupted due the failure of a pipe connection. Over two thousand shrimp (20% of the population) died as a result of the low dissolved oxygen condition. Cannibalism was frequently observed in the nursery and intermediate sections of the three-phase culture tanks. After 179 days the shrimp in the single-phase culture tank, H5-SE averaged 14.6 g and a total of 194.2 kg of shrimp were harvested. Growth rates averaged 0.57 g/shrimp/week. Shrimp harvested from the three-phase culture tanks averaged 15.3 g/shrimp (H5-SW) and 13.6 g/shrimp (H5-NW) after 180 days. The growth rates in H5-SW averaged 0.6 g/shrimp/week, while the shrimp in H5-NW grew at an average rate of 0.53 g/shrimp/week.

Table 1: Summary of production results

Culture System

Date Stocked

Number of Shrimp Stocked

Initial Average Wt. (g)

Culture Period (Days)

Number of Shrimp

Surviving Percent Survival

Final Average Wt. (g)

Final Biomass

(kg)

Final Density

(Shrimp/m2)

Feed Conversion

Ratio

Single-Phase

H5-NE 11/27/98 15,750 .005 180 13,798 88% 10.3 142 201 1.76

H5-SE 2/23/99 17,400 .01 179 13,300 76 % 14.6 194.2 169 1.83

J1 4/28/99 19,000 .003 135 12,807 67% 9.0 115.3 109 1.36

J2 4/26/99 20,000 .03 137 1 18,074 90% 9.5 171.7 153 1.41

Three Phase

H5-NW 11/27/98 10,000 .005 180 4,680 47% 15.1 70.7 108 1.91

H5-SW 2/23/99 11,362 .005 179 6,938 61% 15.3 106.2 142 1.61

H5-NW 2/5/99 10,450 .008 180 4,153 40% 13.7 56.9 95 2.05

H5-NW 4/12/99 9,900 .02 154 6,224 63% 14.6 90.9 144 1.67

J3 4/25/99 10,000 .02 138 8,184 82% 15.0 122.8 121 1.70

1) Nearly all of the shrimp initially stocked on 4/26/99 died on 6/1/99 due to high nitrite levels resulting from stocking the

PLs before the biofilter was conditioned. The tank was restocked on 6/14/99 with 20,000 shrimp of the same age as the survivors from the initial stocking. The re-stocked shrimp were approximately the same size (0.90 g vs. 0.99 g) as the surviving shrimp from the initial stocking. Because the restocked shrimp were the same age and size as the surviving shrimp from the initial stocking, the culture period for J2 given in the table is counted from the initial stocking date (4/26/99).


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Growth rates in all three of the tanks stocked in February were extremely slow during the first ninety days after stocking (Figure 8), averaging less than 0.3 g/shrimp/week. The shrimp in all three tanks measured 4 grams or less after 90 day. For the second 90 days of the culture period, growth rates averaged between 0.80 and 0.85 g/shrimp/week in each of the three tanks. Low temperatures may be partly responsible for the slow growth observed in the spring production trials. Weekly temperature averages were less than 28ºC for more than half of the culture period (Figure 9). Growth rates begin to be affected when temperatures drop below 28ºC. No major ammonia or nitrite problems were observed in any of the three culture tanks. Total ammonia nitrogen levels were maintained, for the most part, at a concentration less than or equal to 0.4 mg TAN/L (Figure 10). TAN concentrations were elevated in H5-NW for a period of three weeks in June, ranging between 0.8 and 1.2 mg TAN/L. Concentrations of toxic unionized ammonia ranged between 0.05 - 0.08 mg NH3-N/L during this period. These concentrations of unionized ammonia are well below the lethal limit for juvenile shrimp, but could have had an impact on the growth rates of the shrimp. Nitrite levels were generally less than 0.4 mg NO2-N/L in all three culture tanks. Nitrite levels were slightly elevated (0.8 – 1.2 mg NO2-N/L) in H5-SE during the last 2 weeks before the shrimp were harvested. However, these levels were more than an order of magnitude less than the lethal limit for adolescent shrimp. Dissolved oxygen levels dropped to about 1.5 ppm in H5-NW on July 7 when the air supply to the tank was interrupted for an undetermined period of time because of the failure of a pipe connection. This incident resulted in the loss of 2,000 shrimp. Aside from this incident, dissolved oxygen levels were maintained above 5 mg/L throughout the culture period in both of the three-phase tanks (Figure 11) . The dissolved oxygen concentrations in the single-phase tank, H5-SE, were maintained above 5 mg/L except for the final 7 weeks, when dissolved oxygen concentrations averaged between 4 and 5 mg/L (Figure 11). This may explain why the growth rate of the shrimp in H5-SE slowed during the final three weeks of the culture period (Figure 8).

System B Summer Trials The System B trials were terminated 6 weeks early on September 13 and 14 because of the threat posed by Hurricane Floyd. As a result, the survival and final harvest size data presented for tanks J1, J2, and J3 in Table 1 are not directly comparable to the System A data because the culture period was much shorter. At the time these tanks were harvested, however, trends were already emerging. Survival in J1 after 135 days was 67%, slightly lower than had been observed in other single-phase culture systems. The large majority of the mortality was observed during the fourth and fifth week of the study, when nitrite levels peaked at 10 mg NO2-N/L. This was due to the fact that, because of the construction delays, we stocked the system without pre-conditioning the biofilters. Nitrite levels were even higher J2, which was stocked with


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larger postlarvae and was receiving more feed. J2 experienced massive mortality four weeks after it was stocked. We restocked J2 on June 17 with juvenile shrimp of the same age as the shrimp that had been initially stocked. Survival of these animals was 90% to the end of the study. One of the three-phase systems, J4, also experienced nitrite levels above 20 mg NO2-N/L during the fourth week of the study and suffered nearly 100% mortality. There were no animals available to restock this tank, so we continued the study with only one three-phase tank in System B. The survival in the remaining three-phase tank, J3, was 82% after 138 days (Table 1). This survival was achieved despite the fact that these shrimp experienced nitrite levels as high as 6.0 mg NO2-N/L during the biofilter conditioning process. In contrast to what was observed in the three-phase tanks in System A, very little cannibalism was observed in J3. The reduction in cannibalism may be related to two system modifications that reduced the encounter frequency between the shrimp. The tank depth in the nursery section of J3 is nearly twice as deep as in the System A nursery sections. In addition, current velocities were higher in the nursery and intermediate sections of J3. Higher current velocities cause the shrimp to move off the bottom and swim in the water column. This, combined with the increased tank depth, reduced the encounter frequency between the shrimp. The shrimp in the single-phase tanks, J1 and J2, averaged 9.0 and 9.5 g/shrimp, respectively, after 136 days (Table 1, Figure 12). These average weights are comparable to the weights observed for shrimp in the System A studies after the same time period (Figures 4 and 8). Growth rates were particularily slow during the first 90 days of the culture period. This was most likely due to the fact that throughout the second month the shrimp were on a restricted diet because of the high nitrite concentrations in the tanks. In addition, these greenhouses were covered with a 95% shadecloth, which virtually eliminated algal growth. Without adequate feed input and without significant natural productivity in the tanks, the shrimp were not adequately nourished during the first half of the two and half months of the study. In contrast, the shrimp in the three-phase tank, J3, grew very rapidly and reached an average weight of 15.0 g/shrimp after only 138 days. The average growth rate of these shrimp over the final 72 days of the growout period was 1.0 g/shrimp/week. Projecting this growth rate out over the next 42 days (to the expected harvest at 180 days), the predicted harvest weight of these shrimp would be 21 g/shrimp. Without carefully controlled experiments, it is difficult to say with certainty why the shrimp in J3 grew at a much faster growth rate than did all of the other tanks in this study. It is possible that the lower stocking density (about 30% less than the stocking density of the System A three-phase tanks) allowed for faster growth rates. However, with the higher survival rate, the final harvest density (122 shrimp/m2) was intermediate in the range of final harvest densities (Table 1) for the three-phase tanks in System A (95-144 shrimp/m2). Yet the growth rates in J3 far outpaced the growth rates in any of the System A tanks. System differences such as tank depth and filtration systems might be partially responsible for some of the observed differences in growth rates. While the differences in the tank depth and water filtration systems could explain differences between the growth rates


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between J3 and the System A tanks, they do not explain the pronounced difference in growth rates between J3 and the single-phase System B tanks (Figure 12). One important difference existed between the greenhouse enclosing J3 and the greenhouse enclosing J1 and J2. The shadecloth covering the J3 greenhouse was removed in the middle of June. After the loss of the shrimp in J4, system J4 was put into algae production for the clam hatchery. This required removal of the shadecloth. Following removal of the shadecloth, a dense algal bloom developed in J3, which was maintained until the end of the study. It is possible that the phytoplankton provided a supplemental food supply for the shrimp. Temperatures in the System B greenhouses (Figure 13) were much warmer than the temperatures that were maintained in the System B winter and spring trials (Figures 5 and 9). Temperatures were maintained above 30ºC in J3 from July until the end of the study. The temperatures in J1 and J2 were slightly warmer than in J3 through the first 60 days. During this period average weekly temperatures in J3 were less than 28ºC. Average weekly temperatures in J1 and J2 remained above 28ºC from the end of the first month to the end of the study (Figure 13). Ammonia was not a problem during the System B trials, despite the fact that the systems were started up without preconditioning of the biofilters. Ammonia concentrations spiked, as expected, about three weeks after the systems were started up (Figure 14). The maxiumum total ammonia nitrogen (TAN) concentrations observed in any of the tanks was 1.2 mg TAN/L. TAN concentrations declined to low levels following the initial peak, and remained low throughout the rest of the studies (Figure 14). TAN levels in J1 never peaked at all. High nitrite concentrations presented the major water quality problem in the System B trials, and resulted in the loss of two crops of shrimp. Establishment of the Nitrobacter population on the biofilter media lagged far behind the establishment of the Nitrosomonas population. For much of the first 60 days, the majority of the ammonia that was converted into nitrite by the Nitrosomonas bacteria, remained in the system. During this period nitrite levels were controlled primarily by water exchange. Initially we were rinsing and tumbling the beads in the solids filter twice a day to remove the solid wastes. In addition, the beads in the biofilter tank were tumbled continuously by aeration. It appears that the vigorous tumbling the beads were subjected to during this procedure interfered with establishment of the Nitrobacter population. Near the end of June we reduced rinses of the solids filter to once every third day. This procedural change was quickly followed by a reduction in the nitrite levels to less than 0.5 mg NO2-N/L in all three tanks. The solids filter thereafter functioned as a biological solids filter, and was the principal site for nitrification of nitrite to nitrate by Nitrobacter. Dissolved oxygen concentrations were maintained above 6.0 mg/L for the first 90 days in all of the System B tanks (Figure 15). Dissolved oxygen concentrations were maintained above 5.0 mg/L throughout the study in J1. Dissolved oxygen (DO) concentrations dropped below 5.0 mg/L in both J2 and J3 during the final month of the study. Morning DO levels occasionally dropped to as low as 3.5 mg/L in system J3, but typically would rise to about 7.0 mg/L in the afternoon. This diurnal swing in DO levels was related to the presence of an


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algal bloom in this system. Dissolved oxygen concentrations never dropped to dangerous levels in any of the System B tanks.

System A Summer Trial The three-phase culture system H5-NW was stocked for the third time during the year on April 12. This crop was not paired with a single-phase System A crop because no single-phase culture systems were available for stocking at that time. To a certain extent, the crop serves as a System A counterpart or control to the summer production trials in System B. This study was also terminated early on September 14 due to the threat of landfall by hurricane Floyd. Production data for this crop are summarized in Table 1. Survival was 63%, with a harvest density of 144 shrimp/m2. Cannibalism during the nursery phase and shrimp jumping out of the tank account for most of the observed mortality. Based on observations of mortality patterns in other crops, very little additional mortality would be expected if the crop had been carried on for three more weeks. Based on the high survival observed during the last three weeks in other tanks, it is likely the survival for a 180-day growout would have been above 60%. The average weight after 159 days was 15.1 grams. While growth rates averaged only 0.67 grams per week for the entire culture period, over the last 72 days of the culture period growth rates averaged 1.0 gram per week (Figure 16). If growth rates had continued at this pace for another 3 weeks, the shrimp would have averaged approximately 18 grams after 180 days. While this is slightly smaller than the projected harvest size of the System B three-phase crop in J3, it is much better than the growth rates that were observed in any of the winter or spring trials in System A. During the first 80 days, weekly average temperatures (Figure 17) were maintained between 26ºC and 28ºC. Weekly average temperatures were maintained between 28ºC and 29ºC during the last 80 days of the study. Warm temperatures during the latter half of the growout period coincide with the 1.0 gram/week growth rates. For the majority of the culture period, TAN concentrations were maintained below 0.4 mg TAN/L (Figure 18). For a two-week period during the second month of the culture period TAN concentrations averaged 0.8 mg TAN/L. During this time frame the pH averaged between 8.0 and 8.3, and unionized ammonia levels rose as high as 0.08 mg NH3-N/L. While these concentrations are well below the lethal level, they are more than double the desired upper limit of 0.03 mg NH3-N/L. Had the unionized ammonia levels remained at this level for long, growth rates would likely have been affected. Nitrite levels were generally less than 0.6 mg/L throughout the study. For a short period in early June, nitrite levels rose to 1.4 mg NO2-N/L. This level is slightly above the desireable upper limit for nitrite, but well below the lower lethal limit for juvenile shrimp. Dissolved oxygen concentrations were maintained above 5.0 mg/L throughout the study (Figure 19). During the first half of the study dissolved oxygen concentrations were generally maintained above 6.0 mg/L.


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Discussion One of the objectives for this study was to compare the productivity of a single-phase production system with that in a three-phase production system. As part of the analysis, a comparison was made of the potential annual production of shrimp in a single-phase and a three-phase production system (Table 2). This comparison was based on average stocking densities, survival rates, and harvest weights achieved during the course of this project. This analysis indicated that, despite lower average survival rates, the total annual production of a three-phase production system system should be 60% higher per unit of production area than the annual production of a single-phase system. The average weight of shrimp harvested from three-phase systems during this study was actually larger than for single-phase systems. Even if the harvest weights of shrimp from three-phase and single-phase systems were equal, the annual productivity of the three-phase system would be 40% higher than that of a single-phase system. If survival and final harvest weight of the two systems were equal, the three-phase system would out-produce a single-phase system by 80%. These results strongly favor the use of three-phase production systems over the traditional single-phase approach. The primary objective for this project was to quantify the key production parameters and associated costs for growing shrimp in greenhouse-enclosed freshwater recirculating systems. An economic analysis based on the data that were collected during the course of this project was performed for a hypothetical 12-greenhouse enterprise (see Appendix A). Because the production potential for the three-phase system was so much greater than that of a single-phase system, only the three-phase system was modeled.

Table 2: Comparison of the annual production potential a single-phase and three-phase system.

Culture System Parameter Single-Phase Three-Phase

Number of shrimp stocked/m2 of culture area/crop 200 120 Average survival rate to 180 days 1 77% 60% Average number of shrimp harvested/m2 of culture area/crop 154 72 Average weight of individual shrimp harvested (g) 2 14.1 15.9 Average total weight harvested/m2 of culture area/crop (kg) 2.17 1.14 Potential crops/year 2 6 Potential total harvest weight/m2 of culture area/ year (kg) 4.34 6.87

1 Predicted 180-day survival rates were used rather actual survival rates for the tanks that were harvested early

due to hurricane Floyd. The 180-day survival rate was predicted by projecting a straight-line survival curve from the initial stocking date to the actual harvest date out to the date when the culture period would have reached 180 days. This probably underestimates 180-day survival rates because most of the mortality occurs during the first 90 days after stocking. The survival rate of the spring trial of H5-NW, which lost 2,000 shrimp due to a disconnected airline, was adjusted upward by assuming that 80% of the shrimp lost in that event would have otherwise survived.

2 Predicted 180-day harvest weights were used rather than actual harvest weights for the tanks that were harvested

early due to hurricane Floyd. The 180-day harvest weights were predicted by assuming that during the time period between the actual harvest date and 180 days the shrimp would continue to grow at the same rate as was observed for the 10-week period prior to the actual harvest.


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Based on the costs and production parameters estimated in this study, the economic model shows that culturing shrimp in systems like those demonstrated in this study would not be profitable. However, the sensitivity analysis shows that if the survival can be improved to 70%, and the growth rate improved so that 18 gram shrimp can be grown in 150 days, a 12-greenhouse enterprise could generate an internal rate of return of nearly 50% (assuming the shrimp can be sold as a fresh, heads-on product for $5.24/lb). How likely is this scenario? There is good reason to believe that growth rates can be improved. As was discussed earlier, the slow growth rates that were observed in many of our production trials were, at least partly related to cool temperatures in the culture tanks during significant portions of the culture period. However, the production tanks were not heated and temperatures were not optimal for growth throughout much of the winter and spring trial culture periods. With optimal culture temperatures there is little doubt the shrimp could be grown to a minimum harvest size of 18 g/shrimp in 180 days. This was demonstrated in the summer trials in tanks J3 and H5-NW (Table 1). We did not, however, demonstrate that shrimp can be grown in tank culture systems to 18 grams in 150 days. It is well known that in ponds, L. vannamei grows best in ponds with high levels of natural productivity (Scura, 1995). Phytoplankton and organic detritus are both important components of the shrimp’s diet (Moss, 1992). L. vannamei has a very inefficient digestive system consisting of short, straight gut. Evidence is accumulating that L.vannamei does not utilize prepared diets efficiently, especially if their feces are rapidly filtered from the system. However, if their feces are allowed to remain in the system, heterotrophic bacteria will colonize the fecal material and convert feed protein into bacterial protein. Shrimp consume the decaying fecal material and the associated bacteria. The shrimp derive significant nutritional benefits from the bacterial proteins and partially digested feed proteins during this second pass. The importance of the detrital food chain to shrimp growth was not fully appreciated until this study was nearly over. The culture tanks were shaded to control algae growth, and solid wastes were quickly removed from the system in the interest of maintaining optimal conditions for biofiltration. As a result, the shrimp were almost completely dependent upon the nutrition they could absorb from the prepared feeds in a single pass through the gut. Recent unpublished work at Harbor Branch has demonstrated that the growth rates of shrimp grown in tanks managed for optimization of the detrital food chain have been up to 50% faster than the growth rates observed in the systems modeled in this report. Similarly, Moss (1999) reported growth rates of L.vannamei cultured in a high density tank-based culture system at the Oceanic Institute (OI) in Hawaii that were double the growth rates observed in the HBOI system. The primary difference between the OI system and the HBOI system was that the OI system was a “greenwater” system, while the HBOI system was a “clearwater” system. The presence of algae and organic detritus in the tanks was credited by Moss for the rapid growth rates that were observed in the OI system. These results suggest that the mediocre growth rates observed in this study were not strictly a function of the tank environment, or the high densities that were used. Rather, the slow


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growth may be related to the scarcity of detritus in the system. Better growth rates might be realized with alternative management strategies. Sand filters are probably too efficient at removing algae and fecal wastes. The filtration system used in System B allows some of the finer particulates remain in the system. These are broken down by bacteria and are potentially reprocessed by the shrimp. Removing the shade cloth and allowing phytoplankton blooms to develop appears to allow for much faster growth during the first 90 days of the culture period. Work is needed to learn how to manage systems with dense phytoplankton blooms so that they are stable. Nevertheless, it is clear that it is possible to grow the shrimp to a size of 18 grams in 150 days. The possibility exists that growth rates are reduced in a freshwater culture environment. Based on our results in this study, we have no basis for evaluating the possibility that the stress of the freshwater culture environment somehow inhibits shrimp growth, because we do not have saltwater controls to compare our results to. Reports from pond culture systems shows that excellent growth is possible at salinities down to 2 ppt. At 0.5 ppt, however, the shrimp are closer to their physiological limits. It is certainly plausible that the increased energy expended on osmoregulation comes at the expense of growth. This is an important question for future investigation. There is good reason to believe that survival rates can be improved in three-phase systems. A major problem encountered in our three-phase culture tanks was the high rate of cannibalism in the nursery and intermediate sections of the three-phase raceways. This is, at least in part, a density-related phenomenon. Post-larvae are very cannibalistic in high density environments, especially when underfed. The high density increases the encounter rate between individuals, increasing the opportunities for aggression to occur. Design modifications such as deeper tanks and greater water movement should help reduce cannibalism by reducing the encounter rate between postlarvae. Increasing the frequency of feedings is another management strategy that may help reduce cannibalism. Shrimp are more likely to cannibalize their peers when they are hungry. This problem can be overcome by feeding aggressively and more frequently to make sure the shrimp are never hungry. There is growing evidence that artificial substrates can help improve both survival and growth rates of shrimp in both ponds and raceways. Artificial substrates provide additional surface area, which lets the shrimp spread out more. In addition, periphyton growing on artificial substrates provides a nutritious supplement to the artificial feeds. The average survival rates achieved in our single-phase culture tanks was 77% . Survival of shrimp in our unshaded System B three-phase system was 82%. There is good reason to believe that improved system design, use of artificial substrates, promotion of algal and detrital food chains and increased feeding frequencies can improve survival rates to 70%. If both growth and survival can be improved, and shrimp can be sold for at least $5.00 per pound, greenhouse culture of shrimp in freshwater recirculating systems could be a profitable business.


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Recommendations While it is clear improvements are needed, we still are confident this is a technology for the future. The following are some recommendations for improving the technology, based on the experiences of this year: 1. Heating systems are required to obtain consistent growth year-round. Studies are

necessary to determine the most economical system for heating a raceway. 2. Deeper raceways with higher velocity should help improve survival, and obtain more

uniform growth rates. 3. Artificial substrates should be investigated to determine if they help reduce cannibalism

and if epiphytic growth can provide an additional source of natural foods. 4. Research should be conducted to determine how to create stable production systems with

rich algal and detritus-based food chains. 5. More research is needed to develop nutritionally complete diets, especially for young

juveniles. 6. Work on improving low-head biofiltration and solids removal systems. 7. More research is needed to determine whether or not near-freshwater systems inhibit

growth due to chronic osmotic stress. Additional research is need to determine if mineral supplements to the water are needed or beneficial.

8. Market research is needed to determine the nature of direct markets for freshwater

shrimp. 9. Work is needed to determine if other types of biofiltration systems are better suited to

shrimp culture (for example, systems utilizing heterotrophic bacteria, in addition to autotrophic bacteria, to control ammonia and nitrite concentrations.).


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Literature Cited Davis, D.A. and C.R. Arnold. 1998. The design, management, and production of a

recirculating raceway system for the production of marine shrimp. Aquaculture Engineering 17: 193-211.

Moss, S.M. (1999). Biosecure Shrimp Production: Emerging Technologies for a Maturing

Industry. Global Aquaculture Advocate 2(4/5): 50-52. Moss, S.M., G.D. Pruder, K.M. Leber, and J.A. Wyban. 1992. The relative enhancement of

Penaeus vannamei growth by selected fractions of shrimp pond water. Aquaculture 101: 229-239.

Scura, E.D.. 1995. Dry season production problems on shrimp farms in Central America

and the Caribbean Basin. In, C.L. Browdy and J.S. Hopkins, editors. Swimming Through Troubled Waters, Proceedings of the Special Session on Shrimp Farming, Aquaculture ’95. World Aquaculture Society, Baton Rouge, Louisiana. pp. 200-213.

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Figures


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Figure 4: System A Winter Production Trial Growth Curve.

Figure 5: System A Winter Production Trial Temperature Data

System A Winter Production TrialGrowth Curve

0.0

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H5-NW (Three-Phase)

System A Winter Production TrialTemperature

18

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24

26

28

30

32

34

0 15 30 45 60 75 90 105 120 135 150 165 180Days After Stocking


H5-NW (Three-Phase)


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Figure 6: System A Winter Production Trial Total Ammonia Nitrogen Concentrations

Figure 7: System A Winter Production Trial Dissolved Oxygen Concentrations

System A Winter Production Trial Total Ammonia Nitrogen

0.0

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System A Winter Production TrialDissolved Oxygen

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Figure 8: System A Spring Production Trial Growth Curves.

Figure 9: System A Spring Production Trial Temperature Data.

System A Spring Production TrialGrowth Curve

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H5-SE (Single Phase)

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H5-NW (Three-Phase)

System A Spring Production TrialTemperature

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34

0 15 30 45 60 75 90 105 120 135 150 165 180

Days After Stocking

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H5-NW (Three-Phase)


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Figure 10: System A Spring Production Trial Total Ammonia Nitrogen Concentrations.

Figure 11: System A Spring Production Trial Dissolved Oxygen Concentrations.

System A Spring Production Trial Total Ammonia Nitrogen

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System A Spring Production TrialDissolved Oxygen

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H5-NW (Three-Phase)


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Figure 12: System B Summer Production Trial Growth Curves.

Figure 13: System B Summer Production Trial Temperature Data.

System B Summer Production TrialGrowth Curve

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J3 (Three-Phase)

System B Summer Production TrialTemperature

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Week

J1 (Single Phase)

J2 (Single Phase)

J3 (Three-Phase)


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Figure 14: System B Summer Production Trial Ammonia Concentrations.

Figure 15: System B Summer Production Trial Dissolved Oxygen Concentrations.

System B Summer Production Trial Total Ammonia Nitrogen

0.0

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System B Summer Production TrialDissolved Oxygen

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Figure 16: System A Summer Production Trial Growth Curve

Figure 17: System A Summer Production Trial Temperature Curve.

System A Summer Production TrialTemperature

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System A Summer Production TrialGrowth Curve

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Figure 18: System A Summer Production Trial Total Ammonia Nitrogen Concentrations.

Figure 19: System A Summer Production Trial Dissolved Oxygen Concentrations.

System A Summer Production Trial Total Ammonia Nitrogen

0.0

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System A Summer Production TrialDissolved Oxygen

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Chapter 1 – Introduction

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Chapter 1 Introduction

by Kevan L. Main and Peter Van Wyk

Harbor Branch Oceanographic Institution This manual was developed to provide an overview of the production strategies for farming marine shrimp in recirculating freshwater, greenhouse-enclosed, raceway production systems. A detailed review of the design and operation of these shrimp farming production systems and an economic analysis will be presented. The manual is intended for field use by shrimp farmers, students and extension agents. The focus is on the postlarval through growout phases of the production cycle and will not address broodstock or larval rearing issues. This publication and the demonstration study was funded by a contract from the Florida Department of Agriculture and Consumer Services (FDACS Contract No. 4520). Research conducted at Harbor Branch Oceanographic Institution demonstrating the technical feasibility of growing marine shrimp in freshwater has resulted in a surge of interest in shrimp farming from the aquaculture, agriculture and business community. Florida farmers have begun to seriously consider shrimp culture as a second crop and a few Florida farms have recently begun producing shrimp. Although Florida has lagged behind South Carolina and Texas in shrimp farming, there are signs that the Florida shrimp farming industry is on the verge of development. New technologies have been developed that make shrimp farming a viable option in Florida. The feasibility of growing L. vannamei in Florida's hard freshwater has been demonstrated and has greatly expanded the potential sites for shrimp farming. The technology for growing shrimp in intensive, enclosed culture systems is being refined and the economic analyses indicate that shrimp farming can be profitable.

An Overview of the Development of Shrimp Farming Litopenaeus vannamei (also known as Penaeus vannamei) is the most extensively farmed species of marine shrimp in the Western Hemisphere. L. vannamei is rapid growing, tolerates high stocking densities, has a relatively low dietary protein requirement and tolerates a wide range of salinities. It is a hardy animal that is highly adaptable to culture conditions. The natural distribution of L. vannamei extends from the Pacific coast of Mexico to northern Peru (Dore and Frimodt 1987). The technology for culturing marine shrimp is relatively new. The initial hatchery culture technology was developed in Japan in the 1930s and 1940s with Penaeus japonicus by Motosaku Fujinaga (Shigueno 1975). Breakthroughs in shrimp hatchery technology in the 1960s and 1970s paved the way for rapid growth of shrimp farming in the 1980s and 1990s. Annual world production of farm-raised marine shrimp has grown from 92 metric tons in


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1982 to 737,200 metric tons in 1998 (Rosenberry 1998). In many areas around the world, shrimp seedstock are still collected from the wild and stocked in large coastal ponds. In the western hemisphere, Ecuador is the leading producer of farm-raised shrimp (Rosenberry 1998). The majority of the shrimp farms in Ecuador continue to rely on extensive growout techniques. In a typical extensive growout system, stocking is accomplished by flooding the ponds and bringing naturally occurring shrimp postlarvae into the pond, along with fish, crabs and other organisms. Little or no feeding is done and shrimp growth depends on the natural productivity of the pond. Extensive systems produce around 50-500 kg/ha/yr. As the shrimp industry matured, producers adopted more intensive production methods. Stocking rates are closely monitored through the use of hatchery-reared postlarvae and the shrimp are fed specially formulated feeds. Most farms in the western hemisphere use a semi-intensive pond production strategy. Semi-intensive growout systems produce around 500-5,000 kg/ha/yr and the natural food in the pond is supplemented with formulated feeds. Intensive production systems produce around 5,000-10,000 kg/ha/yr (Brock and Main 1994). Shrimp are fed large quantities of formulated feeds, pond water is frequently exchanged and supplemental aeration is provided. A two-phase production system may be used, where juvenile shrimp are grown at high densities in small nursery ponds. Juvenile shrimp are later transferred to large growout ponds, where they are reared to harvest. One of the obstacles to the development of commercial shrimp farming in the U.S. has been the lack of a reliable supply of high quality seedstock. It is critical for U.S. hatcheries to be able to rear their own broodstock to produce high-health or specific pathogen free (SPF) seedstock for U.S. farms. Reliance on broodstock and seedstock from other countries increases the risk of introduction of new shrimp diseases. Broodstock that are guaranteed to be free from specific disease-causing organisms are called "Specific Pathogen Free" (SPF) broodstock. Some states, such as South Carolina, now require that all shrimp seedstock sold in the state come from SPF broodstock. Development of new hatcheries should help to overcome the obstacle of a short supply of L. vannamei seedstock in Florida. Over the past twenty years there has been a significant increase in U.S. consumer demand for marine shrimp, while the U.S. commercial catch has remained relatively constant. Increasingly the demand for marine shrimp has been met by farm-raised shrimp. Nearly 30% of the world shrimp supply is now being provided by pond aquaculture (Browdy 1998). Until recently, commercial shrimp farming in the United States has been limited to a few farms located in south Texas, South Carolina and Hawaii. There are several reasons why commercial shrimp culture has been slow to develop in the United States. A combination of regulatory constraints, temperate climate conditions and high labor costs have limited the development of U.S. coastal shrimp ponds. Unlike the tropics, where air and water temperatures allow for year-round shrimp production, low temperatures during the late fall and early spring limit the production of this species to one crop per year (Main and Fulks, 1990). Higher U.S. land and labor costs and are also limiting factors.


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New Approaches and Considerations for Shrimp Farming The limited growing season and higher land values have forced U.S. shrimp farmers to adopt a more intensive approach to production. Researchers and farmers in Florida have recently been investigating intensive tank or raceway recirculating aquaculture systems. These systems are often enclosed in greenhouses to allow for greater control of temperature. A full description of the steps involved in greenhouse construction is presented in Chapter 3. The system design principles for recirculating, intensive raceway or tank production systems are discussed in Chapter 4. Stocking densities in intensive raceway culture systems range from 100-250/m2. Water in the culture system is typically circulated through a water treatment system that removes solids and nitrogenous wastes. Ultraviolet light or ozone is often used to reduce bacteria levels in the water. Blowers or liquid oxygen are used to maintain adequate levels of dissolved oxygen in the water. Recirculating aquaculture systems require a higher level of technical expertise and are more expensive to build and operate than a pond culture system. However, there are several advantages associated with these types of systems. They allow shrimp to be grown commercially in locations where land is limited or land values make pond construction prohibitively expensive. The controlled environmental conditions allow for year-round production in areas otherwise restricted to a limited growing season. Water reuse technology can reduce the water requirements and discharge from the culture system, minimizing the environmental impact and permitting requirements for the operation. Harbor Branch is currently evaluating a relatively low-cost, recirculating production system. The design objectives and system features are described in Chapter 5 of this volume. Providing the animals with the optimum quantities of a nutritionally complete feed is one of the most critical aspects of shrimp husbandry. Feeds remain the single highest expense for farmers that are operating intensive production systems. Feeds still need to be formulated to meet the nutritional requirements of shrimp farmed in intensive raceway production systems. Chapter 7 discusses the nutritional requirements and the feeding strategies that are appropriate for intensive shrimp production. One of the biggest challenges facing the shrimp industry is the control of disease in farmed shrimp populations. Effective strategies to control the occurrence and spread of disease are primarily related to management of the production system. The common health problems, diseases and strategies that farmers can use to control disease in L. vannamei are discussed in Chapter 9. The success of a new aquaculture business is closely correlated with careful planning. Chapter 2 addresses three issues that need to be addressed as you are getting started in shrimp farming. The first issue is business planning. Training, marketing and the preparation of a business plan are all discussed. The second issue that must be addressed is obtaining the required permits to construct and operate a business. At the time this manual was prepared, Florida’s permitting requirements were undergoing revision. The state was working with the


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key stakeholder groups to develop Best Management Practices (BMPs) for each of the commodity groups, including shrimp. A general overview of the changes that have occurred in Florida’s permitting process is also discussed in this chapter. The third issue that needs to be examined is the suitability of your water source for shrimp farming. The parameters that need to be examined are listed and the water testing process is presented. The last chapter of the manual is an analysis of the economic feasibility of culturing marine shrimp in a hypothetical freshwater production system. The analysis in Chapter 10 uses the data gathered by Harbor Branch during the experiments conducted during the demonstration study and certain key assumptions to run the economic model. The assumptions, investment requirements, production inputs and operating costs are presented.

Freshwater Culture of Marine Shrimp The Pacific white shrimp’s tolerance to low salinities has greatly expanded the potential locations for shrimp aquaculture in Florida. That information, coupled with the desire of the agriculture community to diversify their production options, has significantly increased the interest in shrimp farming in Florida. Studies have shown that L. vannamei can be successfully farmed in freshwater raceways and ponds, provided the water is hard enough and has the correct mineral balance (Scarpa and Vaughan 1998; Scarpa et. al. 1999). Other penaeid species have also been shown to be adaptable to low salinities. Shivappa and Hambrey (1997) found that Penaeus monodon can be grown at salinities ranging from 2-3 ppt. Maturation and postlarval production of L. vannamei still requires saltwater. Once the postlarvae reach the PL12 to PL14 stage, they can be acclimated to freshwater. At this stage, the gills are developing and they can withstand the osmotic stress (Scarpa 1998). Chapter 6 describes the procedures for handling and acclimating postlarvae from saltwater to freshwater production systems. The ionic composition of the well water in a number of locations around Florida appears to be suitable to support L. vannamei and the results of a water testing program are presented in HBOI’s 1999 final project report to the Department of Agriculture and Consumer Services. A full discussion of the water quality parameters and requirements for culture of L. vannamei is presented in Chapter 8. Taste tests have shown that shrimp grown in freshwater are well accepted and are difficult to distinguish from those grown in saltwater.


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Literature Cited Brock, J.A. and K.L. Main 1994. A guide to the common problems and diseases of cultured

Penaeus vannamei. World Aquaculture Society, Baton Rouge, Louisiana, USA. Browdy, C.L. 1998 Recent developments in penaeid broodstock and seed production

technologies: Improving the outlook for superior captive stocks. Aquaculture 164 (1-4):3-21.

Dore, I. And C. Frimodt 1987. An illustrated guide to shrimp of the world. Osprey Books,

Huntington, New York. 229 pp. Main, K.L. and W. Fulks 1990. The culture of cold-tolerant shrimp: Proceedings of an

Asian-U.S. workshop on shrimp culture. The Oceanic Institute, Makapuu Point, Honolulu, Hawaii. 215 pp.

Rosenberry, B. (Ed.) 1998. World shrimp farming 1998. Shrimp News International. Scarpa, J. 1998. Freshwater recirculating systems in Florida. In: Moss, S.M. (Ed.)

Proceedings of the U.S. Marine Shrimp Farming Program Biosecurity Workshop February 14, 1998. The Oceanic Institute. pp. 67-70.

Scarpa, J. and D.E. Vaughan 1998. Culture of the marine shrimp, Penaeus vannamei, in

freshwater. Aquaculture '98 Book of Abstracts, pp. 473. Scarpa, J., S.E. Allen and D.E. Vaughan 1999. Freshwater culture of the marine shrimp,

Penaeus vannamei. Aquaculture America '99 Book of Abstracts, pp. 169. Shigueno, K. 1975. Shrimp culture in Japan. Assoc. for Int. Tech. Promotion, Tokyo, Japan. Shivappa, R.B. and J.B. Hambrey 1997. Tiger shrimp culture in freshwater? INFOFISH

International 4/97:32-36.


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Chapter 2 – Getting Started

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Chapter 2 Getting Started

by Megan Davis-Hodgkins, John Scarpa and Joe Mountain

Harbor Branch Oceanographic Institution Introduction Several steps are involved in getting started in an aquaculture business. A thorough background review of the species you are interested in farming should be done. That review should include an evaluation of the production strategies that are appropriate for your site and lifestyle and the market opportunity. The next step is the development of a business plan. The business plan will identify the variables that should be considered before you make a decision to start a new aquaculture business. Be sure to have the water resource evaluated to determine if it is suitable for shrimp farming. The water quality parameters that need to be examined are briefly discussed and the water testing process is presented in this Chapter. Finally, you will need to obtain the required permits to construct and operate an aquaculture business. A short summary of the changes in Florida’s permitting process during the past few years is discussed in this chapter.

Planning Your Aquaculture Business Planning is the key to success in any business and aquaculture is no exception. The planning of your aquaculture business has to be done by you. There is no other business exactly like yours and you are the one who understands and can evaluate your personal and business goals. This section provides you with steps that will guide you in making decisions on starting a new aquaculture enterprise. The ultimate goal of this section is to stress the importance of a well thought out business plan and the methodology to build your business plan. The steps that should be considered before starting a new enterprise are: • Expectations (personal considerations) • Research and Training (know your species) • Production Planning (inputs and outputs) • Market Feasibility (selling the product) • Business Plan (financial feasibility) • Demonstration or Pilot-Scale Operation (test ideas - start small) • Commercial-Scale Operation (expansion to profitability)


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Expectations When starting a new business it is important to balance this business with your personal life style and financial capabilities. If the aquaculture business is a change of career, you need to ask yourself three questions: "How strongly do I want to make a change? What do I want for my future? Am I willing to take a risk in a new business?" These questions may be best answered by considering the following set of questions:

1) Are you interested in shrimp culture? 2) Does your current job or business hold your interest? 3) Do you have skills that you are not using and would like to incorporate in an

aquaculture business? 4) How much money do you expect to make? 5) How much income do you need to operate your household? 6) How much money can you afford to invest? 7) Would you risk losing your savings on the new enterprise? 8) Are you willing to absorb occasional losses? 9) Are you willing to borrow money to finance the new business? 10) How will the new venture impact your family? 11) Is your family willing to relocate? 12) Is your family willing to live on a reduced income until you sell your crop? For how

long? 13) Does your family support your directions and the risk? 14) Will family members work in the business? 15) Will this new venture supplement or replace your current job? 16) Are you willing to devote the daily time and effort required?

Research and Training If you are new to aquaculture and/or new to shrimp aquaculture, it is important to have a complete understand of the biology and the techniques used to farm shrimp. This knowledge can be acquired through several methods such as:

1) attending short courses at training institutes or college/universities 2) experiencing daily work routines as a volunteer or employee 3) contacting local state agencies and extension agents for information 4) attending professional meetings and trade shows 5) reading articles, documents, or journal papers 6) researching the topic through the internet

A combination of all of the above is probably the best way to become familiar with the working knowledge needed to develop a business plan, produce the animals, and operate a business. Experience, through trial and error, is the most valuable way to learn the ins and outs of the business. Setting up a demonstration or pilot-scale operation is one way to gain this valuable experience and is discussed later in this chapter.


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Production Planning A production plan describes each step of the production process and identifies inputs, outputs and time requirements for each phase of the operation. It is based on assumptions that come from documented research, commercial operations or personal experience. It is the first step in developing information about the cost of production (operating and capital costs) and the expected yield (revenues). The emphasis for this production plan discussion is based on the nursery and growout production phase for shrimp. First, it is important to decide on the production system you will be using. This decision will take into account these factors:

1) Biological requirements (environmental and nutritional parameters) 2) Technological and commercial viability 3) Resource availability 4) Environmental impact 5) Permitting requirements

Next the key variables need to be identified. These variables will affect the inputs and outputs of the production operation. Each of these variables need to be assigned values. These values need to be as realistic as possible and based on data reported for similar facilities producing shrimp. These are some examples of key variables for the nursery and growout production phases: • Number of production units • Production unit area or volume • Stocking density • Growth rates • Feed type • Feed conversion ratios • Stocking size • Harvest size • Survival • Water exchange rates • Aeration requirements • Filtration requirements • Energy requirements • Labor requirements • Chemical requirements Here are a few examples of how to use these variables with assigned values. Usually the inputs and outputs are based on one production unit, then the value calculated can be multiplied by the number of units in a facility. In these examples a standard production unit will be one shrimp raceway:


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Example 1: How many postlarval shrimp are needed to stock one unit? Production unit area = 90 m2 Stocking density = 230 shrimp/m2

90/m2 x 230 shrimp/m2 = 20,700 shrimp total

Example 2: What is the survival from the beginning of the operation to the final harvest for one production unit? Number stocked = 20,700 shrimp Survival = 69% 20,700 shrimp x 0.69 = 14,283 shrimp

Example 3: What is the total harvest weight per production unit? Number harvested = 14,283 shrimp Average weight of shrimp = 18 grams (14,283 shrimp x 18 grams) ÷ 1000 grams = 257 kg (566 lb)

Example 4: How much food is needed to feed the shrimp from stocking to harvest? Feed conversion ratio (FCR) = 2 Total weight at harvest = 257 kg (566 lb) 2 x 257 kg of shrimp = 514 kg (1131 lb) of feed

Example 5: How many production cycles per year for a standard production unit? Days from stocking to harvest = 158 days Fallow days = 4 days 365 days/year ÷ 162 = 2.2 cycles/year


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After each of the key variables are calculated dollar values can be assigned. Here are some examples of how to apply dollar values.

Example 1: What is the cost of the postlarvae to stock one production unit? Number of postlarvae = 20,700 shrimp Cost per 1000 acclimated postlarvae = $20/1000 shrimp (20,700 shrimp ÷ 1000 shrimp) x $20 = $414

Example 2: How much does the food cost to grow one crop? Amount of food needed for one crop = 1131 lb Average cost of food = $0.45/lb 1131 feed x $0.45/lbs = $509 Example 3: What is the revenue generated from one crop? Amount of shrimp harvested = 566 lb Revenue = $5.25/lb head-on shrimp 566 lb shrimp x $5.25/lb = $2972 per crop If there are two crops per year from one raceway = $5943 per raceway

You can build a production plan from two directions. You can ask yourself: How much product do I want to produce to get the desired revenue? or How much capital do I have available to invest? In either case, the revenue has to make money or a profit. The next example will assist you in determining how much production area is required to produce 30,000 kg (66,000 lb) of 18 g shrimp per year. Assume 2 crops/yr/raceway, 69% survival, and an average stocking density of 230 post larval shrimp/m2.

Weight Harvested = 30,000 kg/yr ÷ 2 crops/yr = 15,000 kg/crop No. harvested = 15,000 kg ÷ 0.018 kg/shrimp = 833,333 shrimp No. postlarvae stocked = 833,333 shrimp ÷ 0.69 survival = 1,207,729 postlarvae Area required = 1,207,729 postlarvae ÷ 230 shrimp/m2 = 5,250 m2


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Most customers will want to have fresh shrimp delivered every week. How many production units are needed to harvest one unit per week? How big would this unit have to be? These calculations will be based on production of 30,000 kg shrimp harvested per year.

1 harvest/week x 52 weeks/yr = 52 harvests/yr No. of production units = 52 harvests/yr / 2 harvests/yr/production unit = 26 production units

This means 26 production units would be required to allow for 1 harvest per week. In the above example it was determined that the production area to meet annual production target is 5,250 m2. Now you can calculate how large each unit should be:

Area/production unit = 5250 m2 ÷ 26 production units = 202 m2

In summary, approximately 2 raceways (90 m2 each) need to be harvested per week to meet the annual production quota of 30,000 kg of shrimp. If there are 4 raceways per greenhouse, then 13 greenhouses would be needed to grow 30,000 kg of shrimp per year. These examples were used to illustrate how to calculate production numbers, infrastructure size, harvest schedules, and revenue. These numbers should not be used as accurate numbers for your business plan. This is only a guide to help you in the development of your plan. Refer to the last chapter of this volume for additional information on production figures.

Market Feasibility Production must be driven by marketing. If you cannot sell your product or sell your product at the price needed to make the business profitable, then the business will fail. The marketing plan should be developed alongside the production plan. The market analysis is usually the most difficult section because the prospect of marketing and selling your product is a long way off; between 4-6 months after stocking for farm-raised shrimp. Planning ahead will turn your concept into a producing profitable operation with a consistently available and high quality product that people want to buy. There are five steps to market analysis:

1) define product market structure 2) determine relevant market 3) analyze demand 4) segment the market 5) determine market position

Marketing Farm-raised Shrimp Florida farmed shrimp cannot compete with imported, fresh-frozen shrimp prices in the commodity markets. Therefore, the most feasible alternative for Florida farmed shrimp is to sell the product directly to restaurants, retailers and consumers. This marketing strategy is


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called direct marketing and/or niche marketing. Unless a processing facility is going to be included in your business, the product forms that you will most likely be marketing are live shrimp or fresh heads-on shrimp. These product forms will require minimal processing and regulatory requirements. Any processing beyond the live or fresh, heads-on product form will incur additional costs, effort and expertise to realize the full economic potential of producing a processed product. There is limited knowledge on the direct markets for marine shrimp raised in intensive freshwater recirculating systems. Now is the time to investigate the market for Florida farm-raised shrimp. The specific areas that need to be analyzed for the shrimp marketing plan include:

1) Identify and quantify fresh shrimp direct markets. Which of the shrimp direct markets are most attractive for shrimp farmers, in terms of market size and the willingness of the purchaser to buy directly from the farmers?

2) Determine the value, demand and product characteristics for each of the direct

markets. What are the product requirements (shrimp size, price, shelf life, delivery terms) for each of the direct markets?

3) Test consumer and direct market buyer attitudes and acceptance of marine shrimp

grown in freshwater versus domestically wild-harvested or imported farm-raised products, relative to sensory attributes (appearance, flavor, texture) and yield (edible cooked meat from fresh vs. frozen product).

4) Prove the shelf life and product quality/safety parameters for direct farm sales of

whole, fresh shrimp. Include handling parameters and packaging to extend shelf-life, and Hazard Critical Control Point (HACCP) and Sanitation Standard Operating Procedures (SSOP) models to assure product safety.

Business Plan The next step after the production plan and market analysis is the business plan. Why a business plan? The business plan summarizes the business opportunity. It motivates you to find out all the information needed to make a successful business. This process must be done before any purchases are made. You are testing your ideas on paper, which minimizes mistakes and risks. This point cannot be stressed enough!! If you are seeking funding from investors, a bank or venture capitalists, they will not consider your ideas without a well thought out plan. The business plan is the principal sales tool in obtaining a loan or raising equity capital from investors. Loan officers and investors want to be sure that you have thought through your plan carefully, that you know what you are doing and that you can respond effectively to problems (crisis) and opportunities that arise.


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The business plan can also be used as a planning document. It allows you to organize your directions and evaluate the anticipated outcome of your company. You can use your business plan to set goals and assess your progress. There are usually 7 parts of a business plan: Executive Summary, Business Description, Market Analysis, Management Team, Financial Information, Milestone Schedule, and Appendix. Each of these parts are discussed below.

Executive Summary The Executive Summary is presented first, but is usually written last. The investor reads this first - it is the “first impression” of your business. This summary must convince the reader to continue reading the plan. It should be no more than 3 pages in length (preferable 1 page), it contains only information from the body of the business plan and the wording should be active and forward thinking. This summary needs to include: why the business will succeed, what is the market product, what market the product will meet, assessment of the competition, management team’s expertise, amount of money needed in terms of capital and operational expenses, the projected internal rate of return and why this venture is a good risk. This section could be prefaced with a short Mission Statement.

Business Description This is the major section of the business plan. The business is described with the following information: history of the company, product description, ownership, markets being served, location and site, permits and licenses, facilities and equipment, operations and culture techniques, cost breakdown and labor force. In this section, you also need to describe certain risks you may encounter. Some of the primary risks associated with most commercial aquaculture ventures are the following: drops in market price, disease, extremes in weather, management problems and electrical failure. Addressing these risks in the business plan will assist you in determining what to do to avoid risks. It shows the investors your knowledge of the business venture and how you will handle critical risks.

Market Analysis This part discusses market size and trends, competition and regulatory requirements. The market size and trends can be supported with industry trends and graphs. The strengths and weaknesses of competing businesses should be described. These competing businesses will probably include other aquaculture ventures and the commercial fishery. Your product should be compared with the competitor’s product ($, quality, availability). The product form will dictate what regulatory permits and licenses will be required. This part should also describe what markets will be met and how the product will be marketed (i.e., advertisement)

Management Team The management team needs to have a balance of skills in marketing, finances, management and production. A brief review of the duties and responsibilities of each person needs to be outlined. It is important to discuss how each person will be compensated (salary, profit sharing, incentive sharing). Depending on the structure of the company, it may be necessary to discuss the shareholders, board members and professional services. An organizational chart is very helpful.


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Financial Information The financial statements and cash flow balance sheets should include startup years and the projections should continue for 5 years after the commercial business is fully functional. A narrative of the capital costs (land, facilities, equipment - the non recurring costs), operational costs (salaries, feed, postlarvae, electricity, insurance – costs that occur daily, weekly, yearly) and revenue is included along with the spreadsheets. A breakeven and sensitivity analysis shows you what key variables are controlling the success of the company. This is a valuable analysis to present in your business plan. The net present value and internal rate of return are the numbers you and your lenders or investors are most interested in.

Milestone Schedule The schedule shows the projected timeline of the business. It includes all start-up operations, production schedules and harvesting times. It is typical for a start-up shrimp operation to havest the first crop 12-24 months from the time you begin to plan and build your facility.

Appendix (optional) The appendix usually includes additional details to further describe your business. Marketing studies, letters of interest, photographs and technical drawing, resumes, news clippings and character references are a few examples of what can be included in the appendix.

Demonstration or Pilot-Scale Operation A demonstration or pilot-scale operation is a scaled-down version of the commercial business. It is built with the same infrastructure and equipment, but may only include one to four greenhouses instead of 12-20 greenhouses. Even though a demonstration size facility may not make money or breakeven it will save you money in the long run. This facility is a prototype that is used to test ideas, to learn where to make improvements, to find out the demands of the business, to learn the methodologies to culture shrimp and to test your marketing plan. A demonstration facility will prove to you and to your lenders and investors that you know what you are doing. It is highly recommended that this demonstration project proceed the commercial business. This pilot-scale operation should either have it’s own production and business plan or be part of the main business plan. Start small and scale up slowly. Learn as you go and invest as you learn.

Commercial-Scale Operation By the time you get to the commercial- or production-scale business you have done all your homework. You have written a well-thought out business plan and are demonstrating your ideas in a small-scale facility. It may take 2-3 years to scale-up to a commercial size business, but once you are there, you will be operating a production business that will continue to bring you revenue harvest after harvest.


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Water Selection Criteria When you are culturing an aquatic organism, one of the most important variables to consider is the water. In order to select the water you are going to use, you need to know what the water quality parameters are for the organism you will culture. In this chapter, we will consider the culture environment for the marine shrimp, Litopenaeus vannamei, being grown in freshwater. All the parameter levels presented here have not been experimentally evaluated for culturing L. vannamei in freshwater. Having a supply of good water is essential in aquaculture and should meet the specific environmental needs of the organism to be cultured. The following list is a synopsis on the water quality parameters that should be checked during the evaluation of the site for shrimp culture. A more detailed discussion of these parameters can be found in Chapter 8 - Water Quality Requirements and Management. All water samples should be tested for pH, total ammonia nitrogen, nitrite, nitrate, total hardness, calcium hardness, total alkalinity, salinity, chloride and hydrogen sulfide. The acceptable values for each of these variables is listed in Table 2-1. Additionally, the water should be tested for heavy metal, pesticide and herbicide contamination if there is a reason to suspect that the site was previously used for agricultural or industrial purposes (see Chapter 8).

Table 2-1. Water quality variables and acceptable ranges. VARIABLE RANGE pH 7-9 total ammonia nitrogen <0.1 ppm nitrite <1 ppm nitrate <60 ppm total hardness >150 ppm as CaCO3 calcium hardness >100 ppm as CaCO3 total alkalinity >100 ppm as CaCO3 salinity >0.5 ppt chloride >300 ppm hydrogen sulfide <0.002 ppm

Finally, a bioassay should be performed using the source water and shrimp postlarvae. Bioassays use living organisms as indicators. In the case of aquaculture, bioassays should be conducted using the organism to be cultured. Bioassays are an important consideration during the selection of a site for aquaculture. The use of water quality data alone, without including a bioassay, can lead to false conclusions regarding the suitability of the water. Even after basic water quality parameters are identified, including pesticide and herbicide presence, the results of a bioassay with the intended culture organism is insurance that the site will be productive.


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Permitting Before 1990, obtaining the requisite permits to develop an aquaculture project in the State of Florida was an ordeal. Depending on the part of the State one wanted to work in, the degree of difficulty could vary greatly. In April 1990, a publication entitled "Florida Aquaculture Regulatory Sourcebook, Leasing, Licensing and Permitting Requirements for Aquaculture in Florida" was made available. It contained information compiled by James W. Miller, Ph.D., Florida Institute of Oceanography. In its 252 pages, he attempted to outline and clarify regulatory procedures affecting Florida aquaculture, and emphasized the need for and the means of obtaining the more than 50 leases, licenses and permits described therein. According to Dr. Miller, the major aquaculture activities in Florida in the late 1980’s were tropical/ornamental fish, and aquatic plant production. Their overall economic impact approached $150 million. He identified eighteen other types of Florida aquaculture activities involving organisms listed alphabetically from alligators to watercress. However, there was no mention of shrimp. In 1993, it was demonstrated that one particular species of marine shrimp, Litopenaeus vannamei, could be raised in hard "fresh water" in Florida. For the 25 years prior to this time, L. vannamei had been present in aquaculture activities in the State, but always in coastal/saltwater conditions. The fact that this species thrived, grew and tasted normally, under hard "fresh water" conditions, showed that inland shrimp aquaculture was a realistic possibility. In 1999, The Florida Legislature designated the Florida Department of Agriculture and Consumer Services (Department) as the clearinghouse for aquaculture permitting. This makes the process more streamlined, i.e., one phone call to the Division of Aquaculture at (850) 488-4033 initiates the process. Each case is handled individually. The department will provide an Aquaculture Certificate of Registration application and assist you through the environmental permitting process or implementation of the Best Management Practices(BMPs) specific to your situation. The Department has adopted an Interim Rule continuing the above process until BMPs are later adopted. Federal, county and local permitting will be the responsibility of the individual producer. The Division of Aquaculture may be able to provide some guidance. Under the proposed BMPs, shrimp producers must adopt and implement both the required sections of the General BMPs and the required sections of the shrimp specific tabbed section as the BMPs conform to the individual operation. Other suggested BMPs may be adopted to enhance the facility’s operation and production. If the required BMPs are not adopted or implemented, Environmental Resource Permits, NPDES permits and/or other required permits will be the responsibility of the producer and are prerequisites for certification of registration.


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Literature Cited Avault, J. W., Jr. 1996. Fundamentals of Aquaculture, A Step-by-Step Guide to Commercial

Aquaculture. AVA Publishing Co., Inc. Baton Rouge, Louisiana. Harbor Branch Oceanographic Institution. 1995. Aquaculture Options for Commercial

Fishermen. Prepared for Florida Department of Labor and Employment Security. Stromborm, D.B. and Tweed, S.M. 1992. Business planning for aquaculture - is it feasible?

Northeast Regional Aquaculture Center Fact Sheet No. 150.

Chapter 3 – Greenhouse Construction

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Chapter 3 Greenhouse Construction

by Megan Davis-Hodgkins

Harbor Branch Oceanographic Institution

Introduction Greenhouse structures have traditionally been used in agriculture, to house systems for growing crops such as tomatoes, cut flowers, and ornamental plants. Greenhouses are now widely used in aquaculture to house systems to culture aquatic plants, fish, and shrimp. Aquaculture greenhouses may be set up with a variety of different tank configurations depending on the species and stage of production. Numerous small volume tanks (less than 2500 gallons) are typically employed for culturing organisms during the hatchery phase. Large raceways (usually 1, 2, or 4 per greenhouse) are frequently used in commercial nursery and growout operations. Greenhouse buildings and the enclosed systems provide many advantages to the land-based aquaculturist. These systems provide cost-effective protection from adverse weather conditions and afford some level of control over environmental parameters such as temperature, light intensity, and relative humidity. Greenhouses are inexpensive to operate because they allow natural light to enter, which reduces lighting and heating costs. In temperate climates the use of greenhouses can often extend the growing season of tropical aquaculture species. Greenhouses offer the additional advantage of excluding predators, which often cause major losses in unenclosed systems. When enclosed recirculating systems are used in the greenhouse it offers the culturists a higher degree of biosecurity compared to open culture systems. Closed systems also ensures the grower that their crop unlikely to escape from the raceways. Greenhouses feature a simple design which is strong and inexpensive to construct. A common greenhouse design employs a frame made from a series of ribs or arches of galvanized steel alloy tubing anchored into the ground at their bases. They may be built either as single free-standing units, or as multiple units connected at the gutters. They are often covered with either a flexible plastic film made of UV stabilized polyethylene, vinyl, or mylar, or with rigid panels made of acrylic, polycarbonate, fiberglass reinforced plastic or aluminum sheeting. Polyethylene is the least expensive of these materials. However, it will need to be replaced after approximately 10 years, because it becomes brittle from exposure to the sun’s ultraviolet rays. The greenhouse covering may either be clear, to maximize the amount of light entering the building, or tinted or opaque to limit the amount of light. Opaque coverings are sometimes used in situations where photoperiod manipulation is required. Shade cloth hung internally from the cross trusses or secured on the outside of the greenhouse can reduce incident light levels which helps control excessive algae growth in the


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raceways. It is typical to keep the shade cloth on during the summer and roll the shade cloth off during the winter months. The ability to control the lighting conditions in greenhouses makes them suitable for culturing a wide variety of aquatic organisms.

Zoning and Permitting Greenhouses in this country are typically located in rural areas. However, not all rural land is zoned for agricultural use, therefore, you need to check on zoning or on acquiring a zoning variance. By building a greenhouse on agricultural land, the owner is likely to benefit from lower property taxes. Usually a greenhouse which does not have a permanent foundation (slab or footing) and which has a nonpermanent roof (polyethylene) is considered a temporary structure. In most places in Florida greenhouses are considered agricultural temporary buildings. The permitting process for greenhouses and temporary buildings varies from county to county. Therefore it is best to check with your county building department regarding placement of greenhouses.

General Construction Every greenhouse kit comes with a set of detailed instructions. Depending on the manufacturer, they may be willing to come out to the job site and assist you with the installation. They will certainly be available by telephone to assist you on details. The following is a general guide for the erection of a standard greenhouse with raceways.

Site Preparation A greenhouse is like any other building, it is no better than the site and foundation on which it is placed. Before beginning the erection of the greenhouse the site needs to be completely ready. In choosing a location for your new greenhouse try to pick an area which receives at least three to four hours of sunlight daily in the winter. If possible place the greenhouse ends north and south in order to take advantage of the maximum amount of sunlight in a day. Try to avoid locating a house under overhanging power lines and trees with limbs which might damage the house. Most important, avoid areas where poor drainage or water shed may be a problem. Try to avoid high areas or exposed areas which may subject the house to high prevailing winds which may cause damage and or high heat loss in winter. The site needs to be graded level for the length and width of the house plus a minimum of five feet on either side and ends. Always check the site with a transit or other reliable surveying instrument and grading should be done with the use of grade stakes. A fall of 12" every 100 ft will aid in drainage. This is the time to cut all drainage trenches and install all water lines and electrical services, before the house has been erected.

Anchor Layout The site preparation along with anchor placement are the most important steps of the greenhouse erection. These steps control the success of the constructed greenhouse and cannot be corrected later. To determine the perimeter of the greenhouse, four anchors are cemented in the corners of the site. By measuring both sides, both ends, and diagonally you


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can check the “square” of your house. The perimeter of the house can be defined by pulling a string around the 4 corner posts. The rest of the anchors are spaced every 4 ft for a standard greenhouse. Once they are level and properly spaced they are cemented into place with 10" of the anchor protruding above the ground. The arches must fit down over the anchors, therefore, they must be carefully installed and properly spaced. For instance, never drive an anchor into the ground without the protective cap installed. The concrete serves two purposes. First it provides a base on to which the arches can rest, therefore, the pads must be level laterally and longitudinally. Second, the concrete makes anchor movement unlikely. Allow the concrete to set up over night before installing arches.

Assembling Arches Arches typically come in two halves and any two halves will match. Before assembling the arches together at the apex with a sleeve, you must install the clamps over the arches that will be used for the horizontal cross trusses. There will be two end arches that need clamps for the gable end set. The number of upright pieces on the gable ends will depend on the greenhouse width and whether or not there is a premounted door, exhaust fans, and louvered vents. Prior to assembling the arches, lay down plastic on level ground to prevent soiling the arches. Using level ground will avoid the arches from becoming"warped". The arches should not be placed over the anchors until all arches are assembled. As an arch is assembled, place it in the proper location on the job site. Be certain that gable end arches and cross truss arches are properly sequenced. There is usually a cross truss every third arch if evenly spaced. Beginning on one end, select the gable end arch and place it over the anchors. Each arch and anchor have predrilled holes. Line these holes up and secure the arch to the anchor with a bolt. Erect the other arches in this manner. When assembling the arches do not be concerned that the arches are slightly wider than the greenhouse width ordered. One of the keys to the high load rating and strength of the greenhouse is the prestressing of arches as they are placed over the anchors. By allowing for prestressing in the manufacturing process, there is an increase in rigidity for increased static and wind loading. Once the arches are in place the center pipe ridge, cross trusses, and side purlins can be installed.

Gable Ends During the summer months the greenhouses may need to be cooled. This is typically accomplished by creating a draft through the length of the greenhouse using fans mounted in one gable end and louvered vents mounted in the opposite gable end (Figure 3-1). With a gable end kit, you need to determine where doors, fans, and louvers will be located. The fans and louvers should be placed between the uprights for support or bracing. With a premounted door, the door uprights may take the place of one or two of the uprights. The


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doors should be installed to open outward. Since the gable end uprights may be located in different spacing arrangements, you may find that you need to cut some of the uprights. Once the gable ends are installed and the uprights are cemented in place, the ends can be covered with polyethylene plastic or reinforced fiberglass sheeting.

Covering the Greenhouse As mentioned earlier there are many different coverings that can be used on the outside of the greenhouse. Polyethylene is the least expensive covering and is the one chosen for descriptive purposes. Prior to covering the greenhouse, extrusion tracks need to be installed along the bottom of the sides, on the gable ends, and the door ways. Covering the greenhouse with 6 mil UV stabilized polyethylene plastic will require a minimum of 10 people. The greenhouse should be covered on a day with absolutely no wind. The cover comes on a roll in two layers, which are folded to make it easier to handle. It is recommended that the corners of the plastic be secured with a triangular-shaped piece of plywood with a rope attached to aid in pulling the plastic over the greenhouse.

Figure 3-1: Gable end with mounted fans and door frame.


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There are two methods for covering the greenhouse. The first one is to have people situated inside the greenhouse near the top (Figure 3-2). The roll of plastic is then fed up to the people over the front of the arches, then unfolded down the sides of the greenhouse. People at the bottom need to grab the sides and temporarily secure the excess plastic, using sand bags, shovels of dirt, or themselves. Temporarily securing the plastic is a very important step, because if a slight gust of wind comes along it can take the plastic off the greenhouse. Care should be taken at all times not to force the plastic in any one direction because it tears easily. The polyethylene kit does come with tape to cover accidental holes. The second method is to unroll the cover approximately 30’ from the side of the greenhouse. The cover is then unfolded and several people pull the plastic over the sides of the arches (Figure 3-3). It is important to be careful not to pull too hard to avoid ripping the plastic. The sides need to be temporally secured in the same manner mentioned above. Once the plastic is on the greenhouse arches and temporarily secured it needs to be pulled tight. A long, thin rod is placed over the plastic and fits into a groove in the extrusion. Several people on both sides will secure the plastic in place with bolts and washers. This makes an air tight seal and removes any wrinkles in the cover. Once all of the washers are in place on the bottom sides, gable ends, and door frames, the blower can be turned on. It is best to place the blower in the center of the building above your head. The air pocket between the two layers helps to keep the building cool and keeps the cover structurally sound.

Figure 3-2: Installing plastic cover over the arches of the greenhouse. Raceway anchors are in place.


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Flooring There are many substances which make suitable flooring in the greenhouse (cement, sand, gravel, brick). Gravel (3/8" diameter stone) makes for an ideal walkway around the filtration units and in between the raceways. The flooring is typically laid down after the raceways are in place.

Raceway Installation Raceways can be used for culturing shrimp in the nursery and growout stages. A raceway is a long, narrow tank, which for shrimp, can be made of a 24" high wooden frame lined with a plastic liner. In a typical greenhouse 1, 2 or, 4 raceways can be installed in a single or three-phase configuration. The length of the raceways can vary depending on the amount of space you want in the center-middle or end of the greenhouse for plumbing, filters and equipment storage. It is recommended that you use increments of 4’ to stay within the spacing of the arches. Each raceway needs to be graded and surveyed in a similar manner to surveying the initial site for the greenhouse. Anchors are placed every 4 ft along the perimeter of the raceway. If you plan ahead, the raceway anchor next to the arch anchor can be cemented at the same time.

Figure 3-3: Installing the plastic cover of the greenhouse over the sides of the arches.


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Once the anchors are in place and the cement has dried over night. The wooden frame for the raceway can be put into place (Figure 3-4). The frame is constructed out of two 2” x 12” pressure treated lumber. Each piece of wood is fastened to the inside of the anchor using bolts. It is important to countersink the heads so that they do not damage the plastic liner. Prior to lining the raceways they need to be graded with a sand base. The sides of the bottom are graded to slope down to create a V-shape, this will assist in good drainage. It is easier to remove the bulk of the dirt prior to building the raceway frame. To ensure complete drainage of the raceways, the tanks should be sloped so that the drains are the lowest points in the tank. The wooden framed raceways are then lined with a plastic liner (30-40 mil thick PVC) (Figure 3-5). This liner will shape to the desired slopes and contours created during grading. The liner usually comes as one big sheet. It is recommended not to cut the liner in the corners, because it is difficult to seal with glue. Instead just fold the liner to fit in the corners. If corners are desired, it is best to ask the manufacturer to do this for you. Bulkhead fittings are used to connect drains to the liner. The liner can be secured to the wooden frame with PVC sleeves. You may consider building the raceways and installing the filtration system prior to covering the greenhouse. This will enable you to easily move items in and out of the greenhouse.

Figure 3-4: Raceways are built from 2’ x 12’ pressure treated lumber.


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Systems and Filtration Equipment The following is a list of systems you will need to consider installing in your greenhouse (see Chapters 4 and 5 for more details). These components will require space which must be designed into the floor plan. 1) Source of water 2) Filtration prior to the greenhouse 3) Filtration in the greenhouse 4) Aeration 5) Drainage for raceways 6) Dimensions of pvc pipe for incoming and outgoing water and for aeration

Figure 3-5: Installing raceway with PVC liner.

Chapter 4 – Principles of System Design

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Chapter 4 Principles of Recirculating System Design

by Peter Van Wyk


Introduction The design of the production system is one of the most important factors determining the success or failure of an aquacultural operation. The primary design objective should be to provide an environment that supports the consistent production of the targeted density of healthy, market-size shrimp in as short a time period as possible, while minimizing system capital and operating costs. Each element of the design should be considered in the context of this primary design objective. Recirculating aquaculture systems are composed of many components: culture tank, solids filter, biofilter, aeration system, pump(s), water distribution system, and drainage system. Multiple options are available for each of these components. Choosing between the available options is not simply a matter of deciding which option works the best. Each component must be selected to function as part of an integrated system, taking into account how each component will affect the operation of every other component in the system. In many cases one component option may be incompatible with certain other component options. In addition to choosing pieces of equipment that are compatible with the other components in the system, one must also make sure that each equipment item is properly sized with respect to the system volume, loading rates, and flow rates. The challenge is to select a set of appropriately sized components and set them up in a configuration in which each piece of equipment is able to perform optimally, thereby enhancing the performance of the other components in the system. Achieving this objective is not easy, especially considering that the system must be economical to build and operate. The goals of this chapter are to identify the basic components that make up a recirculating aquaculture system, to acquaint the reader with the different component options that are available, and to summarize the basic principles of recirculating system operation.

The Culture Tank The culture tank is a critical component of the production system, for this is where the shrimp live. The culture tank should provide enough space for the shrimp to grow to market size and should create a healthy environment for the shrimp to live in. The water quality in the tank should be relatively uniform, and solid wastes should not accumulate on the bottom of the tank. The uniformity of the water quality in the tank and the efficiency of solids transport out of the tank are largely determined by the hydraulic characteristics of the tank.


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These are a function of the shape of the tank, the location of the drain, and how the water is introduced into the tank.

Circular Tanks Circular tanks are good culture vessels because they provide virtually complete mixing and a uniform culture environment. When properly designed, circular tanks are essentially self-cleaning. This minimizes the labor cost associated with tank cleaning. Typically, water is introduced into a circular tank at the perimeter and is directed tangential to the tank wall. The incoming water imparts its momentum to the mass of water in the tank, generating a circular flow pattern. The water in the tank spins around the center drain, following an inward spiral to the center of the tank. Centrifugal forces and the inward, spiraling flow patterns transport solid wastes to the center drain area where they are removed. Once the mass of water in the tank is set into motion, very little energy is required to maintain the velocity of water movement in the tank. The momentum of the water circling the center drain helps sustain the circular flow. One can easily demonstrate this fact by swirling the water in a bucket with one's hand. The water will continue to spin for several minutes after the hand is removed. A good rotational velocity can be generated in the tank through the use of spray bars or airlift pumps positioned at the perimeter of the tank. The primary disadvantage of circular tanks is that they do not use space efficiently. A circular tank of a given diameter will have about 21% less bottom culture area than a square tank whose sides are the same length as the diameter of the circular tank. This means that if circular tanks are used there will be a 21% loss of potential production area in a given amount of space. If production is being carried out inside buildings or greenhouses the cost of enclosing a given amount of production area will be higher for circular tanks than for rectangular tanks.

Raceways A raceway is a rectangular tank in which water is introduced at one end and drains at the other end. The flow pattern generated in a raceway is called "plug flow". In raceways, the water travels down the raceway at close to a uniform velocity across the entire cross-sectional area of the raceway, so very little mixing occurs. As a result, water quality may vary significantly from one end of the raceway to the other. The best water quality will be found near the water inlet, while at the tail end of the raceway, oxygen levels will typically be lower and ammonia and carbon dioxide levels will be higher. It is important to turn over the volume in the raceway quickly enough so that water quality is within the acceptable range throughout the entire length of the raceway. Solids transport down the length of the raceway is dependent primarily upon the velocity of water flow. In order for the raceway to be self-cleaning, the water velocity must be sufficient to push the particulate wastes down the length of the raceway. The required water velocity, termed the minimum cleaning velocity, increases with increasing particle size and density


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(specific gravity). The minimum cleaning velocity can be estimated based on the following expression (Youngs and Timmons, 1991):

Vclean ≥

1

2 d

49 (G – 1)

12 Eq. (4.1)

where, Vclean = cleaning velocity (ft/sec) G = specific gravity of the material d = particle diameter in mm Minimum cleaning velocities for different types of particles are given in Table 1. Table 1: Minimum cleaning velocities for different types of materials (after Young and

Timmons, 1991). Material Particle Size Vclean (ft/sec) Feed/feces (G = 1.19) 0.1 mm 0.08 Feed 1.6 mm 0.12 Silt 0.002 mm 0.03 Fine Sand 0.05 mm 0.13

The required flow rate to produce a self-cleaning tank can be calculated by multiplying the cross-sectional area of the tank (in square feet) by the required cleaning velocity (ft/sec). The result will be the required flow rate in cubic feet per second. In practice, the minimum cleaning velocities will be somewhat less than the values calculated in equation (4.1) because the shrimp continually stir up and resuspend the particulate wastes, allowing the lower density fecal wastes to gradually work their way down the raceway. A difficulty with raceways is that when the solids reach the end of the tank, the hydraulic forces do not efficiently concentrate the solids around the drain. Water reflected off of the end wall generates turbulence, causing eddies to form that may keep the solids from going down the drain.

Racetrack Configuration Rectangular tanks can be set up with a "racetrack" configuration, which is essentially a hybrid between a circular tank and a rectangular raceway. Rectangular tanks set up in the racetrack configuration have two drain outlets at either end, centered between the end wall and the sides of the tank. A center divider is placed between the two drain outlets and functions to separate water flowing down one side of the "racetrack" from water flowing down the opposite side. The water in the tank flows in an elongated oval pattern, traveling down one side of the tank, circling around the drain outlet at one end, then traveling up the other side of the raceway and circling around the opposite drain. Baffles placed in the corners of the tank will prevent


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Figure 4-1: Airlift Pump

WaterWater

Airstone

Lifter Tube

Water

Air Bubbles

eddies from developing in the corners. The baffles help create a semi-circular flow pattern at each end of the tank so that the water pivots about the drain outlets. This flow pattern generates centrifugal forces as the water circles the drain, concentrating the solid wastes in the area around the drains. The water should be introduced through spray bars positioned at the head of the straight runs down each side of the tank. Mixing the incoming water with the water circling the raceway helps maintain uniform water quality throughout the tank. The spray bar serves three additional functions: (1) the spray bar increases the velocity of water flow around the tank because the water enters the tank at a higher velocity, thus increasing the velocity of the water in the tank; (2) the water is aerated as it enters the tank; and (3) excess carbon dioxide is degassed as the water passes out of the spray bar. Rectangular tanks in a racetrack configuration offer some of the same advantages as circular tanks with less loss of growing area. The water in a racetrack tank is well mixed, providing nearly uniform water quality. The semi-circular flow patterns around the ends of the racetrack provide a more efficient means of concentrating the solid wastes around the drain outlet than in a rectangular raceway configuration. It is critical, however, that that the velocity of the water circulating around the racetrack approaches minimum cleaning velocity. This will ensure that fecal wastes will be transported down the straight runs of the racetrack and concentrated close to the drain outlets. Airlift pumps are sometimes used to generate additional water velocity to assist with the movement of solid wastes down the straight runs of the raceway. An airlift pump consists of a vertical section of small diameter PVC pipe, called a riser tube, that is open at bottom and is fitted with a PVC elbow at the surface (Figure 4-1). Air is introduced into the lifter tube near the bottom through either an airstone or through a hose barb tapped into the side of the lifter tube. As the air bubbles rise inside of the lifter tube they expand, pushing water out ahead of them through the top of the airlift. The elbow serves to direct the flow in the desired direction. Introducing the air through an airstone slightly reduces the pumping efficiency of the airlift, but provides better aeration of the water than can be achieved without airstones. For maximum effect, airlift pumps should be set up at intervals of three or four feet along both sides of the center divider and the sidewalls of the tank.


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Water Depth The depth of water in the culture tank has many important effects on the culture system. The ideal water depth should maximize the space available for the shrimp, provide sufficient volume to provide for stable water quality and temperatures, maximize aeration efficiency, while minimizing the ratio of construction cost to system carrying capacity. Generally speaking, shallower tanks are cheaper to build. The question becomes then, what is the minimum depth necessary to provide a stable culture environment that can support high densities of shrimp? From a space standpoint, water depth is not that critical at densities less than 100 shrimp/m2 because the shrimp have room to spread out on the bottom of the tank. Water depths greater than 20 cm (10 inches) should provide sufficient space for the shrimp. However, as densities exceed 100 shrimp/m2, the shrimp will begin to utilize the water column to a greater extent as the bottom area becomes overcrowded. Strictly from a space standpoint, water depths between 45-60 cm (18-24 inches) are sufficient for densities of up to 150 shrimp/m2. Higher densities may require additional water depth. The thermal characteristics of the tank will be determined by the volume and surface area of the tank. Shallow tanks will have a high surface area to volume ratio. As a result, they will be more subject to rapid temperature change and will be in near equilibrium with the air temperature. Air temperatures are subject to wide daily fluctuations in greenhouses, and these fluctuations will be reflected in the water temperatures. Deeper tanks will have lower surface area to volume ratios and will be more resistant to temperature change. Over a period of time the water temperature will more nearly reflect the average air temperatures, rather than the extremes. Water depths of at least 45 cm (18 inches) are required to prevent extreme fluctuations in temperature. Tanks with depths of 90 cm (36 inches) have relatively stable thermal characteristics. System water quality will generally improve as water volume increases relative to the system biomass because of the dilution factor. Diluting a given amount of waste product in a larger volume of water results in a lower concentration of that waste product in the water. The detrimental effects of the waste products are a direct function of their concentrations in the water. All else being equal, a deeper tank will hold a greater volume of water and will provide a greater dilution factor for the waste products produced by the shrimp. Water quality in recirculating aquaculture is also related to the efficiency of solids transport out of the culture tank, system turnover time (the amount of time it takes to process 100% of the water in the system through the water treatment system) and the effectiveness of the water treatment system. Increasing the depth and volume of the culture tank increases the flow rates that will be required to generate minimum cleaning velocities. Increasing tank volume without increasing flow rates may negate much of the water quality benefit gained from the greater dilution factor because waste volume is often a function of time. For example, if tank volume is increased 10% without a corresponding increase in flow rates, tank turnover time will also increase by 10%. If waste production is constant over time, then the concentration


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of waste products in the tank will be the same in both situations. In order to get a proportionate benefit in water quality from an increase in water volume, the flow rate through the water treatment system must be increased proportionately as well. This implies that horsepower requirements increase with increasing water depth and system volume. Water depth also affects oxygen absorption efficiency in systems where air is supplied to the culture tank through submerged air diffusers. Oxygen absorption efficiency is a measure of the percentage of oxygen transferred from the air bubbles into solution. Absorption efficiency increases with depth because of the increased contact time the bubble has with the water while rising to the surface. The relationship between depth and absorption efficiency is linear (Figure 4-2) with the slope dependent on the percent saturation of dissolved oxygen in the water. Absorption efficiency in water less than two feet deep is very low. Regenerative blowers supply large volumes of air at low pressures. Typically, blowers operate most efficiently at pressures less than 60 inches of water. Blower horsepower requirements increase rapidly at greater pressures. The maximum effective diffuser depth of most blower based aeration systems is around 4 feet. This means that culture tanks relying on submerged diffusers for aeration should be between 2 to 4 feet deep. Taking all factors into account, it appears that the ideal culture tank depth is between 2 to 4 feet deep. If harvest densities greater than 150 shrimp per square meter are anticipated, tanks of at least a 3-foot depth are recommended.

Artificial Substrates Artificial substrates are structures placed in the culture tank to provide additional surface area for the shrimp to utilize so as to avoid overcrowding on the tank bottom. In theory, the use of artificial substrates allows for higher culture densities by reducing stress and cannibalism, which is sometimes observed at high densities. Artificial substrates may also allow for a certain amount of in-tank biofiltration to take place by providing additional surface area for nitrifying bacteria to colonize. The biofilm on the surface of these subtrates may also provide some additional nutrition for the shrimp. On the down side, artificial substrates potentially

DO(% Saturation)

Figure 4-2: Oxygen Absorption Efficiency as a Function of Depth and DO. (after Huguenin and Colt, 1989)

2 4 6 8 10 1200

1

2

3

4

5

6

7

0%50%

60% 70%

80% 90%

100%

Diffuser Depth (ft)


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may interfere with solids transport, the removal of dead shrimp and molts, feed distribution, and harvest activities. At this point in time, little formal research has been carried out to determine whether or not artificial substrates really increase the carrying capacity of a culture system or whether the increased capital and operating costs associated with their use is justified. It is likely that the answers to these questions depend upon the number and types of substrates used and the way that the substrates are deployed in the culture tank.

Standpipes and Drain Structures The drain outlets of the culture tank should be equipped with standpipes. The function of the standpipe is to set the minimum water level in the culture tank. The standpipe prevents the tank from being completely drained if the return flow of water to the tank is interrupted for some reason. Ideally, the water draining out of the tank should be pulled from the bottom of the tank, where the solid wastes accumulate. This prevents the solid wastes from building up in a pile around the drain. An outer sleeve is placed over the standpipe to allow the water flowing out of the drain outlet to be drawn from the bottom of the tank. The outer sleeve is usually a PVC pipe with a slightly larger diameter than that used for the standpipe. The pipe is scalloped or screened at the bottom to allow bottom water to pass to pass through (Figure 4-3). The pipe used for the outer sleeve should be longer than the operational depth of the tank so that surface water will not normally overflow the top of the pipe. The pipe should be shorter than maximum height of the tank so that if the openings at the bottom of the sleeve become clogged the tank itself will not overflow. Drainpipes should be laid with sufficient slope (at least a 100:1 pitch) to ensure good return flow to the water reconditioning system. The pipe diameters of the drainage system must be sufficient to allow for adequate return flow. If, for example, a flow rate of 150 gpm is required to provide one complete turnover of the tank volume in one hour, then the drainage system must be capable of transporting 150 gpm under the working head pressure. Often, a drainage pipe will empty into a sump whose water level may only be a few inches below that of the tank. After friction losses are taken into account, the effective head pressure driving the water through the drainpipe may only be one or two inches. Over-sizing the drainage pipe and eliminating unnecessary pipe fittings will maximize the available head pressure by minimizing friction losses.

Figure 4-3: Standpipe with Outer Sleeve


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Solids Filtration

Sources and Types of Solid Wastes Solid wastes in a recirculating shrimp culture system consist of uneaten feed, fecal wastes, shrimp molts, decomposing plant and animal matter, as well as inorganic particulates such as sand. The solids are characterized by particle size and density. Different technologies are required to efficiently filter solids of different sizes. Settleable suspended solids are particles with a diameter greater than 100 µm and will readily settle out of the water column. Non-settleable suspended solids are those particulates with a diameter between 1 and 100 µm. Particles in this size range take more than 15 minutes to settle out of a 1-foot deep water column, so mechanical filters must be used to remove them. Solids less than 0.45 µm in diameter are classified as dissolved solids. These are removed either by foam fractionation or by ozone treatment.

Consequences of Excessive Solid Wastes Effective solids removal is critical to the success of high density recirculating aquaculture systems. Accumulation of solid wastes in a system will produce a variety of adverse consequences. High concentrations of fine particulates in the water can cause direct damage to the gills of the shrimp, making them less tolerant of low dissolved oxygen conditions and more susceptible to bacterial infections. Accumulations of uneaten feed and fecal material on the bottom of the culture tank and elsewhere in the system will provide a substrate for heterotrophic bacteria to grow on. These bacteria can consume tremendous quantities of oxygen, resulting in low dissolved oxygen levels. Some of these bacteria, especially those in the Vibrio group, may be shrimp pathogens. If the shrimp ingest large numbers of pathogenic bacteria, they may develop potentially lethal bacterial infections. Accumulations of solid wastes in the system will also have a detrimental effect on the functioning of the biofilter. Accumulations of organic material in the biofilter can literally smother the beneficial nitrifying bacteria, which live in a thin biofilm on the surfaces of the biofilter media. Organic matter accumulated in the biofilter will be colonized by heterotrophic bacteria, which will compete with the nitrifying bacteria for available space and oxygen. These bacteria will also contribute to higher levels of ammonia nitrogen within the system. Heterotrophic bacteria break down the proteins in the organic waste matter and utilize the amino acids from that protein as building blocks for their own proteins. Ammonia is a by-product of this process and is excreted by the bacteria into the water. The production of ammonia by the heterotrophic bacteria and the reduced efficiency of the biofilter due to fouling may have the combined effect of generating toxic concentrations of ammonia within the culture system. The accumulation of solid wastes in a recirculating system may also result in an off-flavor developing in the shrimp. This most often occurs when solid wastes are allowed to accumulate in the bottom of sumps, creating an anoxic environment. Anoxic conditions favor the growth of a group of bacteria, called Actinomycetes, that release a substance called methyl iso-borneol (MIB) into the water. MIB accumulates in the shrimp, imparting a muddy off-flavor to the shrimp. Although MIB can be purged from the shrimp by placing them in


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clean water for three to four days, it is far better to avoid the problem altogether by effectively filtering the solids out of the water before they have a chance to accumulate.

Solid Waste Filters The first step in the water treatment process is to filter the suspended solids out of the water. Filtering the solids will be much easier if the fecal strings and uneaten feed particles reach the solids filter intact. Rough treatment will break the fecal strings and uneaten feed into very fine particles, which will be difficult to filter out of the water. Ideally, the settleable and suspended solids wastes should be filtered before they ever pass through a pump. Adherence to this principle limits the choice of filters, because some types of solids filters have a high pressure requirement and the water must be pumped through the filter. Some system designers use a two-step solids filtration strategy, using a low-head filter to remove the larger particulates followed by a high-pressure filter to remove the finer particulates. The choice of which solids filter to use depends upon many considerations: (1) filter pressure requirements (2) size range of particles efficiently filtered (3) volume of water that can be filtered per unit time (4) length of time solids are retained in the filter between backwashes (5) volume of effluent generated (6) space requirements (7) labor and maintenance requirements

(8) initial capital cost

The ideal solids filter would have a very low-pressure requirement, would be able to handle very high flow rates, would efficiently filter down to a small particle size, and the filtered solids would be eliminated from the system continuously, thereby minimizing the opportunity for heterotrophic bacteria to colonize the wastes. The effluent from the filter would be highly concentrated, thereby minimizing the makeup water required for the system. The filter would have a small footprint and would do its job reliably with little labor input. And it would cost next to nothing. Obviously, there is no filter that meets all of these criteria. Each type of filter has its advantages and disadvantages. Descriptions of some of the more common types of solids filters, and discussion of the relative strengths and weaknesses of each are provided below.


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Sedimentation Tanks Sedimentation tanks create conditions that are conducive to passive settling of the larger particulate materials out of the water. The water velocity is slowed in the tank and the water is retained in the tank long enough for the settleable solid wastes to settle to the bottom of the tank. The effectiveness of the sedimentation tank will depend upon four factors: 1) retention time; 2) water velocity and flow; 3) the density of the particulate waste, and 4) water depth (Piper et al. 1982). Figure 4-4 illustrates the relationship between settleable particle size, retention time, and tank depth. The particle size removal efficiency is a logarithmic function of retention time (the time required to exchange the entire volume the tank at a given flow rate) and is an inverse function of tank depth. To effectively settle out 100 µm particles, a one meter deep sedimentation tank must provide a retention time of at least 15 minutes. If the flow rate through the tank was 150 gallons per minute, the tank would need to occupy about 8.5 square meters of floor area. This illustrates the major disadvantage associated with sedimentation tanks. Sedimentation tanks require a very large amount of floor space and do not efficiently remove wastes with particle diameters less than 100 µm. Ideally, a sedimentation tank should have a conical bottom or some other shape that serves to concentrate the settled wastes. Otherwise, a large volume of water will be lost when the solids are eliminated from the tank. If the wastes are not removed frequently, bacteria acting on the wastes may release significant amounts of ammonia into the water. The main advantages associated with sedimentation tanks are that they require very little head pressure to operate and are relatively inexpensive.

Figure 4-4: Required Retention Time for Settlement of Solid Particles as a Function of Particle Diameter and Tank Depth (after Chen & Malone, 1991).

1 0 - 3

1 0 - 2

1 0 - 1

1 0 0

1 0 1

1 0 2

1 0 3

1 0 4

1 0 5

1 0 6

1 0 0 1 0 1 1 0 2 1 0 3 1 0 4

D e p t h = 0 .0 5 m

D e p t h = 0 .5 m

D e p t h = 1 .0 m

D e p t h = 2 .0 m

Ret

entio

n Ti

me

(min

utes

)

Particle Diameter (microns)

1 0 - 3

1 0 - 2

1 0 - 1

1 0 0

1 0 1

1 0 2

1 0 3

1 0 4

1 0 5

1 0 6

1 0 0 1 0 1 1 0 2 1 0 3 1 0 4

D e p t h = 0 .0 5 m

D e p t h = 0 .5 m

D e p t h = 1 .0 m

D e p t h = 2 .0 m

1 0 - 3

1 0 - 2

1 0 - 1

1 0 0

1 0 1

1 0 2

1 0 3

1 0 4

1 0 5

1 0 6

1 0 0 1 0 1 1 0 2 1 0 3 1 0 4

D e p t h = 0 .0 5 m

D e p t h = 0 .5 m

D e p t h = 1 .0 m

D e p t h = 2 .0 m

Ret

entio

n Ti

me

(min

utes

)

Particle Diameter (microns)


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Hydrocyclones Hydrocyclones, also known as vortex filters or swirl separators, concentrate particulate wastes by centrifugal force. Hydrocyclones are cylindroconical tanks with a bottom drain, a water inlet near the bottom of the sidewall, and a water outlet near the top of the sidewall. Water and solid wastes from the culture tank enter the tank tangential to the sidewall, generating a spinning motion to the water in the tank. The solid wastes are forced to the center of the tank where the denser particles settle to the bottom of the separator. The filtered water exits the separator through a side outlet near the top of the hydrocyclone. The effectiveness of a hydrocyclone depends on the velocity of the water entering the separator, the particle size and its density. Hydrocyclones are most effective in removing particulates greater than 80 µm in diameter. Pumping is usually required to generate the water velocity required for effective separation of the particulates from the water. Unfortunately, pumping the water breaks up the solids into smaller particle sizes, which are less likely to be removed by the swirl separator. Another problem with hydrocyclone filters is that they are ineffective when particle densities are similar to the density of the water. The particulate wastes in many recirculating aquaculture systems are characterized by low densities (Chen and Malone, 1991). The efficiency of hydrocyclone filters was evaluated by Scott and Allard (1983), who found that these filters are capable of removing up to 56% of the dry solids in their system. In the system they were evaluating, 90% of the particulates measured greater than 77 µm in diameter. The hydrocyclone filter was able to remove 70% of the particulate wastes above this size.

Tube Settlers Tube settlers are sedimentation tanks that have been modified to make them more efficient. Water entering the sedimentation tank passes upward through a matrix of parallel inclined tubes or plates. These tubes promote settlement of the particulate wastes in a number of ways. Laminar flow through the tubes minimizes turbulence and facilitates settlement of the particulates. The small diameter of the tubes reduces the distance the particles have to settle before contacting the interior wall of the tube where they stick. The more solids there are on the insides of the tubes, the stickier the tubes become. The steep angle of the tubes helps promote contact of the particles with the walls of the tubes. Although the same factors that determine the efficiency of sedimentation tanks also determine the efficiency of tube settlers (particle size, particle density, and residence time), tube settlers are much more efficient than sedimentation tanks. They efficiently remove particulates down to 70 µm (Chen and Malone, 1991). Because the distance the particles must settle is so much less than in the typical sedimentation tank, the required residence time is also much less. This means that a tube settler tank does not need to be as large as a traditional settling tank, which makes them a much more attractive option for indoor aquaculture systems. Other advantages of tube settlers are their relatively low capital cost, and the fact that they require very little head pressure to operate. Typically, they are gravity-fed.


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There are several drawbacks, however, to using tube settlers. Tube settlers do not effectively remove particles less than 60 µm in size. Other filters must be placed downstream from a tube settler if one desires to remove smaller particles as well. Tube settlers must be cleaned frequently because bacterial breakdown of the accumulated organic wastes may contribute to higher ammonia levels in the system. The bacteria growing on the solid wastes will also create an additional oxygen demand, contributing to lower system dissolved oxygen levels. Removing the solid wastes from a tube settler can be difficult and time-consuming. To clean a tube settler the tank must be completely drained, and then each tube must be rinsed out. This is best accomplished by inserting the wand of a pressure washer into each of the tubes. The water required to eliminate the solid wastes from a tube settler can be significant. If the makeup water must be heated, this can significantly increase energy costs.

Microscreen Filters Microscreen filters intercept solid waste particles as they pass with the water through a fine-mesh screen. The minimum particle size removed is determined by the mesh size. Microscreen filters are available with mesh sizes as small as 10 µm, but most commercial models utilize mesh sizes of 40 µm or larger. There is a tradeoff between the hydraulic loading rate (the volume of water that can be passed through a given area of screen per unit time) and the mesh size. Because each pore of the screen can be clogged by just a few particles, microscreens require nearly continuous cleaning. For this reason, microscreen filters are designed with self-cleaning mechanisms built in. Usually this involves rotating the screen past a series of high-pressure spray nozzles. The microscreen is typically set inside of a filter tank in such a way that the water entering the tank is obligated to pass through the screen before exiting the tank. Rotating disk filters feature a microscreen stretched over a disk-shaped plate that rotates in a channel of a semi-cylindrical tank. The lower half of the disk is submerged while the upper half comes up out of the water. When the screen becomes clogged, the water level on the incoming side of the filter tank rises and trips a float switch. This causes the disk to rotate past a series of high-pressure nozzles positioned on the clean side of the screen. The water is jetted through the screen, dislodging the trapped particles. These are collected in a trough and carried off to the final waste treatment system.


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The microscreen drum filter is perhaps the most popular filter using microscreen technology. In these filters the microscreen is stretched over a cylinder set inside of a semi-cylindrical filter tank (Figure 4-5). Typically, the drum rotates on rollers. In most microscreen drum filters the water enters the inside of the drum and passes through the screen before exiting the filter tank. When the screen becomes clogged, the water level inside the drum rises, tripping a float switch that causes the drum to rotate past a spray bar located outside of the drum. The particulate wastes collect in a trough and are carried off to the final waste treatment system. Self-cleaning microscreen filters offer several advantages. Because they are low head systems, the wastewater does not have to be pumped through these filters. This keeps the fecal strands and feed particles intact, facilitating their removal. The continuous cleaning feature of these filters eliminates the solid wastes from the recirculation system before leaching and bacterial action have time to release nutrients contained in the wastes into the water. Few other solids filtration technologies feature continuous removal of the solid wastes. Most of the other technologies trap the solids so that they do not return to the culture tank, but the solids remain in contact with the system water until the filter is backwashed. The interval

Figure 4-5: Microscreen Drum Filter


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between backwashes may often be as much as 24 hours. The labor required for daily maintenance of microscreen filters is generally very small compared to many other technologies, because these filters are self-cleaning. There are a few disadvantages associated with microscreen filters. The motor, float switch and high-pressure pump are subject to mechanical failure, especially if not properly maintained. The water requirement for cleaning the filters is typically 2-5% of the system volume per day. The pressure sprayer needs a clean water supply to prevent the spray nozzles from becoming clogged. For this reason, system water is not usually used for the pressure sprayers. Most of the water used to clean the microscreen filters is carried off in the waste stream. Unless significant denitrification is taking place somewhere in the system, an additional 3-5% water must be added to the system to prevent nitrates from building up to toxic levels. Perhaps the biggest disadvantage associated with microscreen filters is their high capital cost. Microscreen drum filters capable of filtering 150 gpm often cost $5,000 or more. Recently, however, some cheaper models have appeared on the market. The high capital cost of microscreen filters may be justified because the filters save on labor costs.

Bead Filters Bead filters use a packed bed of floating plastic beads to intercept particulate wastes as they pass through the bed. Raw water enters the bead filter from below the bed of beads, flows up through the bed, and exits at the top of the filter. Wastes particles are removed by several different mechanisms. The larger particles (> 80 µm) are physically strained by the beads. Particles in the range between 20 and 80 µm are removed by interception, a process caused by collisions between the particles and the beads. Some particles smaller than 20 µm may be removed by adhesion to the biofilm on the surface of the beads. Backwashing is accomplished by vigorously agitating the beads to dislodge the trapped solid wastes and by draining the water out of the filter. There are two types of bead filters (Figure 4-6)

Figure 4-6: Two types of bead filters

Bubble-washed Bead Filter Propeller-washed Bead Filter


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commercially available that are distinguished by the mechanism used to agitate the beads during the backwash cycle. Bubble-washed bead filters have an hourglass shape. During the filtration cycle the beads are packed in a bed at the top of the upper half of the hourglass. Backwashing is accomplished by draining the water out of the filter vessel while bubbling air through the beads. The beads are washed by the turbulent flow caused by bubbles and cavitation as the beads pass through the throat of the filter. Propeller-washed bead filters have an upside down teardrop shape and use a motor-driven propeller to agitate the beads. During the backwash cycle the flow of water into the filter is interrupted and the propeller stirs the beads vigorously for a few minutes to dislodge the solid wastes. The propeller is then turned off and the beads float back up to the top of the filter vessel while the solid wastes settle to the bottom of the filter. The concentrated solid wastes are drained off and the filtration cycle is resumed. Commercial bead filters are available with a timer-actuated valving system that carries out the backwash cycle automatically at user-programmed time intervals. Models with automatic pressure-actuated backwash cycles are also available. There are several advantages associated with bead filters. They are capable of filtering down to a fairly small particle size, operate efficiently at high hydraulic loading rates, and are economical with respect water usage. Models with automatic backwash systems require very little labor to operate and maintain, which saves on operating costs. Bead filters offer another significant advantage due to the fact that the beads create a great deal of surface area for the nitrifying bacteria that are responsible for carrying out biofiltration. In some recirculating aquaculture systems bead filters are used primarily as a biofilter, while in others they are used primarily as a solids filter. In many systems all of the solids filtration and biofiltration are carried out in a single bead filter. The use of bead filters as biofilters will be discussed later in this chapter. The primary disadvantage associated with bead filters is that they are expensive. Bead filters capable of filtering 150 gpm begin at about $3,000 for bubble washed models and at $5,000 for propeller washed models. Only propeller washed bead filters are capable of handling flows greater than 150 gpm. Automatic backwash systems add considerably to the purchase price. Another disadvantage associated with bead filters is that they require a significant amount of energy to operate. The water must be pumped through these filters. Head losses may be as high as 15 psi through bubble washed filters and 25 psi through pressure washed filters. Head loss depends on the hydraulic loading rate and backwash frequency.

Sand Filters Like bead filters, sand filters depend on a packed bed of granular media to intercept solids from the water. Raw water enters the upper part of the filter, flows downward through the packed bed of sand and exits the filter through a manifold of finely slotted pipes. The solid wastes are trapped in the sand bed through the same processes of straining, interception, and bioadhesion that trap the solids in a bead filter. Sand filters are typically filled with #20 to #30 silica sand. This media is much finer than that used in bead filters, so sand filters are more efficient in filtering solids less than 30 µm than are bead filters. Trapped solids are


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removed from sand filters by backwashing. Backwashing involves reversing the flow through the filter to fluidize the sand bed. This releases the solid wastes that are then flushed out of the top of the filter and sent to waste. The main advantages of sand filters are their low cost, small footprint, and high efficiency in removing small diameter particulates. A 36-inch sand filter costs less than $1,000. Like bead filters, sand filters provide significant surface area for nitrifying bacteria to colonize. With proper daily maintenance, sand filters perform the dual function of solids filtration and biofiltration reasonably well when feeding rates are less than 2 kg of feed per day. There are a number of disadvantages associated with the use of sand filters in high- density applications. The packed bed of fine sand is easily clogged with solid wastes, creating substantial pressure losses through the filter. High capacity pumps are required to push the water through the filter, driving up energy costs. Frequent backwashing is necessary to maintain adequate flow rates through the filter. Backwashing results in substantial water losses from the system. A sand filter requires at least 20 volumes of water to adequately backwash. A 36-inch sand filter requires more than 1,000 gallons to backwash. Heavy growths of heterotrophic bacteria within the media bed result in the sand grains becoming very sticky. Under pressure the sand grains are virtually cemented together. This has several consequences. Water flow through the interstitial spaces between the sand grains is impeded. The water will form channels through bed, bypassing the media without passing through the interstitial spaces. When this occurs, solids removal efficiency drops off significantly. Compacted sand beds are not readily fluidized during the backwash sequence. To overcome this problem the sand bed must be broken up using a high pressure wand prior to the backwash sequence. This is an extremely labor-intensive process. When feed rates exceed 0.5 kg/day, this procedure must be performed at least twice a day. This is extremely labor intensive. When sand filters begin to cake, they do not function very effectively as biofilters because the nitrifying bacteria are smothered by the solid wastes and must compete with the heterotrophic bacteria for space and oxygen. For all of the above reasons, sand filters are not recommended for systems in which feed rates exceed 1 kg of feed per day.

Foam Fractionators Foam fractionators (also known as protein skimmers) are devices which use a bubble adsorption process to remove surfactant and fine particulates from the water (Figure 4-7). In a typical foam fractionator a fine stream of air bubbles is introduced at the bottom of a vertical column through which water is flowing. As the bubbles rise to the top of the column, surface active organic particles are adsorbed onto the surface of the bubbles. The head of bubbles that forms at the top of the column is drained off, carrying with it the adsorbed organic material. Foam fractionation is only effective in removing surfactant substances, which are characterized by having a polar, hydrophilic (water loving) end, and non-polar, hydrophobic (water fearing) end. Surfactant molecules become adsorbed onto the surface of the air bubble when the hydrophobic end inserts itself into the interior of the bubble while the hydrophilic end remains in the water on the outside of the bubble. The solids


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removed by foam fractionators are mostly fine particulates with a diameter less than 30 µm. Most of the solid wastes removed are proteins of fecal and feed origin. Foam fractionators remove the fine particulates that are not easily removed by other methods of filtration. They are meant to supplement rather than replace other types of solids filters. There are two basic types of foam fractionators: co-current foam fractionators, and counter-current foam fractionators (Figure 4-7). In co-current models, raw water enters the bubble column at the bottom and exits at the top. The direction of flow of the water and the air bubbles is the same. In counter-current models, raw water enters at the top of the bubble column and exits at the bottom. The water flow is counter to the direction of the air flow. All else being equal, counter-current foam fractionators are more efficient than the co-current models because the contact time of the bubbles with the water is increased. This is because the downward flowing water slows the rate of ascent of the air bubbles. The efficiency of foam fractionators is determined primarily by the contact time between the air bubbles and the water, the size and number of air bubbles, and the foam drainability (the property that allows water to drain away from the foam condensate). The contact time between the air bubbles and the water is determined by the air diffuser submergence depth, the air flow rate, and the water flow rate. The foam overflow height (the height the foam must rise above the level of the water before it is removed) is an important variable affecting the volume of foam condensate removed and the concentration of solid wastes in the condensate. Generally, higher foam overflow heights reduce the volume of condensate removed, but increase the concentration of solid wastes in that foam. Foam fractionators can be positioned either in a sump or in the culture tank itself, or externally. When positioned in the water column, an airlift pump is generally used to pull the water through the device (Figure 4-7). Raw water must be pumped through the bubble column of foam fractionators positioned outside of the tank. It is important to note that there is a pressure break in the system when water is pumped through a fractionator, because the top of the bubble column is open to the atmosphere. This means that return flow of water

Figure 4-7: Counter-current Foam Fractionator


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from the foam fractionator to the culture tank must be by gravity or that the water must be pumped a second time. The performance of a foam fractionator is a function of the surface tension and viscosity of the water being filtered. Bubble stability (and hence, foam condensate formation) is enhanced in water with higher surface tension. For this reason, foam fractionators work more efficiently in saltwater systems than they do in freshwater systems. The efficiency of foam fractionators in freshwater can be increased by reducing the bubble size. Foam fractionators which utilize venturi air injectors to create the bubble stream tend to work best in freshwater systems.

Ozone Ozone is a highly unstable gas composed of three oxygen atoms held together by weak bonds. The weak bonds make ozone highly reactive and one of the strongest oxidizing agents available. In recirculating aquaculture systems, ozone is often used to oxidize refractory, non-biodegradable organic compounds into smaller, biodegradable compounds and to reduce pathogen loading. Ozone must be generated on-site because it is very difficult to store. It has a very short half-life (<12 hours at room temperature) and can spontaneously explode if gas concentrations exceed 70% ozone. Ozone may be generated by exposing oxygen to a high energy source, such as ultraviolet light (photozone), or a corona discharge between two dielectric metals. Corona discharge ozone generators produce much higher concentrations of ozone than the photozone generators. For this reason the corona discharge units are much more widely used in high density recirculating aquaculture systems. Ozone can be generated using either air or pure oxygen gas as the feed gas for the generator. For a given amount of energy input, two to three times as much ozone is produced using pure oxygen rather than air as the feed gas. Using pure oxygen as the feed gas, corona discharge ozone generators can produce up to 12% ozone in the off gas, although ozone concentrations are more often limited to about 2-3%. The way in which ozone is introduced into the water is very important. A very high percentage of the ozone must be absorbed into the water, otherwise large amounts of ozone may be released into the atmosphere. If the building is not well ventilated ozone concentrations in the building may reach unsafe levels. High absorption rates are also desirable to keep costs down. Care must be exercised in the application of ozone to ensure that residual ozone does not remain in the water returning to the culture tank. Ozone is toxic to fish and shrimp at very low concentrations. Ozone damages the gill lamellar epithelium, which leads to ionic imbalances. In extreme cases, this can lead to death. Gill pathology has been reported (Wedemeyer et al., 1979) in rainbow trout at residual ozone concentrations as low as 5 µg/liter (5 ppb). The half-life of ozone depends on the concentration of organic matter in the water, ranging from <15 seconds in systems with high concentrations of organic matter to more than 165 minutes in clear water (Bullock et al. 1996).


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There are several options regarding where ozone should be injected into the system (Figure 4-8). Probably the most economical way to introduce ozone into the system is to inject the oxygen-ozone mixture as a fine bubble stream into some type of oxygen contact device (Brazil, 1996). This allows the oxygen contactor to serve for both oxygen and ozone injection, eliminating the need for a separate ozone contact chamber. Oxygen contactors are most often positioned just before the point where the treated water returns to the culture tank. This location is advantageous for ozone treatment because it allows the ozone to act upon bacterial pathogens just before the water is returned to the tank. This location does present additional risk of residual ozone being introduced into the culture tank. An alternative location for ozone injection is between the solids filter and the biofilter. Injecting ozone at this point can help reduce the potential for biofouling of the biofilter, and also minimizes the chance of ozone residuals harming the shrimp.

Biofiltration

Sources of Ammonia and Nitrite One of the consequences of feeding shrimp is that nearly 10% of the protein in the feed is converted into ammonia. There are a number of different pathways these proteins can take before being converted into ammonia (Figure 4-9). Most of the feed that is added to the culture tank is consumed by the shrimp. The protein in the feed that is eaten may be assimilated by the shrimp or may pass through the gut and be excreted in the feces. The shrimp convert some of the feed protein into shrimp protein (e.g., muscle), but ammonia is produced as a by-product of this process. This ammonia is then excreted by the shrimp through their gills. Heterotrophic bacteria metabolize proteins in the uneaten feed and fecal material and also excrete ammonia as a by-product of protein metabolism.

Figure 4-8: Possible locations for introducing ozone into a recirculating system.

Option 1: Introduce ozone into oxygen contactor immediately prior to culture tank.

Option 2: Introduce ozone into ozone contact chamber located between solids filter and biofilter.

Biofilter

SolidsFilter

O3

Ozone Contact Chamber

OzoneGenerator

CultureTank

OxygenContactor

O2 gas

Biofilter

SolidsFilter

O3

OzoneGenerator

CultureTank

OxygenContactor

O2 gas

O2 + O3


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Nitrifying bacteria utilize inorganic nitrogen as nitrogen sources for protein synthesis. Nitrosomonas and Nitrobacter are the two most important genera of nitrifying bacteria. Nitrosomonas uses ammonia as its source of nitrogen and excretes nitrite (NO2

-). Nitrobacter uses nitrite as its source of nitrogen and excretes nitrate (NO3

=). All three forms inorganic nitrogen (ammonia, nitrite, and nitrate) are present in the water at any given time.

Ammonia and Nitrite Toxicity Ammonia exists in two different forms in the water: unionized ammonia (NH3) and ionized ammonia (NH4

+). Both of these forms of ammonia are generally present in the water simultaneously. The combined total concentration of unionized and ionized ammonia is termed the “Total Ammonia Nitrogen” concentration, or TAN for short. TAN is what is measured by most ammonia test kits. The fraction of TAN in the unionized form of ammonia is dependent upon the pH and temperature of the water. At a pH of 7.0 most of the TAN is in the ionized form. The fraction of TAN in the unionized form increases with both temperature and pH. At a pH of 9.0 about 50% of the TAN is in the unionized form. The unionized form of ammonia is highly toxic to shrimp, while ionized ammonia is relatively non-toxic. The unionized ammonia 48-hr LC50 for juvenile shrimp (the concentration that will kill 50% of the shrimp in 48 hours) was reported by Wickins (1976) to be 1.26 mg NH3-N/liter. Concentrations below 0.1 mg/L of unionized ammonia are considered “safe”, in the sense that these concentrations are non-lethal. However, chronic exposure to concentrations above 0.03 mg NH3/liter will produce a number of non-lethal

Figure 4-9: Transformations of nitrogen in feed within a recirculating aquaculture system.


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effects on the shrimp, including reduced growth rates, increased feed conversion, swollen gills, reduced tolerance to low dissolved oxygen conditions, and decreased resistance to disease. Nitrite is also toxic to shrimp. The 96-hour LC50 of nitrite for shrimp is about 13.6 mg NO2

- /liter (Chen and Chin, 1988). These authors recommend 1.36 mg/L as the safe level of nitrite for shrimp. The mechanism for nitrite toxicity in shrimp is not as well understood as it is in fish. In fish, high nitrite levels interfere with the uptake of oxygen by hemoglobin in the blood of fish and can cause the fish to die from asphyxiation. Shrimp have a different blood pigment, hemocyanin, which transports oxygen to the tissues. It is likely that the mechanism for nitrite toxicity is similar to the mechanism in fish.

Mechanisms for Controlling Ammonia There are three primary mechanisms to control ammonia: 1) water exchange, 2) plant uptake, and 3) nitrification,.

Water exchange Water exchange is a crude, but effective means of controlling ammonia. While water exchange is an effective way of rapidly reducing ammonia levels in an emergency, it should not be the primary mechanism for controlling ammonia levels in a recirculating system. Very high exchange rates would be required to adequately control ammonia levels in a high density system. During the several months out of the year when the water needs to be heated to maintain suitable water temperature, the cost of heating the water would be exorbitant for what would amount to be a flow-through system. In addition, exchanging large volumes of water would greatly increase the required size of the post-treatment system for treating effluent from the culture system.

Plant Uptake Plants are able to extract inorganic nitrogen directly from the water and use it as a nitrogen source for protein synthesis. In simple terms, ammonia, nitrite, and nitrate can be utilized by plants as a nitrogen fertilizer. In aquaculture systems with good exposure to sunlight, the nutrients from the feed, including nitrogen and phosphorus, will generally develop phytoplankton blooms, consisting of many different species of algae. These algae will extract inorganic nitrogen from the wate, and help maintain lower levels of ammonia and nitrite. Macrophytic algae and rooted plants are also able to extract inorganic nitrogen from the water, and some recirculating aquaculture systems depend on these types of plant for ammonia removal. In these systems, the water is circulated through beds of macrophytic algae or vascular plants growing external to the culture tank. In aquaponic systems, hydroponic cultures of a variety of vascular plants utilize the nutrients from a recirculating aquaculture system as fertilizer and simultaneously help control the ammonia and nitrite levels in the aquaculture system. In other systems, the effluent from the culture tank is passed through a water treatment pond, which contains a variety of aquatic plants. The nutrients in the water are extracted by the aquatic plants, and the treated water is reused.


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It is generally not a good idea to depend on phytoplankton blooms to control ammonia in high density recirculating systems. Microalgae live for a very short time. Once a phytoplankton bloom is established, old microalgae cells die at about the same rate as new cells form. The dead cells are decomposed by heterotrophic bacteria, which will break down the algal proteins and excrete ammonia. If the algae bloom crashes, there may be a rapid release of ammonia into the system as the algae undergo bacterial decomposition. Another reason why it is not a good idea to depend on algal blooms for ammonia control is that daytime pH levels will often be very high in systems with dense algae blooms. During the day, algae extract carbon dioxide from the water to utilize for photosynthesis. As carbon dioxide concentrations in the water fall, pH rises. In systems with dense algae blooms, pH levels of 9.0 or greater are not uncommon. At this pH level, a very high percentage of the TAN will be in the toxic, unionized form. Even relatively low concentrations of TAN may be lethal to the shrimp at high pH.

Nitrification Most recirculating aquaculture systems depend on nitrification processes to control ammonia and nitrite levels. Nitrification is a two-step process in which ammonia is converted first to nitrite by Nitrosomonas bacteria and then to nitrate by Nitrobacter bacteria. Nitrosomonas and Nitrobacter are normally present in all aquaculture systems, growing on surfaces such as tank walls and the inside walls of pipes. The numbers of these bacteria can be greatly enhanced, however, by the addition of a biofilter to a system. A biofilter is simply a device that provides vast amounts of surface area on which nitrifying bacteria can grow, and a chemical environment suitable for nitrification to take place. There are many different types of biofilter media and biofilters. The more common types of biofilters are discussed below. There are several conditions that must be present for the nitrifying bacteria to be able to efficiently carry out the process of nitrification. Nitrification is an oxidative process, so there must be an adequate supply of oxygen. The minimum oxygen level at which nitrification can efficiently proceed is about 2 mg/L (Hochheimer and Wheaton, 1991). A stable pH in the range from 7.0 - 9.0 should be provided. There are many different species of Nitrosomonas and Nitrobacter. Each species has its own optimum pH range, which is narrower than the range given above. A stable pH will allow the dominant species to function near its optimum pH. Bicarbonate ion is required by the nitrifying bacteria as a source of carbon. Nitrification consumes 7.14 g of bicarbonate ion for each gram of ammonia that is converted to nitrate (Hochheimer and Wheaton, 1991). However, it is difficult to give an exact minimum bicarbonate level that must be maintained, because bicarbonate chemistry in the water is very complex. To be on the safe side, bicarbonate alkalinity should be maintained above 50 mg/L as calcium carbonate. Light levels have also been found to have an important influence on the nitrifying bacteria. Olson (1981) found that nitrifying bacteria are inhibited at light levels less than 1% of full sunlight intensity, and that complete darkness was superior to diurnal cycling of light regimes. Stable salinities are also important for efficient biofilter operation.


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Akai et al. (1983) found that rapid changes in salinity significantly slowed the growth rate of nitrifying bacteria. Nitrification occurs most efficiently when the bacteria are arrayed in a thin film on the surface of some type of media. Over time, older bacteria die and are replaced by newer bacteria. If the dead bacteria are allowed to accumulate on the surface, heterotrophic bacteria will attach to the surface and begin to decompose the dead nitrifying bacteria. These heterotrophic bacteria begin to compete with the nitrifying bacteria for space and oxygen. Solid particulates may also adhere to the surface of the biofilter media, smothering the nitrifying bacteria and providing additional organic material for the heterotrophic bacteria to grow on. In order to maintain a healthy, thin film of nitrifying bacteria on the biofilter media, some type of shearing force must be applied to the biofilter media to slough off the dead bacteria and to keep organic material from accumulating on the surfaces of the media. The shearing force may be the hydraulic force of water flowing over the media surface or abrasion caused by collisions between individual units of biofilter media.

Types of Biofilters There are many different types of biofilters and biofilter media that can be used in recirculating aquaculture systems. The most common types of biofilters include submerged biofilters, trickling biofilters, rotating biological contactors, bead filters, sand filters, and fluidized bed biofilters. These biofilters differ with respect to media type, head and energy requirements, labor and maintenance requirements, space requirements, and capital requirements.

Submerged Biofilters The distinguishing feature of a submerged biofilter is that the biofilter media is completely submerged beneath the water surface (Figure 4-10). Water enters one end of the tank and exits at the other. A wide variety of biofilter medias may be used in a submerged biofilter, but may be broadly classified as random media or fixed media. Random media consists of many small units of a material which provides surface area for the bacteria to grow on. Typically, random media consists of molded plastic balls, barrels, wheels, or beads with surface area enhancing features such as fins, pores, fingers, etc. The individual units of media are randomly dispersed throughout the biofilter tank. Examples of some common random media types include, bioballs, biobarrels, plastic rings, and gravel. Fixed media consists of sheets or blocks of biofilter material set in a fixed array. The media may consist of blocks of corrugated plastic sheets (e.g., Biodek and Munter’s media) or sheets of fibrous material. The fixed media is fixed into place in a regular or uniform distribution inside of the biofilter tank.


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Figure 4-10: Submerged Bed Biofilter

The oxygen for the submerged biofilter is supplied by the dissolved oxygen in the water passing through the filter. Often this is supplemented by applying vigorous aeration within the biofilter tank (see Figure 4-10). In addition to supplying oxygen to the nitrifying bacteria, aeration generates strong water currents within the biofilter tank that help to shear off accumulated organic material from the biofilter surfaces. In some aerated submerged bed biofilters, random media is tumbled by aeration. The tumbling of the media creates constant collisions between the surfaces of the media, thus helping maintain a thin, healthy biofilm. Submerged filters utilizing fixed media, or static beds of random media, have a greater potential for biofouling. The bacterial populations in aerated submerged bed biofilters can be kept alive indefinitely during interruptions of water flow. For this reason, an aerated submerged bed biofilter is the biofilter of choice in a situation where only the blowers are powered by the backup generator. Head loss through submerged bed biofilters is generally quite low. Frequently water gravity-feeds into a submerged bed biofilter from the solid filter and the biofilter tank serves as a sump or head tank for the pump returning the water to the system. An alternative configuration is to have the submerged biofilter elevated above the level of the culture tank. The water is then pumped into the submerged biofilter and gravity-feeds back to the culture tank. Labor requirements and maintenance requirements for submerged biofilters depend on the degree of biofouling that takes place. Aerated random media biofilters tend to be self-cleaning and require little maintenance. The media in fixed or static media filters should be removed and cleaned periodically. These filters are not subject to breakdown since they have no mechanical parts. Submerged bed biofilters are relatively inexpensive, although the cost may vary widely depending on the cost per square foot of surface area of the biofilter media used and the size and type of tank used to contain the media.


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Trickling Biofilters Trickling biofilters often use a similar type of media as is used in a submerged biofilter, but the media is supported in a column above the water surface. Water is introduced into a trickling biofilter through a water distribution system at the top of the column and trickles down in a thin film over the surface of the biofilter media. Ideally, biofilter media will provide a high amount of surface area with large void spaces. The void spaces create a lot of splashing as water trickles down through the media bed and helps prevent solid materials from becomes trapped in the media. Because the biofilter column is filled with air, and the media is wetted by a thin sheet of water, large amounts of oxygen are available for nitrification. Often the water exiting a trickling biofilter is saturated with oxygen. Biofilter columns also serve as effective degassers, stripping excess carbon dioxide from the water. The head requirement for a trickling biofilter depends upon the height of the column. In most trickling biofilters the water is pumped to the top of the biofilter, trickles down through the column and collects in a sump. If the sump is at or below the culture tank water level, the water must be pumped a second time to return the water to the culture tank. Hobbs et al. (1997) circumvented the two-pump requirement for a trickling filter by sinking the trickling column below ground, allowing the water to flow by gravity from the culture tank to the top of the trickling column. Water collected in an underground sump and was pumped from there back to the culture tank. The two-pump requirement can also be circumvented by elevating the trickling column so that the water can gravity drain from the bottom of the column back to the culture tank. Trickling biofilters are not subject to mechanical breakdown but may require regular maintance to prevent biofouling. Trickling biofilters depend on hydraulic forces to prevent biofouling. Many trickling biofilters are subject to fouling because the sheering forces generated by water travelling in thin sheets are insufficient to dislodge the solids from the surfaces of the biofilter media. If trickling biofilters are used it is recommended that multiple trickling columns be set up in parallel. That way, the media from an individual column can be removed and cleaned without complete disruption of the biofilter. Maintenance of the water distribution system at the top of the column may also be required. A common system used to evenly distribute the water over the cross-sectional area of the filter is an orifice plate. The orifices may require frequent maintenance to prevent clogging. Rotating spraybars are also used to distribute the water over the surface of the biofilter. The orifices in the spray bar may also require periodic maintenance to prevent fouling. As is the case with submerged biofilters, the capital cost of trickling biofilters varies widely. The cost of a trickling biofilter will depend on the cost per square foot of biofilter media, the cost of the tank or structure supporting the biofilter media, and the cost of the collecting sump. The cost of a typical trickling biofilter is likely to be the same as or slightly higher than the cost of a comparable submerged biofilter utilizing the same type of media. The


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energy requirements are likely to be slightly higher for a trickling biofilter because of the need to pump water. Two pump confurations will have significantly higher energy costs than one pump systems.

Rotating Biological Contactors Rotating Biological Contactor (RBC) filters consist of a cylindrical block of biofilter media attached to a rotating shaft (Figure 4-11). The biofilter media is 40-50% submerged in a filter tank and slowly rotates so that the media is alternately submerged and then exposed to the air. In a sense, an RBC is a hybrid between a submerged biofilter and a trickling biofilter. While the media is submerged in the filter tank, it picks up water with ammonia nitrogen that then trickles over the surface of the media as the media is rotated out of the the water. The thin film of water allows for excellent gas exchange. RBC filters are generally self-cleaning. Dead bacteria and solid wastes are knocked off the media when the media dips into and rotates through the filter tank. Water flows through the filter tank parallel to the axis of the shaft. In a well-designed RBC the filter tank is semi-cylindrical, with a narrow clearance between the media and the walls of the filter tank. This minimizes filter media by-pass.

While a number of different types of media have been used in RBC biofilters, most commercially available units use BioDek, a media consisting of laminated sheets of corrugated plastic. On some models the RBC shaft is rotated by motor drive. These models are subject to mechanical failure. The drive motor is located in a corrosive environment that necessitates a fair amount of maintenance and drive pulleys sometime break. The biggest

Figure 4-11: Rotating Biological Contactor


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problem associated with these models is shaft breakage due to the weight of the water and organic material on the media. Newer models have replaced motor drives with air driven paddles integrated into the cylinder of the RBC. To overcome the shaft breakage problem, styrofoam flotation disks have been integrated between panels of biofilter. In these models the ends of the shaft sit in vertical channels on either end of the filter tank. These channels fix the vertical axis of rotation, but not the horizontal axis. As the media gets heavier the shaft drops a little lower in the channel, preventing an increase of torque on the shaft. RBC filters have very low head requirements. Water typically flows through an RBC by gravity. The main energy requirement is the energy required by the drive motor or air lift that makes filter rotate. RBC filters are generally more expensive per square foot of surface area than are submerged or trickling biofilters. This is because of labor that goes into constructing the RBC and the custom shape of the filter tank.

Bead Filters Bead filters, described above in the section on solids removal, are gaining popularity as a biofilter. The beads in these filters provide 1,300 square meters of surface area per cubic meter of volume, which is among the highest specific surface areas of all man-made media. The high specific surface area of the plastic beads allows for a great deal of nitrification to take place in a relatively small area. The fact that these filters double as a solids removal filter makes bead filters among the most compact filtration technologies available. This is an important advantage in indoor aquaculture systems where space is a precious resource. The surfaces of bead filters are cleaned during the bead-washing portion of the operating cycle. Propeller-washed bead filters have been reported in the past to do a poor job of nitrite removal because many of the Nitrobacter bacteria are knocked off by the vigorous agitation by the propeller. Nitrobacter spp. are not very “sticky”. Experiments with alternative bead shapes indicate that tubular beads and beads with indentations are more effective at nitrite removal than the spherical beads (Beecher et al., 1997). This is because these alternative bead shapes provide protection for the bacteria against abrasion during the propeller-wash cycle. The bubble-washed bead filters do a better job of nitrite removal than the propeller-washed bead filters. This is because bubble-washing action is apparently less violent than the propeller wash action, so Nitrobacter cells are less likely to be dislodged. Pumps are typically used to force the raw system water through a bead filter. However, DeLosReyes et al. (1997) described a recirculating system powered only by an airlift pump which featured a bubble-washed bead filter as a combined solids/biofilter. Bubble-washed bead filters can withstand pressures of up to 15 psi, while the propeller-washed units can withstand pressures of up to 20 psi. The pump selected should provide the desired flow rate at about half of the maximum filter operating pressure. The rule of thumb is that bead filters can handle up to 1 lb of feed/day and 5 gallons per minute (gpm) per cubic foot of media. The bubble-washed units are more appropriate for smaller recirculation systems. The largest bubble-washed filters (10 cubic feet of beads) can handle up to 10 pounds of feed per day and


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flow rates of up to 50 gpm. Another rule of thumb for recirculating systems is that 100% of the culture tank volume should be turned over at least every 90 minutes. Based on this criteria, the largest system volume that a 10 cu.ft. bead filter could handle would be 4,500 gallons (50 gpm x 90 minutes/turnover). Propeller-washed bead filters are more appropriate for medium to large systems. Propeller-washed bead filters range in size from 10 to 100 cubic feet of biofilter media. A disadvantage of bead filters is that the flow through the filter declines as the filter becomes loaded with solid wastes. This may affect the operation of other filtration components downstream from the bead filter, so flow sensitive devices (e.g., foam fractionators) should not be placed downstream from a bead filter. The decrease in flow rate through a heavily loaded bead filter will increase the turnover time for the culture tank. The beads must be backwashed frequently enough to maintain adequate system turnover times. The flow through a bead filter is interrupted during each backwash sequence. Because of the high bacterial loading of a bead filter, oxygen concentrations can be expected to decline by up to 4 mg/liter during a pass through a bead filter. Provisions should be made to re-aerate the water before or as it is returned to the culture tank. Bead filters require 10-20 minutes of daily maintenance to perform the bead washing sequence. As mentioned earlier, timer or pressure controlled valves are available to automate the bubble-wash sequence. The additional capital cost of this addition must be balanced against the savings in labor costs. The automatic valves will only be cost effective if they allow a reduction in the number of employees. Although bead filters can function as both a solids filter and as a biofilter, at higher loading rates it probably is best to use it primarily for only one of these functions. For example, a bead filter may be set up as a the primary solids removal system in a system using a trickling filter as the primary biofilter. Alternatively, a bead filter may be used as the primary biofilter in a system that relies on a microscreen filter as the primary solids removal system. In a system like this, the bead filter will be able to run for longer periods of time between backwash sequences and will operate more efficiently because there will be fewer solids to interfere with the nitrification process. The biggest disadvantage of bead filters is their high capital cost. A 10 cubic foot bubble-washed unit costs about $3,000. A propeller-washed unit of the same size costs about $5,000. The next larger size, a 25 cubic foot unit, costs about $7,500. If the unit doubles as a solids filter, the cost of an additional filter unit is avoided.

Sand Filters Sand filters, like bead filters, are solids filters that can double as a biofilter. Silica sand makes a wonderful biofilter media because it provides such a high specific surface area (about 3,000 square feet per cubic foot), and the nitrifying bacteria adhere well to the irregular surfaces.


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Sand filters are inexpensive, typically costing less than a $1000 for a 36-inch unit, and sand is by far the cheapest biofilter media. Unfortunately, these filters have a number of shortcomings that limit their utility as a biofilter for high density systems. The dense packing of the sand results in substantial pressure losses through a sand filter. High head centrifugal pumps are required to push the water through a sand filter. A heavily loaded sand filter may generate as much as 35 psi (80 feet) of head pressure. This greatly reduces the flow through the filter and increases the tank turnover time. Other system components whose performance is flow-rate dependent (eg. foam fractionators or oxygen contactors) should not be placed on the same plumbing circuit as a sand filter. The performance of a sand filter as a biofilter declines at heavier loading rates. The accumulation of organic wastes within the sand bed supports heavy growths of heterotrophic bacteria. The heterotrophic bacteria compete with the autotrophic nitrifying bacteria for space and oxygen. The dissolved oxygen concentration of the water may drop by 3-4 mg/L while passing through a heavily loaded sand filter. Mucopolysaccharides are released as a result of the decomposition of the organic matter in the sand filter. Mucopolysaccharides are extremely sticky and can cement the sand grains together so that water cannot pass through the interstitial spaces. Under pressure, the water carves out channels through the sand bed, reducing contact of the water with the nitrifying bacteria. To prevent this from happening it is necessary to break up the filter bed frequently (possible twice a day) by jetting the filter bed with a high pressure stream of water prior to the backwash cycle. This is a very time consuming process, requiring 15 minutes per filter, twice a day. Jetting and backwashing a sand filter may require 1,000 gallons of water. Sand filters work best in systems with daily feed rates of less than 1 kilogram of feed per 600 pounds of media. With frequent jetting and backwashing, a 36-inch sand filter with 600 pounds of sand can support up to 3 kg of feed per day, but performance declines and water usage increases at these higher loading rates.

Fluidized Bed Biofilter Like sand filters, fluidized bed biofilters use sand as the biofilter media. The performance characteristics of a fluidized bed filter are very different from those of a sand filter because the sand in a fluidized bed is suspended in the water, rather than compacted in a dense bed. Water is pumped into a fluidized bed through a manifold at the bottom of a tall cylinder and flows up through the sand bed at a velocity sufficient to suspend, or fluidize, the sand grains (Figure 4-12). The water flows out of an outlet near the top of the filter. The flow rate of water through a fluidized bed filter must be carefully controlled. The velocity of the water must be sufficient to fluidize the bed, but not so great that the sand is washed out of the filter. Typically, flow rates are adjusted to provide about a 50% expansion in the volume of the sand bed (relative to the compacted volume of the sand). Water velocity requirements are a function of sand grain diameter and the desired expansion of the bed. Timmons and Summerfelt (1998) reported the velocities required to expand sand beds by different amounts for different sizes of sand grains (Table 4-2).


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Table 4-2: Velocities of water flow required to achieve different percent expansions of the sand bed of a fluidized bed biofilter expressed as a function a sand grain size (after Timmons and Summerfelt, 1998).

Required Velocity (cm/sec) as Function of Retaining Sieve Mesh Sizes

40/70 30/50 20/40 18/30 20% Expansion 0.45 0.8 1.2 1.6 50% Expansion 0.9 1.4 2.05 2.8 100% Expansion 1.4 2.2 3.2 4.4 150% Expansion 1.9 2.9 4.1 5.6

The required flow rates may be quite high for large fluidized beds. Initial fluidization of the sand bed requires a pump capable of generating significant head pressure, but once fluidized, pressure losses through a fluidized bed filter are very low. Because the flow rate of a fluidized bed filter must be controlled so closely, it is not advisable to configure these filters together with other system components which will result in significant changes in flow rates. Fluidized beds are high performance biofilters because they provide a nearly perfect environment for the nitrifying bacteria. The rough, irregular surfaces of the sand grains provide excellent attachment surfaces for the nitrifying bacteria. The shearing forces created

Figure 4-12: Fluidized Bed Biofilter

ManifoldH2O In

Fluidized Coarse Sand Media

H2O Out


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by the gentle collisions between sand grains slough off dead bacteria, making fluidized bed filters self-cleaning. The high rate of nitrification and replication of the bacterial populations within a fluidized bed filters often results in the sloughing off of substantial amounts of biofloc, mostly consisting of old and dead bacteria. In some systems the quantity of biofloc generated by fluidized bed biofilters is so great that a solids filter is needed downstream from the fluidized bed. Fluidized bed biofilters are capable of removing 2.7 kg of TAN/day per cubic meter of sand at 25°C (Timmons and Summerfelt, 1998). However, these authors recommend using a value of 1.0 kg of TAN removed per day per cubic meter of sand for design purposes. The high specific surface area and high nitrification rates that take place within a fluidized bed filter make this one of the most compact biofilters available. The footprint of a fluidized bed filter is substantially smaller than that of most other types of biofilters. Compared to other types of biofilter media, sand is by far the cheapest. The major cost associated with a fluidized bed filter is the filter tank and perhaps other required equipment. The oxygen for a fluidized bed biofilter is supplied by the incoming water. The decline in dissolved oxygen concentration through a fluidized bed filter may be substantial. Effluent D.O. concentrations may even approach zero, so it is essential that the water be re-aerated after it passes out of the fluidized bed. Often this is accomplished by placing a packed column after a fluidized bed filter. A packed column is simply a column filled with plastic rings or other media that has a large amount of voids. The media is supported at the bottom by a screen or perforated plate. Water is distributed evenly over the top of the column and splashes down through the media, picking up oxygen. To function properly a packed column should provide a gas:liquid ratio in the range between 3:1 and 5:1. A packed column also serves to strip the water of supersaturated gases such as carbon dioxide and nitrogen. There are a few disadvantages associated with fluidized bed filters. Relatively large pumps are required to generate the velocity of water necessary to fluidize the bed. If water flow is temporarily interrupted to a fluidized bed, it may be difficult to refluidize the sand media. The biofilm on the surfaces of the sand grains is quite sticky, causing the sand grains to stick to one another when compacted. Simply starting up the pump again may not provide enough pressure to break up the compacted sand bed. It may be necessary to sequentially apply very high pressure water flow to small portions of the bed to get it to refluidize. Prolonged interruptions in flow may cause an even more serious problem. The high BOD of the bacteria in the sand bed may completely deplete the oxygen within the filter within 30 minutes. Anoxic conditions will quickly kill the nitrifying bacteria. If one is planning to use a fluidized bed biofilter, select a backup generator with enough capacity to power the pumps supplying water to the fluidized bed. The backup generator should have an automatic switching mechanism to power up the generator in the event of power outage.


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Sizing a biofilter One of the most critical steps in the design process is determining the correct size for the biofilter. If the biofilter is too small, ammonia and nitrite levels will not be controlled and poor water quality conditions will prevail. Oversizing a biofilter will unnecessarily drive up the capital cost in the system. An oversized biofilter might require larger pumps, which would drive up operating costs as well. To determine the correct size of biofilter to use in a system it is necessary to calculate the amount of filter-media surface area that will support the numbers of nitrifying bacteria required to convert the daily ammonia production to nitrate. The required surface area can be easily calculated if there is a good estimate of the expected nitrification rate for the type of biofilter being used under the conditions the biofilter will be operated. Nitrification rates are generally expressed as grams of NH4-N converted to nitrate per square meter of filter surface area per day. Ideally, empirical data will be available for the exact type of biofilter you will be using and under the same operating conditions (e.g., same feeding rates, hydraulic loading rates, pH, temperature). Prototype production systems are set up for gathering exactly this kind of data. In the absence of these kind of data, the designer will have to rely on published nitrification rates for similar types of biofilters operating under similar conditions. Biofilter media vendors are another possible source of information regarding the nitrification rates one might expect with a particular type of biofilter media. Two of the variables that strongly influence nitrification rates are biofilter influent ammonia concentrations (TAN) and temperature. Wheaton et al. (1994) developed a linear regression expression relating nitrification rates in RBC biofilters to influent TAN concentrations: NR = –16.6 + 163.3 TAN Eq. (4.2) where, NR = nitrification rate (mg NH4-N removed/m2 media/day) TAN = Total Ammonia Nitrogen concentration (mg NH4-N/liter) Wheaton et al. (1994) also observed that the nitrification rate declined by 10% for every 5°C drop in temperature below 30°C. The following is a temperature-corrected regression equation for nitrification: NRt = (–16.6 + 163.3 TAN) x (1 - 0.1 x (30-T)/5) Eq. (4.3) where, NRt = temperature corrected nitrification rate TAN = Total Ammonia Nitrogen concentration ( mg NH4-N/L) T = Temperature (°C).


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The nitrification rates calculated using equation (4.3) agree quite well with published nitrification rates for other types of biofilters where the TAN concentrations and temperatures at which the nitrification rates were measured were also reported. The media surface area requirement for the biofilter can be calculated by dividing the amount of ammonia nitrogen produced per day by the nitrification rate:

Biofilter Surface Area (m2 ) = TAN production/dayNitrification Rate

= g NH4 - N Produced / day

g NH4-N consumed /m2 filter media/day

Eq. (4.4)

Once the required biofilter surface area has been determined, it is a simple matter to calculate the volume of biofilter media required. Most vendors of biofilter media can provide information on the Specific Surface Area (SSA) for the different types of media they sell. Specific surface area is a measure of the number of square meters of surface area per cubic meter of media. SSA values are sometimes reported as square feet of surface area per cubic meter of media. One ft2/ft3 is equivalent to 3.28 m2 /m3. Dividing the required biofilter surface area by the SSA value for the biofilter media that will be used will give the required volume of biofilter media:

Volume of Biofilter Media Required (m3 )

=

Required Biofilter Surface Area (m2 )

SSA of Biofilter Media (m2 /m 3 ) Eq. (4.5)

Pumps The heart of a recirculating aquaculture system is the pump (or pumps), which circulate the water through the system. There are a wide variety of pumps available, each suited to a particular set of conditions. Every pump is design to operate efficiently within a relatively narrow range of flow rates and head conditions. For pumps of a given horsepower rating, there is an inverse relationship between flow capacities and the amount of head pressure that a pump generates. The performance characteristics of a pump are a function of the shape, diameter, and revolutions per minute (RPMs) of the impeller and the shape of the volute or pump casing. If the shape and RPMs of two pumps are the same, the pump with the larger impeller diameter will generate more head pressure. If impeller diameter and shape are the same, the pump rotating at a higher RPM will generate more head pressure. If we hold impeller shape, and head pressure constant, flow rates increase with increasing impeller diameter and RPMs. Impeller and volute shapes influence the amount of slippage of water past the impeller vanes when there is resistance to flow. Generally speaking, there is an inverse relationship between


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the amount of head that can be generated by a pump of a given impeller diameter and RPM, and the volume of water the pump can generate against low head pressures. There are three main types of mechanical pumps that are used in aquacultural systems: 1) centrifugal pumps 2) axial flow pumps 3) submersible pumps Centrifugal pumps are pumps in which water enters the impeller casing at the center of and perpendicular to the impeller blades, and exits tangentially. These pumps are very versatile, with designs available for a wide range of flow rates at medium to high head pressures. Because centrifugal pumps are capable of pumping against high head pressures, they are the pump of choice for pumping water through system components and water distribution systems. Centrifugal pumps come in flooded intake and self-priming models. The flooded intake models have little self-priming capability, so these pumps are typically used in situations where there is a positive suction head. Axial flow pumps are pumps which feature a propeller-shaped impeller at the bottom of a cylinder that is open at both ends. The cylinder is at least partially immersed in the water reservoir supplying water to the pump. Water enters the bottom of the center and passes through the impeller and is pushed up to the top of the cylinder travelling in the same axis as the impeller shaft. At the top of the cylinder the water is deflected so that the water exits travelling in a horizontal plane. Axial flow pumps are able to efficiently pump very large volumes of water against low head pressures. Flow rates drop off rapidly as head pressure increases. Axial flow pumps are best suited for situations where the water only needs to be lifted a short vertical distance before it is discharged. In low head situations, an axial flow pump can deliver a given flow rate using far less energy than would be required for a centrifugal pump. Submersible pumps (often called sump pumps) are pumps in which the motor and the impeller are submerged in the water. Submersible pumps are sort of a cross between a centrifugal pump and an axial flow pump. The water enters the impeller casing centrally and perpendicular to the impeller, and exits near the perimeter of the impeller (like a centrifugual pump), but travelling in the axis of the impeller shaft (like an axial flow pump). For pumps of a given horsepower, RPM, and impeller diameter, submersible pumps generate a lot more head pressure than an axial flow pump, but less head pressure than a centrifugal pump. Submersible pumps are typically used for dewatering types of applications. Small, portable submersible pumps are often used to empty a tank or to pump water from a culture tank to transport tank. Heavier duty submersible pumps are often placed in the bottom of drainage sumps when there is not sufficient elevation to permit gravity draining out of the sump. Pumps used for these types of applications have a very open impeller which will allow relatively large objects to pass through the pump without jamming the impeller. These types of submersible pumps are sometimes known as trash pumps.


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Required Flow Rates The first step in selecting a pump is to determine the volume of water the pump will need to deliver. There are several factors that influence this choice. The most important consideration is the rate of turnover of the system volume through the water treatment system. A mass balance approach can be used to determine what the minimum flow rate should be through the biofilter in order to maintain unionized ammonia concentrations within the desired range. In order to keep ammonia from accumulating in the system, the rate of ammonia removal by the biofilter, water exchange, or both must equal the rate at which ammonia is generated in the system. Ammonia generation will be a function of the feeding rate and the percentage of protein in the feed. A procedure using the mass balance approach for calculating the required flow rate to the biofilter is included in Appendix A. The following rule of thumb provides a simpler way to calculate the required flowrate for a system: Provide at least one complete turnover of the system volume through the biofilter every 90 minutes. To illustrate, a system with a total volume of 9,000 gallons would require a flow to the biofilter of at least 100 gallons per minute (9,000 gallons ÷ 90 minutues = 100 gallons/minute). This rule of thumb works well for densities of up to 0.1 lb of shrimp per gallon. In more heavily stocked systems the turnover time should be decreased. Flow rates providing one turnover per hour should be sufficient for densities of up to 0.33 lbs/gallon. Systems loaded with up to 0.75 lbs/gallon require flow rates that will provide one system turnover every 45 minutes. Of course shrimp raceways are not likely to ever be loaded this heavily. Even with 15-gram shrimp held at a density of 200 shrimp/m2 in a 0.5 meter deep culture tank, the density equates only to 0.05 lbs of shrimp per gallon. So a 90 minute turnover time should be sufficient for most intensive shrimp culture situations.

Calculation of Friction Losses and Total Head In order to properly select the correct pump for a system, one must first calculate the Total Dynamic Head (TDH) that will be generated at a given flow rate (Q) through the system. The TDH is the sum of the elevation head (the vertical distance the water must be pumped) and the friction losses in the pipe and pipe fittings at the desired flow rate. The friction loss in the pipe can be calculated using the Hazen-Williams formula:

) dQ ( x )

C100 ( x 0.2083 = f 8655.4

i

1.8521.852

Eq. (4.6)

where: f = friction head in feet of water per 100 feet of pipe. Q = flow rate through the pipe, in gallons per minute di = inside diameter of the pipe, in inches

C = Roughness coefficient for inside surface of the pipe. (C = 150 for PVC and CPVC pipe)


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A spreadsheet can be set up to calculate friction losses for pipe as a function of pipe inside diameter and flow. Published friction loss tables based on the Hazen-Williams formula are also readily available (see Appendix B). The following is a procedure for calculating the friction losses through PVC plumbing. Step 1: Draw a diagram of the plumbing system, showing all pipe diameters, lengths of pipe

through each section, and all of the pipe fittings in the system. Step 2: In many systems the water leaving the pump can follow several alternative paths.

Identify the path that has the highest combined elevation and friction head losses. Divide the plumbing along this path into numbered sections with each section having only one pipe diameter and one flow rate.

Step 3: Add up the lengths of straight pipe for each section, measured in feet. Step 4: Determine the equivalent length of straight pipe (feet of pipe) for each fitting in each

section. These values can be found using either tabulated values or the nomograph in Appendix B.

Step 5: Add up the equivalent lengths of straight pipe for the fittings in each numbered

section, and add to that the total length of straight pipe for each section. Divide by 100.

Step 6: Consult the Flow Velocity and Friction Loss Table (Appendix B) for the type of pipe

used in each section (Schedule 40, Schedule 80, etc.). Look up the friction loss, given in feet of water/100 ft, for the pipe diameter (inches) and flow rate (gpm) of each numbered section. Multiply these values by the number of 100 ft. equivalent lengths of straight pipe for each section (calculated in Step 5). These values are the friction losses, in equivalent feet of water, for each of the numbered sections.

Step 7: Add up the friction losses for each of the numbered sections. Add to this value the

elevation head. The result is the Total Head, or Total Dynamic Head. This value can now be used to select the correct pump for the application.

Friction loss tables (Appendix B) can also be used when designing your plumbing system to help determine the correct size of pipe that should be used in each section of the plumbing system. The objective in selecting pipe diameters is to minimize friction losses in the plumbing, while controlling the cost of the system. In a well-designed system, the velocity of water through any section of pipe should not exceed 5 ft/sec, especially when pipe diameters are 6 inches or greater. This will minimize the potential for pipe failure due to hydraulic shock when pumps are started up.


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To select the correct diameter of pipe, scan across the table to find the minimum diameter of pipe that will pass the desired gallons per minute of water at a velocity of 5 ft/sec or less. Water velocities of up to 8 ft/sec are probably safe for small diameter pipes (2” diameter or less), but the friction losses will be high at these velocities.

Pump Sizing Procedure After you have calculated the Total Head for the desired flow rate in your system, you are ready to select the proper pump for the job. The objective here is to choose a pump that will operate at its optimal efficiency under the expected operating conditions of discharge and head. To do this you will need to be able to read a pump performance curve.

Pump Performance Curves In its simplest form, a pump performance curve is simply a graph showing the relationship between pump discharge, plotted on the x-axis, and total head, plotted on the y-axis. The first job, then, is to find pumps whose performance curves indicate that it will deliver the desired number of gallons per minute at the total head that it will be operating against. There may be many pumps that will meet these criteria. However, some of these pumps will not be suitable because under the required conditions, these pumps may not be operating within their range of optimal efficiency. Sometimes a pump vendor will indicate the best efficiency range on the pump performance curve. Select the pump of the lowest horsepower that will deliver the desired flow rate against the calculated total head while operating within its best efficiency range.

Trimmed Impellers Some performance curves will give a family of curves for the same pump operating with different sizes of impellers. As a general rule, the smaller the impeller diameter the less flow that can be generated at a given head. Stated in another way, for a given flow rate, larger diameter impellers will be able to pump against higher head pressures. The duty point is the point on the curve where the flow and the head match the calculated requirement for the application. Some performance curves will show the horsepower requirements for different impeller diameters at different points on the performance curve. A series of upward sloping Brake Horsepower (BHP) lines may be positioned below the pump performance curves. Each line corresponds to a given BHP. The right end of each BHP curve will fall directly under the right end of one of the impeller curves. This will be the correct BHP curve to read from for the corresponding impeller. To determine the correct horsepower for the pump motor, draw a vertical line from the duty point to the correct BHP curve, and then draw a horizontal line over from the point of intersection to the BHP scale on the right. This will indicate the correct horsepower for the pump.


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Literature Cited Akai, D., O. Miki, and S. Ohgaki. 1983. Nitrification model with inhibitory effect of sea

water. Ecological Modeling 19:189-198. Beecher, L.E., A.A. DeLosReyes, Jr., D.G. Drennan II, and R.F. Malone. 1997. Alternative

Media for Enhanced Nitrification Rates in Propeller-Washed Bead Filters. In: M.B. Timmons and T.M. Losordo (eds.) Recent Advances in Aquacultural Engineering (Proceedings). Northeast Regional Agricultural Engineering Service, Ithaca, NY. NRAES–105, pp. 263-275.

Brazil, B.L. 1996. Application of Ozone to Recirculating Aquaculture Systems. In:

Successes and Failures in Commercial Recirculating Aquaculture. (Conference Proceedings). Northeast Regional Aquaculture Engineering Service NRAES−98, pp. 373-389.

Bullock, G.L., S.T. Summerfelt, A.C. Noble, A.L. Weber, M.D. Durant, and J.A. Hankins.

1996. Effects of Ozone on Outbreaks of Bacterial Gill Disease and Numbers of Heterotrophic Bacteria in a Trout Culture Recycle System. In: Successes and Failures in Commercial Recirculating Aquaculture. Proceedings from the Successes and Failures in Commercial Recirculating Aquaculture Conference. Northeast Regional Aquaculture Engineering Service NRAES−98, pp. 598-610.

Chen, J.C. and T.S. Chin. 1988. Acute toxicity of nitrite to tiger prawn, Penaeus monodon,

larvae. Aquaculture 69: 253-262. Chen, S. and R.F. Malone. 1991. Suspended Solids Control in Recirculating Aquacultural

Systems. In: M.B. Timmons and T.M. Losordo (eds.) Recent Advances in Aquacultural Engineering (Proceedings). Northeast Regional Agricultural Engineering Service, Ithaca, NY. NRAES−49, pp. 170-186.

DeLosReyes, A.A., Jr., L.E. Beecher, and R.F. Malone. 1997. Design Rationale for a Air-

Driven Recirculating System Employing a Bubble-Washed Bead Filter. In: M.B. Timmons and T.M. Losordo (eds.) Recent Advances in Aquacultural Engineering (Proceedings). Northeast Regional Agricultural Engineering Service, Ithaca, NY. NRAES–105, pp. 287-294.


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Hobbs, A., T. Losordo, D. DeLong, J. Regan, S. Bennett, R. Gron, and B. Foster. 1997. A Commercial, Public Demonstration of Recirculating Aquaculture Technology: The CP&L/EPRI Fish Barn at North Carolina State University. In: M.B. Timmons and T.M. Losordo (eds.) Recent Advances in Aquacultural Engineering (Proceedings). Northeast Regional Agricultural Engineering Service, Ithaca, NY. NRAES–105, pp. 151-158.

Hochheimer, J.N. and F.W. Wheaton. 1991. Understanding Biofilters, Practical

Microbiology for Ammonia Removal in Aquaculture. In: M.B. Timmons and T.M. Losordo (eds.) Recent Advances in Aquacultural Engineering (Proceedings). Northeast Regional Agricultural Engineering Service, Ithaca, NY. NRAES−49, pp. 57-79.

Huguenin, J.E. and J. Colt. 1989. Chapter 12: Aeration and Degassing. In, Design and

Operating Guide for Aquaculture Seawater Systems. Elsevier Science Publishers B.V.. Amsterdam, Netherlands. 264 pp.

Olson, R.J. 1981. Differential photoinhibition of marine nitrifying bacteria. Journal of

Marine Research 39(2): 227-238. Piper, R.G., I.B. McElwain, L.E. Orme, J.P. McCraren, L.G. Fowler, and J.R. Leonard. 1982.

Fish Hatchery Management. U.S. Department of Interior, Fish and Wildlife Service, Washington, D.C.

Scott, K., and L. Allard. 1983. High-flowrate recirculations sytem incorporating a

hydrocyclone prefilter for rearing fish. Progressive-Fish Culturist 46:254-261. Wedemeyer, G.A., N.C. Nelson, and W.T. Yasutake. 1979. Physiological and biochemical

aspects of oxone toxicity to rainbow trout (Salmo gairdneri). Journal of the Fisheries Research Board of Canada 36: 605-614.

Wheaton, F.W., J.N. Hochheimer, G.E. Kaiser, R.F. Malone, M.J. Krones, G.S. Libey, and

C.C. Easter. 1994. Chapter 5: Nitrification Filter Design Methods. In, M.B. Timmons and T.M. Losordo, editors. Aquaculture Water Reuse Systems: Engineering Design and Management. Elsevier Science, B.V., Amsterdam, Netherlands. pp. 127-171.

Wickins, J.F.. 1976. The tolerance of warm-water prawns to recirculated water. Aquaculture

9: 19-37.


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Youngs, W.D. and M.B.Timmons. 1991. A Historical Perspective of Raceway Design. In: M.B. Timmons and T.M. Losordo (eds.) Recent Advances in Aquacultural Engineering (Proceedings). Northeast Regional Agricultural Engineering Service, Ithaca, NY. NRAES−49, pp. 160-169.

Chapter 5 – Harbor Branch Shrimp Production Systems

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Chapter 5 Harbor Branch Shrimp Production Systems

by Peter Van Wyk


Design Objectives Shrimp culture in the Western Hemisphere is based on semi-intensive production of shrimp in large coastal lagoons. The success of this approach to shrimp culture depends upon the availability of cheap coastal land suitable for pond culture, a year-round growing season and a permissive regulatory environment that permits release of large volumes of untreated effluent into coastal waters. The traditional approach to shrimp culture is not feasible in Florida because of the high cost of coastal property, a short growing season (less than 220 days per year) and a strict regulatory environment that prohibits the release of untreated effluents from aquacultural facilities into state waters. The best candidate for shrimp culture in Florida is a non-native species, Litopenaeus vannamei. Culturing this species in open coastal ponds could result in accidental introductions into Florida waters and a potential negative impact on our state’s native shrimp populations. Clearly, if a shrimp culture industry is to develop in this state, different technologies will have to be used. In recent years, there has been renewed interest in shrimp culture in Florida resulting from new technological developments. These developments have made it possible to culture Litopenaeus vannamei indoors in near-freshwater, recirculating aquaculture systems. Recently it has been demonstrated that L. vannamei can be successfully produced in water with chloride concentrations as low as 300 ppm. This chloride level corresponds to a salinity of only 0.5 ppt. Water with low chloride levels can safely be used to irrigate most crops. The significance of these findings is that shrimp production can now be practiced in areas where less expensive, non-coastal agricultural land is available. New advances in the technology for producing L. vannamei indoors in high-density recirculating aquaculture systems can also enable us to produce this species year-round, even in temperate climates with relatively cold winters. Year-round production improves the economic potential of an enterprise in several ways. The annual revenues of the operation are increased because year-round production increases annual productivity. Continuous harvesting facilitates direct marketing to retail outlets, which may allow for a higher price to be received for the product. Producing shrimp indoors in recirculating systems benefits the producer by significantly reducing the risk of exposing the shrimp to viral diseases that have wreaked havoc in open coastal ponds throughout the world. An indoor production system also protects the crop from losses resulting from predation or theft. In addition, these systems significantly reduce the risk of accidental release of non-native shrimp into Florida’s coastal waters.


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The objective of the Harbor Branch Oceanographic Institution (HBOI) shrimp culture program has been to develop a cost-effective indoor, freshwater production system based on a recirculating water treatment system. The principle that guided HBOI in the development of new system designs is: “Keep It Simple”. The ideal system should be simple to build using inexpensive, readily available materials, and should be operable by individuals with limited training specific to systems operation. With this in mind, HBOI focused its efforts on designing an inexpensive system capable of growing shrimp at moderately high densities (up to 150 shrimp/m2). These densities are significantly lower than the highest densities (>600 shrimp/m2) that have been reported for L. vannamei in more sophisticated recirculating systems (Davis and Arnold, 1998). The shrimp production systems utilized in the State of Florida sponsored project represent two generations of system design. First generation systems at HBOI (System A) feature above-ground raceways and sand filters. Given their simplicity, these systems perform surprisingly well and support loading rates as high as 4 kg shrimp/m3. However, sand filters are expensive to operate because they require inefficient, high-head pumps to deliver the water through the compacted sand filter media and because they require a lot of labor to maintain. The cost of operating the pumps on these systems can be quite high because they operate continuously. The second-generation system at HBOI (System B) features an in-ground raceway and a low-head water treatment system. The in-ground raceway should be less prone to heat-loss, reducing heating costs in the winter. Using a low-head filtration system reduces pumping costs, because the pump horsepower requirements are less than half of the required horsepower to deliver water through a sand filter. The following is a description of the two HBOI shrimp production systems used during the experimental trials conducted in 1999.

System A

Greenhouse System A is housed in a 30 ft x 152 ft Quonset-style greenhouse (Figure 5-1). The greenhouse consists of a series of arches or bows made with 2” diameter galvanized steel pipe (see Figure 5-2). The bows are anchored in concrete at their bases. The arches are

Figure 5-1: Quonset-style greenhouse Figure 5-2: Greenhouse frame


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supported by purlins running the length of the greenhouse connected by clamps to each rib. Cross-struts span every second arch providing additional support. A double layer of 6 mil, clear, UV-resistant polyethylene plastic material covers the greenhouse. The space between the two layers of plastic is inflated using a small blower. Though slightly more expensive than a single layer, the two layers of plastic are highly efficient at collecting and retaining solar heat. Single layer systems have high heat loss at night through a process known as nocturnal radiation, which allows heat collected in the daytime to escape at night. The heat retention in the double-layered roof generally more than pays for the insulating second layer of plastic sheeting and air support system. The captured heat is reflected back into the greenhouse. During the summer months, an 80% shade cloth covers the outside of the greenhouse. The shade cloth minimizes algal growth within the raceways. The greenhouse is ventilated with two 1.5-hp extractor fans and with one 0.5-hp extractor fan mounted at one end of the greenhouse. The ventilating air enters at the greenhouse at the opposite end through two mechanical louver windows (Figure 5-3). The fans and the louvers are thermostatically actuated, providing a measure of automated temperature control to the greenhouse.

Culture Tanks The greenhouse contains four culture tanks, each operating on separate filter systems. The culture tanks in HBOI’s demonstration system measure approximately 13.5 ft x 60 ft, with a three-foot wide walkway down the middle between the tanks (Figure 5-4) This greenhouse was designed to provide open space to accommodate tours and training classes. A similar greenhouse that is not used for tours contains four raceways measuring 13.5 ft x 70 ft. This leaves only a 12 ft x 30 ft open area in the center of the greenhouse for sumps, pumps and filtration equipment.

Figure 5-3: Greenhouse Ventilation

Figure 5-4: Culture tank layout in System A. Upper two culture tanks are single-phase systems. The lower two culture tanks are three-phase systems.


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The culture tanks consist of a wooden frame supporting a black, 30-mil, high-density polyethylene liner. The wooden frame is two board widths high and is built using 2”x12” pressure-treated lumber boards supported by galvanized pipe set vertically in a concrete anchor. The vertical pipe supports are set on 4-ft centers. The arches forming the frame of the greenhouse support the outside walls of the culture tanks. All rocks are cleared from the earthen floors inside of the tank frame to insure that there is nothing underneath the liner that can puncture it. River sand is spread over the floor of the raceway prior to laying in the liner. The sand protects the liner and is easily molded to give the desired contour to the floor of the tank. The floor of the tank slopes slightly downward from the sidewalls to the center of the tank and from one end of the tank to the other (100:1 slope). The culture tanks are rectangular in shape, and have been set up in a "racetrack" configuration, essentially a hybrid between a circular tank and rectangular tank. Each culture tank has two drain outlets at either end, centered between the end wall and the sides of the tank. A center divider baffle (Figure 5-5) is positioned between the drain outlets, and functions to separate water flowing down one side of the "racetrack" from the water flowing down the opposite side. The water in the tank flows in an elongated oval pattern, traveling down one side of the tank, circling around the drain outlet at one end, then traveling up the other side of the raceway and circling around the opposite drain outlet. Baffles have been placed in the corners of the tank to prevent eddies from developing in the corners. The baffles help create a semi-circular flow pattern at each end of the tank so that the water pivots about the drain outlets. This flow pattern generates centrifugal forces as the water circles the drain, concentrating the suspended solid wastes in the area around the drains. Water is introduced into the tank through spray bars spanning the width of the straight run of the tank. Mixing the incoming water in this fashion with the water circling the raceway helps create relatively uniform water quality throughout the tank. The spray bar serves three important functions: (1) it causes the water to enter the tank at a high velocity, which increases the velocity of the water flow around the tank; (2) it aerates the water as it enters the tank; and (3) excess carbon dioxide is de-gassed as the water passes out of the spray bar. Two of the culture tanks in System A are set up as three-phase culture systems. In a three-phase system the raceway is partitioned into three sections, which correspond to three distinct phases of the production cycle: 1) nursery phase, 2) intermediate growout phase, and 3) final growout phase. The nursery phase section is small (7 ft x 13.5 ft), occupying only 12% of the growout area. Postlarvae are stocked into the nursery section and remain there

Figure 5-5: Culture tank with racetrack configuration and center baffle.


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for 50 to 56 days. The intermediate growout section measures 17 ft x 13.5 ft and occupies approximately 28% of the culture area. Juveniles averaging 1.5-2.0 grams are transferred into the intermediate growout section and remain there 50-56 days. The final growout section occupies 60% of the culture area and measures 36 ft x 13.5 ft . Juvenile shrimp averaging about 6.5 grams are transferred into the final growout section and remain there for another 50-56 days. By this time they have been in the system between 150 and 168 days and will have reached a size of 15-18 grams. The walls dividing the three sections, as well as the side walls, are constructed from 2” x 12” treated lumber. A 4-inch bulkhead fitting passes through the liner and the divider walls separating each section. These are normally plugged with a short section of pipe and a 4-inch cap to keep the shrimp from passing from one section to the other. The plugs are removed when the shrimp are ready to be transferred from one section to another. The shrimp pass through the bulkhead fittings, eliminating the need for handling the animals. A small percentage of the shrimp need to be netted from one section to the next. Each of the three sections is also set up in the “racetrack” configuration, with center and corner baffles, and drains at either end of the center baffle. The water is circulated with spray bars located at the top of the straight runs down each side of the raceway.

Drains The single-phase culture tanks each have two drain outlets, one located at either end of the center baffle. The drain outlets are centered between the sidewalls and the end wall of the tank. All drain outlets are 4-inches in diameter, with the exception of those in the nursery tank, which are 2-inches in diameter. A 12-inch standpipe in each drain outlet sets the minimum water level in the culture tanks. The top of this standpipe is fitted with a cylindrical screen extending to 6-inches above the maximum water level in the tank. The purpose of this screen is to exclude shrimp from the drain outlets. An outer sleeve is placed over the standpipe to allow the water flowing out of the drain outlet to be drawn from the bottom of the tank. The outer sleeve consists of a PVC pipe with a slightly larger diameter than that used for the standpipe. The pipe is scalloped or screened at the bottom to allow bottom water to pass through it (see Figure 4-3). The pipe used for the outer sleeve should be longer than the operational depth of the tank, so that surface water will not normally overflow the top of the pipe. The pipe should be shorter than maximum height of the tank, so that if the openings at the bottom of the sleeve become clogged, the tank will not overflow. All drain outlets consist of a bulkhead fitting, which passes through the liner. The liner is sandwiched between the threaded locking ring of the bulkhead fitting and the flange of the bulkhead. A rubber gasket is placed between the flange (which sits on the inside surface of the tank liner) and the liner. When the locking ring is tightly threaded, the rubber gasket seals the liner, so that no water can leak between the flange and the liner. A non-glued extension pipe connects the bulkhead fitting to the underlying drainpipe. All of the drain outlets feed into a common 4-inch central drainpipe. The central drainpipe discharges into a


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4 ft x 3 ft x 4 ft polyethylene sump located outside of the tank at one end of the raceway. The top of the sump is flush with the top of the tank. The drain pipe enters the sump through a bulkhead fitting in the sidewall located just above the bottom of the sump. The sump serves as a settling basin and pump well. The sump itself contains a drain outlet through a 4-inch bulkhead fitting located in the bottom of the sump. The sump drain connects to a common drain line, which carries effluent from the culture tanks to a 1-acre retention pond. A 4-inch diameter standpipe is inserted into the bulkhead fitting of the drain outlet. This standpipe extends to within 4-inches of the top of the sump and serves to set the maximum water depth in the culture tank. During water exchanges, excess water will overflow the standpipe and exit to the retention pond.

Pumps Water is pumped from the sump to the sand filter. The intake for the pump is located near the bottom of the sump and is fitted with a check valve to prevent the pump from losing its prime when it is turned off. The pumps used in System A are 2-hp centrifugal pool pumps. The pumps are self-priming and are fitted with a strainer basket located at the entrance to the pump. These pumps are capable of operating against high heads, which is important if the pump is paired with a sand filter. The pump delivers about 75 gpm against 40 ft of head pressure (17 psi) when the filter has just been cleaned or backwashed. When the sand filter becomes clogged with solids, the head pressure increases to 80 ft of water (30 psi). Against that head pressure the pump delivers water at about 20 gpm. This variation in flow between cleaning cycles is one of the reasons sand filters were replaced in HBOI’s second generation system.

Sand Filters The sand filters (Figure 5-6) are 36-inch diameter high-rate downflow sand filters. Each sand filter is filled with 600 lbs. of No. 20/30 silica sand. The sand filter serves as both the solids filter and the biofilter for the system. Sand filters work well at low feeding rates (less than 1.5 kg of feed/day). At these feeding rates the filters do an excellent job of removing solids and capturing most of the particles above 25 µm in diameter. They also capture a significant percentage of the particles down to 10µm in diameter. Sand filters function well as biofilters at lower feeding rates; however, when feed rates exceed 2 kg of feed/day, the performance drops off in both of its

Figure 5-6: Sand filter and sump.


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roles. The sand bed will begin to cake within a short period of time after backwashing. This results in a sharp reduction in the flow rates through the sand filter and will also lead to channeling of the water through the bed. The sand bed needs to be broken up with a high pressure wand twice a day prior to the backwash procedure. It may take twenty minutes to do a good job of breaking up the sand bed and backwashing the filter. Although we have pushed feeding rates to as high as 4 kg of feed/day on systems with sand filters, at these feeding rates total ammonia levels will often rise to 2 mg TAN/liter or higher. Sand filters are not recommended for systems in which stocking densities exceed 75 shrimp/m2.

Aeration A 2.5 hp regenerative blower supplies air to the culture tanks (Figure 5-7). Each raceway is provided with 30 1” x 3” medium pore diffusers. Each diffuser supplies approximately 0.3 standard cubic feet per minute (scfm) of air to the raceway, providing each culture tank with a total of 9.0 scfm of air. The airstones are distributed at 4-foot intervals along the side walls of the culture tanks. A beltdrive blower powered by a 9-hp diesel motor serves as an emergency backup (Figure 5-8). The backup blower has a pressure-actuated switch that starts the blower motor whenever the pressure in the air system drops to zero.

System B

System B represents a second generation in HBOI shrimp production system design. The design objectives for this system were to:

1) reduce the cost of the greenhouse structure 2) reduce the construction costs for the culture tanks 3) make the systems more energy efficient 4) reduce the labor required to maintain the systems 5) increase the carrying capacity of the systems, while keeping system costs low 6) provide for consistent circulation of water throughout the system.

Figure 5-7: Regenerative Blower Figure 5-8: Diesel-powered backup blower


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Greenhouses The System B culture tanks are housed in a 30 ft x 96 ft Quonset-style greenhouse (Figure 5-9). In many respects, these greenhouses are similar to the System A greenhouses, but there are some important differences. The System B greenhouses also have a series of anchored arches; however, they lack the cross-strut supports. The result is a less expensive greenhouse, but one that has a lower wind tolerance than the System A greenhouses. System A greenhouses are rated to be able to withstand winds of up to 120 mph (with the greenhouse cover removed). System B greenhouses only are rated for winds of 80 mph or less. However, the System B greenhouse kits (including frame, doors, gable ends, exhaust fans, shuttered windows and double polyethylene covering) cost about 54% less per square foot than similarly-equipped System A greenhouse kits (approximately $1.57/ sq. ft vs. $2.90/sq ft). During the summer months, a 95% shade cloth is placed on the outside of the greenhouse. This provides significantly more shade than the System A shade cloth, which provides 80% shading. The greenhouse ventilation system consists of two, 42-inch, 3/4-hp exhaust fans and two 51-inch shuttered windows. A single thermostat controls both the windows and the exhaust fans. An 8-ft x 8-ft sliding door is located at one end of the greenhouse. This door allows large pieces of equipment or harvest boxes to be easily moved into the greenhouse.

Culture Tanks The culture tanks in System B are similar to those in System A, except that they are partially excavated below ground level. Instead of having the floor of the culture tank at ground level and the tank depth determined by the height of the wooden frame, the floor of the System B culture tanks is excavated to a depth of 18 inches below grade. A wooden frame surrounds the perimeter of the excavated area adding an additional 12 inches to the depth of the raceway. The wooden frame is similar to the frame used to create the System A culture tanks, except that it is only one board (6”) high. A berm with a 1:1 slope extends from the bottom of the wooden frame down to the floor of the tank. The overall tank depth, when it is filled with water is 24 inches, or 6-inches deeper than the System A tanks. The culture tank is

Figure 5-9: System B 30 ft x 96 ft greenhouses (B&K Installations, Inc.)


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lined with the same 30-mil high density polyethylene liner material as is used in System A. There are several advantages to this approach to raceway construction. Because much of the volume of the raceway is below ground level there should be less heat loss from the raceways during the winter. The raceways can be made slightly deeper without appreciably increasing the cost of construction. A deeper tank will be able to sustain a higher biomass, and will also have more stable temperature and water quality characteristics. Each of the System B greenhouses is occupied by two culture tanks, each with its own water treatment system. The culture tanks lie side-by-side in the greenhouse, sharing a common central wall (Figures 5-10 and 5-11). The 3-foot walkway between the tanks has been replaced with a 1-ft wide catwalk above the tanks. In this configuration, approximately 90% of the available area in the greenhouse is under cultivation, compared to about 80% in System A. Reducing the number of systems per greenhouse from four to two reduces the labor requirement by nearly half, without sacrificing any production.

Single-Phase and Three-Phase Production Systems One greenhouse in System B contains two, single-phase shrimp production systems (Figure 5-10). Each single-phase culture tank measures 14.5 ft x 88 ft. Except for the fact that they are in-ground culture tanks, most of the details of their construction are essentially the same as in System A. The tracks are configured in “racetrack” configuration with a central baffle and corner baffles. A second greenhouse in System B contains two, three-phase shrimp production systems (Figure 5-12). The culture tanks in the three-phase system have the same design as the single-phase culture tanks, except that they are separated into three discrete sections by divider walls. The divider walls are made from pressure-treated 2”x 12” boards and the walls are two boards high. The bottom of the intermediate growout section is about 6-inches higher than the bottom of the final growout section, and the floor of the nursery section is 6-inches higher than the bottom of the intermediate growout section. These elevation differences allow the shrimp to be transferred between sections by draining the water from

F i gu re 5 - 10 : Sys te m B S ing le-Phase Raceways with Axial Flow Pump


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one section to the next through a 4-inch bulkhead fitting. The nursery sections measures 10 ft x 14.5 ft (11% of the culture area). The intermediate growout section measures 14.5 ft x 27 ft (31% of the culture area) and the final growout section measures 14.5 ft x 51 ft (58% of the culture area). The shrimp spend equal amounts of time in each of the three phases of the growout operation. In our trials, it typically takes between 150 and 168 days for the shrimp to reach an average size of 15 to 18 grams. Market-size shrimp will be harvested from the final growout section every 60 days. In one year, a single three-phase system will produce six crops of shrimp. The three-phase system allows for higher production levels than a single-phase system, because in a three-phase system the biomass of each raceway at the time of stocking is much closer to the final harvest biomass. Raceway space is used more efficiently than in a single-phase system, where raceways are maintained at low densities throughout the early part of the growout cycle. Assuming survival and growth rates are equal between a single- and three-phase system, the three-phase system should be able to produce about 1.8 times as many shrimp per year as the single-phase system. Although the area harvested for each crop is only 60% of the harvest area in a single-phase system, the final growout section of the three-phase system is harvested three times as often as the single-phase system.

Figure 5-11: Interior of System B greenhouse showing shared center wall and catwalk.


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Water Treatment Systems The water treatment system must provide sufficient water management; however, we also wanted to improve the energy efficiency over the performance of the sand filter based systems. Towards this end it was decided to incorporate a low-head filtration system into the design, that flowed by gravity through the solids filter and biofilter. The solids filter in System B consists of cylindro-conical sump (4 –ft diameter x 4-ft deep, 1,200-liter capacity), filled with 16 cubic feet of biofilter beads (Figure 5-13, 5-14). The beads are polyethylene cylinders 7 mm long by 10 mm in diameter. These beads are positively buoyant. The tank is plumbed so that the raw water from the culture tank enters the filter tank through a 4-inch bulkhead fitting cut into the conical portion of the tank. A second bulkhead fitting is cut into the sidewall about 12-inches below the culture tank water

F i gu re 5 - 12 : Sys te m B 3 -P ha s e Production Systems, powered by a 3/4 hp c e n t r if ugal pump.

Figure 5-13: Low Head Bead Filter

Figure 5-14: Preparing to flush a low-head biofilter.


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level and connects the solids filter to the biofilter tank. A 4-inch pipe with 5-mm slots cut into its upper surface is inserted into upper bulkhead fitting on the inside of the tank. This pipe collects filtered water from near the surface of the water and allows it to pass into the biofilter tank. The water must take a tortuous pass through the filter bed before it reaches the collecting pipe at the top of the water column. In the process, settleable solids and larger suspended solids are trapped on the sticky surfaces of the beads. The solids are flushed from the system once a day by plugging the raw water inlet and the filtered water and removing a 2-inch standpipe from the drain outlet in the bottom of the cone (Figure 5-14). A 6-inch diameter outer standpipe keeps the beads from draining out of the system. This standpipe is fitted with stabilizing bars at the top to maintain firm downward pressure so the pipe seals at the bottom. Screened windows near the bottom of the outer standpipe allow the water and solid wastes to drain out of the tank when the inner standpipe is pulled. The tank is completely drained before the plugs are pulled, and water is allowed to flow back into the solids filter. When the water is allowed to flow back into the filter the initial head differential between the culture tank and the filter results in a good flushing of the main drain pipe from the culture tank. The turbulence created by the incoming water washes the beads, releasing most of the solids that remain on their surfaces. If significant amounts of solid wastes are flushed out of the drain line when the filter tank is refilled, the solids filter is flushed a second time. Two flushes of the tank requires less than 2,000 liters, or about 3% of the system volume. An aerated, submerged biofilter receives water after it flows out of the low-head bead filter. The biofilter also uses the beads, but in this application the beads are tumbled by air bubbles introduced into the bottom of the filter bed through a grid of 10 medium pore airstones. The beads are contained within a cage made of 1-inch Schedule 40 PVC pipe and 1/4” square-mesh polyethylene mesh screen (Figure 5-15). The cage serves to contain the beads, so that they do not get sucked into the pump or go out down the drain. The cage sits about 4-inches off of the bottom in a rectangular, polyethylene sump (4-ft x 6-ft x 4-ft). The biofilter beads provide a large amount of surface area, with a specific surface area of 259 ft2/ft3. A total of 20 cubic feet of media are contained in the biofilter tank. In reality, the solids filter

Figure 5-15: Aerated submerged bed biofilter.


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is part of the biofilter too. Between the solids filter and the biofilter, there are a total of 36 cubic feet (approximately one cubic meter) of biofilter media in the system. The distributor of this media reports that biofilter beads can process the nitrogenous wastes from up to 12 kg of feed per cubic meter per day. The maximum feed rate for this system will be approximately 6 kg per day, or half of what the beads can handle. Our early experience indicates that Nitrobacter populations are difficult to establish on this media. Aggressive aeration of the submerged bed of biofilter beads may have the effect of sloughing Nitrobacter off of the exposed surfaces of the beads.

Pumps The biofilter sump doubles as a pump reservoir for the main system pump. The amount of head required to return the water to the culture tanks is minimal, since there are no filter components between the pump and the culture tanks, and the elevation head that must be overcome is less than 6 inches. Two different types of low-head pumps are being used in System B. A 1/4-hp axial flow pump performs the pumping duties in the single-phase culture systems. This pump utilizes a 3-inch plastic fan blade as an impeller. The pump column is made of 4-inch PVC pipe. A tee half-way up the column directs the flow out of the pump and into the culture tank. The pump inserts into a bulkhead fitting that passes through the wall of the sump and the culture tank just below the tank water surface. There is essentially zero head pressure. These axial flow pumps are capable of moving large volumes of water with very little energy expenditure. The pump discharges 160 gpm of water in this application. Despite the high discharge volume of these axial flow pumps, the water is discharged with very little pressure or velocity. As a result, the return flow does not generate a great deal of circulation within the culture tank, nor does the return flow provide additional aeration or degassing. An axial flow pump cannot be used in the three-phase systems because the return flow had to be piped to the opposite end of the greenhouse to the nursery and intermediate growout sections. The discharge out of these axial flow pumps drops off rapidly as head pressure increases. At only 18-inches of head the pump discharge is less than one-quarter of the discharge at zero feet of head. As an alternative we selected a low-head 3/4-hp centrifugal pump, which delivers 150 gpm of water against a 10-ft head. This pump is much more efficient than the high-head, 2-hp pool pump used in System A, which only pumps 100 gpm at this head and requires more than twice the horsepower. Using a centrifugal pump in the three-phase systems permits the return flow to be introduced through spray bars, which spanned the raceways (Figure 5-12, Figure 5-16). The spray bars perform several useful functions. The spray bar causes the water to enter the tank at a high velocity, which increases the velocity of the water flow around the tank. This, in turn, facilitates transport of solid wastes out of the tank. We have seen that, although the discharge of the centrifugal pumps is slightly less than that of the axial flow pumps, the velocity of


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water flow rate in culture tank is much higher. This is because the water enters the culture tank at a high velocity and its momentum sets the entire mass of water in the tank moving. Another important benefit derived from spray bars is that the water is aerated as it enters the tank and excess carbon dioxide is de-gassed as the water passes out of the spray bar. The degassing that occurs when the water is sprayed into the tank also decreases the risk of developing gas bubble disease due to nitrogen supersaturation. The additional cost for these benefits is about $1.00/day/system, assuming electricity costs $0.08/kw-hr. Configuring the system so that the pump is the last component before the water is returned to the culture tank guarantees that the flow rates through the system are constant over time, regardless of the pumping system that is used. This is in sharp contrast to System A, where flow rates decline by 50-75% between sand filter backwashes.

Aeration A single 2.5-hp regenerative blower supplies the air supply for the four culture tanks in System B. This blower supplies approximately 100 scfm of air against a head pressure of 50 inches of water. Each system is supplied with 25 scfm of air, which is delivered through submerged 3”x 1” medium pore diffusers. A total of 44 diffusers are positioned in the culture tanks at 4-ft intervals on either side of the central baffle. An additional ten airstones are set into an air manifold at the bottom of the biofilter cage and both aerate and tumble the biofilter media. A belt-driven blower powered by a 9-hp diesel engine provides emergency backup aeration. This blower is twice as large as it needs to be to deliver 200 scfm of air at 50-inches of water, but was sized to accommodate future expansions. A pressure switch turns the diesel engine on whenever it detects a loss of pressure in the air system.

Figure 5-16: Return flow to culture tank through spray bar.


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Water Supply At Harbor Branch, freshwater and seawater are supplied by wells. Wellwater is a desirable water source because it is virtually free from bacterial, viral, or parasitic pathogens. However, the water does have some undesirable chemical characteristics. Like a lot of wellwater, HBOI’s wellwater is high in hydrogen sulfide, carbon dioxide, and ammonia and is low in oxygen. Before we can use it, the water must pass through a series of pretreatments. The pretreatment process is the same, whether we are treating freshwater or saltwater. The first step in the pretreatment process is to pass the water through a degassing tower. The degassing tower consists of an eight-foot tall, six-foot diameter, polyethylene tank. Inside the tank is a screened plate spanning the entire cross-sectional area of the tank. This plate supports plastic coiled packing media, which fills the volume of the tank above the plate. The water distributed over the packing media at the top of the tank trickles down through the media in thin sheets and small droplets. A 1/2-hp blower pumps air into the bottom of the column. The column is open at the top, allowing the air to escape. By increasing the area of the air-water interface, gas exchange between the air and the water occurs at an accelerated rate. Supersaturated gases in the wellwater, such as hydrogen sulfide and carbon dioxide, are transferred from the water to the air, and gases that are under-saturated in the water, such as oxygen, are transferred from the air to the water. The water passing out of the degassing tower should be close to the saturation level for all of these gases. The next step in the treatment process is to remove the majority of the ammonia and nitrite by passing the water through a large biofilter tank filled with barrels containing crushed oyster shell. Inside of each barrel is an airlift mechanism that pulls water from outside the barrels into the tops of the barrels and down through the oyster shell media. At the bottom of the airlift, both water and oyster shell are pulled into the airlift and lifted to the top of the barrel. This allows the water to be circulated through the oyster shell media and the oyster shell is periodically fluidized, allowing fouling growth to be sloughed off. The biofilter tank has a total volume of 12,000 liters. The flow rate through the tank is approximately 200 liters per hour (this tanks serves other systems besides the HBOI shrimp systems). The total residence time in the biofilter is approximately one hour. During this time the ammonia is reduced from nearly 1 ppm to about 0.05 ppm. The nitrite concentration of the water leaving the biofilter is typically less than 0.01 ppm. The treated water flows by gravity into one of two 20,000 liter water storage reservoirs. The water storage reservoirs are enclosed polyethylene chemical storage tanks that have been given a double coat of paint to keep them dark inside to prevent algal growth. A 2-hp centrifugal pump draws water from the reservoirs and pumps it through a sand filter and out to the culture systems. At Harbor Branch, the water delivery pump operates continuously, so that water is available upon demand. A return line to the reservoir tank is present to circulate the water and protect the pump when there is little or no demand for water.


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Retention Ponds The effluent from the shrimp production operation is discharged into a series of three, one-quarter acre, retention ponds at Harbor Branch. All effluent from the facility discharge into one corner of the first pond in the series. Overflow pipes pass through the levees separating each of the three ponds. The first retention pond is the primary solids settling pond, and typically has the densest growths of algae and aquatic plants. The plants absorb nitrogenous wastes from the water, using it for protein synthesis. Evaporation and seepage account for virtually all of the losses of water from the retention ponds. The second and third ponds in the series provide extended residence time for the water to guarantee that the water has enough time to evaporate, or seep out of the ponds. Every few years it may be necessary to pump out the sludge that collects in the bottom of the ponds. One of the advantages associated with producing shrimp in near-freshwater systems is that this sludge can be used as fertilizer for certain vegetable or row crops. Retention ponds similar to the ones used by Harbor Branch are likely to be required of all feed-based aquaculture operations in the state of Florida.

Literature Cited Davis, D.A. and C.R. Arnold. 1998. The design, management, and production of a

recirculating raceway system for the production of marine shrimp. Aquaculture Engineering 17: 193-211.

Chapter 6 – Receiving and Acclimation of Postlarvae

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Chapter 6 Receiving and Acclimation of Postlarvae

by Peter Van Wyk


Purchasing Postlarvae The success of your operation is dependent upon the quality of the postlarvae you stock into your system. Strong, healthy postlarvae placed into a healthy environment will grow well and give good survival. Weak or diseased postlarvae will not perform well, and potentially can place every shrimp in your facility at risk. Clearly, the choice of hatcheries from whom you buy your postlarvae is a critical decision. One of the main advantages of growing shrimp in indoor recirculating culture systems is the high degree of biosecurity these systems provide. The main biosecurity risk for this type of facility is the health of the postlarval (PL) shrimp brought in from hatcheries. Only high health or specific pathogen free postlarvae should be purchased! Specific pathogen free (SPF) postlarvae have been produced in a breeding facility that employs very strict quarantine and screening procedures to exclude specific viral diseases from the facility. Postlarval shrimp from an SPF facility are guaranteed to be free from specific known pathogens of Litopenaeus vannamei, including Taura Syndrome Virus (TSV), Yellow Head Virus (YHV), Infectious Hypodermal Hematopoietic Necrosis Virus (IHHNV), Monodon Baculovirus (MBV or Whitespot), and Baculovirus Penaei (BP) (Lotz et al., 1995). Only a few SPF nuclear breeding facilities exist worldwide. A high health status (HHS) facility uses broodstock obtained from an SPF facility and maintains strict quarantine and monitoring procedures. However, the list of pathogens monitored is less complete. Not only is it important to find a hatchery that can supply your facility with SPF or high health seedstock, it is also important to find a hatchery that can reliably supply the number of postlarvae you need to keep your facility fully stocked. Empty tanks don’t earn money. For this reason it is important to communicate with the hatchery well in advance to make sure that they know what your postlarvae needs will be, and when. Don’t just assume that because a particular hatchery produces good numbers of high health postlarvae, that you will be able to buy them from that hatchery. Because there is a limited supply of high health postlarvae relative to the demand, the hatchery may already have orders for all the animals they can produce. Anticipate your postlarvae requirements for a year in advance and get your order in as early as possible. It is also a good idea to establish relationships with several hatcheries. That way, if one hatchery has production problems, you will have an alternative source of postlarvae. If you are planning to culture your shrimp in a low salinity water, find out whether or not the hatchery can acclimate the postlarvae to your salinity prior to shipment. If so, what will the charge be for this service? Currently, the Harbor Branch Oceanographic Institution shrimp hatchery is the only one that routinely acclimates postlarvae to low salinities for their clients.


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Low salinity shrimp producers should be able to perform the acclimation of shrimp to low salinities at their own facilities so that they have the flexibility to purchase seedstock from any SPF or high health hatchery. Regardless of the salinity the postlarvae are in when they are shipped by the hatchery, the shipping procedure the hatchery follows is about the same. Postlarvae are chilled to about 18ºC and packed in a double plastic shipping bag filled with 10 liters of clean water. The water in the bags is saturated with pure oxygen and the bags are inflated with pure oxygen before they are sealed. The shipping bags are shipped in styrofoam shipping boxes. The number of postlarvae packed per shipping bag depends upon the age of the postlarvae and the duration of the shipment. Up to 20,000 PL8/10 (eight to ten day old PLs) can be shipped in a 10-liter shipping bag with good survival up to 12 hours. Lower densities are used for longer shipments. Up to 5,000 PL20+ can be shipped in a 10-liter shipping bag with good survivals after 12 hours. Postlarvae acclimated to very low salinities ship as well as postlarvae in full seawater.

Preparations for Receiving Postlarval Shrimp Careful planning and preparation are necessary prior to receiving a shipment of postlarvae. The postlarvae will need to be acclimated to the conditions in your system. The water in the tank the shrimp will be stocked into will most likely differ from the shipping water with respect to temperature, pH, alkalinity, hardness, and possibly salinity. The postlarvae will need to be gradually acclimated to the water conditions in the receiving tank. The acclimation procedure helps minimize stress due to sudden changes in salinity and temperature and is critical for obtaining good survivals. Preparation and forethought are important to the success of the acclimation procedure. Part of the preparation involves communication with the hatchery. Let the hatchery know the salinity of your receiving tank. If possible, shrimp should be adjusted to that salinity before the shipment. If the hatchery is unable to adjust the salinity, then the producer should attempt to prepare the receiving tank to the salinity of the water the postlarvae will arrive in. This eliminates one more variable and makes it possible to accelerate acclimation and stocking if the water quality in the shipping water is very poor. It is also important to get the final shipping count before the shipment arrives. This information will help guide your preparations for receiving the postlarvae. There are many different methods for acclimating shrimp to new water conditions. The choice of how to acclimate the shrimp depends on personal preference and the facilities available.

Acclimation Systems There are two basic approaches to acclimation. The acclimation can be carried out either in the shipping bags or in an acclimation tank. The advantage of performing the acclimation in shipping bags is that this method requires less capital expenditure, as no special tanks or facilities are required, and the acclimation is performed at the tank where the postlarvae are to stocked. The disadvantage of this approach is that if a large number of postlarvae are being acclimated it may be difficult to adequately control the acclimation procedure for the large number of shipping bags being acclimated simultaneously. If the postlarvae have been in the shipping bags for a long time, ammonia may also be a problem in the shipping bags. It


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may be necessary to get the postlarvae out of the shipping water much faster than would otherwise be ideal. The alternative approach is to transfer the postlarvae, shortly after they are received, to an acclimation tank. The water in the acclimation tank is prepared to match the salinity and temperature of the shipping water so the animals can be released quickly into the acclimation tank without stressing them due to a sudden temperature or salinity shock. The density of the shrimp in the acclimation tank is much lower than in the shipping bag, so the acclimation can be performed at a more leisurely pace than is possible when the acclimations are performed in the shipping bag. Acclimation tanks should be used when acclimations are expected to take a long time, such as is the case when postlarvae are being acclimated to low salinities.

Acclimation Equipment Requirements Whichever acclimation method is used, all of the necessary equipment and supplies should be assembled beforehand. The following is a list of equipment that will be required regardless of the acclimation method: √ Thermometer √ Refractometer √ D.O. meter √ pH meter √ Calculator √ Dissecting scope √ Beakers (1-3 liter) √ Oxygen Bottle or Blower √ Manifold for airhoses with as many ports as you have shipping bags √ Airhoses with airstones √ 100-mL beaker √ Hand Counter √ Acclimation Control Report If the acclimation is to be performed right in the shipping boxes, each shipping bag must be provided with an airstone. This will require a manifold with at least one air outlet per shipping bag. In most indoor systems this manifold can be hooked up to the existing aeration system. If this is not possible, an oxygen bottle can be hooked up to the airstone manifold. Water from the receiving tank can be introduced into the shipping bags either by setting up a slow siphon of water from the receiving tank into the shipping bag or by transferring water from the receiving tank to the shipping bag with a beaker or pitcher. It will usually be necessary to drain off water from the shipping bag as new water is added. This is most easily accomplished by siphoning water out of the bag through a 3/8” or 1/4” diameter length of vinyl tubing. Postlarval shrimp can be excluded from the siphon by attaching a medium or course-bubble diffuser to the end of the siphon hose that is in the shipping bag. The flow rate


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out of the incoming and outgoing siphons can be adjusted by means of a roll clamp fitted onto the tubing.

Acclimation Stations If acclimation tanks are used, attention must be given to the design of the acclimation station. The volume of the acclimation tank should be sufficient to accommodate the number of postlarvae that will be acclimated as a single batch. For example, if the standard nursery or growout tank is stocked with 40,000 postlarvae, the acclimation tank should be of sufficient size to accommodate that number of animals. The density should not exceed 500 postlarvae per liter for young postlarvae (PL6/7) or 50 postlarvae per liter for advanced postlarvae (PL15+). If the 40,000 postlarvae in our example are to be held in the acclimation tank until they reach PL15, the minimum volume of the tank should be 800 liters (40,000 PLs ÷ 50 PLs/liter). For extended acclimations it may be desirable to lower the density even further to minimize cannibalism. The acclimation tank(s) should be easily harvestable by draining. Cylindro-conical tanks work well, although tank shape is not critical. The drain should be fitted with an interior screened standpipe (700 - 1000 µm, depending on the size of the PLs) to allow water to be drained out while excluding the shrimp. An exterior standpipe allows for continual water exchange while pulling water from the entire water column rather than just the surface (see

100200

300400500600

700800900

1000 Freshwat er

Salt wat er

Figure 6-1: Acclimation Tank Setup


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Figure 6-1). Ideally the inside wall of the tank should be marked to indicate the depth corresponding to 100 liter volume increments. The tanks should be supplied with both a freshwater and saltwater source to allow the salinity to be adjusted. This water should be filtered to eliminate the majority of the suspended solids. Air should be supplied from a blower. The air is introduced through airstones suspended in the tank. Figure 6-1 shows a suitable setup for the acclimation tank.

Receiving the Postlarvae Immediately upon receiving the shipment, inspect each shipping bag to determine the condition of the postlarvae. The animals should be swimming actively and exhibit good color; the cephalothorax should have a yellowish to pinkish color. Determine whether any of the bags contain a significant number of dead shrimp. The dead shrimp will be opaque and white. Record the number of any shipping bag that appears to have a problem. If most, or all, of the postlarvae in a given shipping bag are dead upon arrival you may be able to get a credit from the hatchery. After moving the shipping boxes to an area adjacent to the receiving tank, randomly select three or four bags from the shipment for conducting a water analysis. Open up these bags and immediately measure the pH and dissolved oxygen. Because the shrimp are shipped at high densities, the dissolved oxygen in the shipping bag will begin to drop rapidly after the bag has been opened. After measuring the pH and dissolved oxygen, place an airstone in each bag and begin to aerate the water. Once aeration has been initiated, measure the temperature and salinity of each of the four shipping bags and remove a small water sample for an ammonia analysis. If you suspect that the number of shrimp in the shipping bag do not match what the hatchery says it packed, you may want perform a postlarvae count on a few bags. Counts should also be performed in order to determine the number of surviving animals if significant numbers of dead shrimp are observed in the bags. Typically, two or three 100-mL samples are taken from the bags and the number of larvae in each sample is counted. When taking the sample, the water in the bag should be vigorously agitated with the hands to homogenize the distribution of postlarvae in the bag. The sample beaker is dipped into the bag during agitation or immediately after and quickly removed. The shrimp in the sample can be counted using a hand counter. It is often easier to count postlarvae if they are killed using a microwave. This immobilizes them and turns them opaque.


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Acclimation Procedures

Acclimation in Shipping Bags There are several methods for performing an acclimation in the shipping bag. As mentioned earlier, a siphon can be set up to introduce water from the receiving tank into the shipping bag. Another siphon is set up to drain off water to keep the bag from filling up and overflowing. The siphon hoses should be fitted with roll clamps that permit control of the flow rates. The flows through the inlet and outlet siphons should be closely matched so that the bag does not overflow or have all of the water drained out of it. Initially the flow rates should be just a trickle to avoid changing the temperature too rapidly. The flow rates can be increased as the temperatures and salinities in shipping bags get close to the levels in the receiving tank. The biggest problem with this method is that it is difficult to control precisely the rate of acclimation. Another method for acclimating in the bags is to simply add receiving tank water to the shipping bag using a beaker or pitcher. Sufficient water is added at to the bag to bring about a 1°C temperature increase. A thermometer is placed in the shipping bag while adding the water to monitor the temperature change. The bag is allowed to adjust to this change for 15 –20 minutes before more water is added for another step temperature change. Water is drained off by siphoning whenever the bag begins to fill up. A third method for acclimating in the shipping bag is to float the shipping bag in the receiving tank. The temperature in the shipping bag gradually equilibrates to the temperature of the receiving tank. This method works well if the temperature differences between the shipping bag and the receiving tank are not very pronounced. However, if the temperature difference is large the temperature in the shipping bag will rise very quickly and stress the shrimp. Another problem with this method is that it does not acclimate the shrimp to water quality parameters other than temperature. While it is true that the bag can be opened and water can be added to the bags, in practice this is difficult to do. The bags tend to be unstable and often will flip over, dumping the larvae prematurely into the tank.

Acclimation in Acclimation Tanks If an acclimation tank is to be used the tank should be adjusted to the average temperature and salinity of the shipping bags that were sampled. Ideally the hatchery will have previously informed you of the salinity the larvae were shipped in, so the acclimation tanks should already be filled with water adjusted to that salinity. If this is the case, the only adjustment that will need to be made to the acclimation tank will be to cool the water down to the temperature of the shipping water. This is done by floating bags of ice in the acclimation tank. If a chiller is available, the water can be cooled by circulating it through the chiller. Once the temperature and salinity of the acclimation tank matches the temperature and salinity in the shipping bags, the shipping bags should be opened and the shrimp transferred to the acclimation tanks. Water exchange in the acclimation tank can either be accomplished by performing batch exchanges or by continuous exchanges.


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Acclimation Schedules The rate of temperature change during acclimation should not exceed 3°C per hour. Sudden temperature changes (more than 3-4°C/hr) thermally shock the shrimp and can be lethal. The rate of change of salinity is dependent upon the initial salinity (see Table 5-1). At high salinities, acclimation can proceed at a faster rate than at low salinities. The major factor determining the acceptable rate of salinity change is the percentage change in salinity per unit time. The rule of thumb is never to exceed a 10% change in salinity per hour (or a 50% change in 5 hours). If possible allow eight hours for each 50% change in salinity. Note, following this rule it will take just as much time to acclimate the larvae from 2 ppt to 1 ppt as it does from 32 ppt to 16 ppt. Gill development should be checked prior to acclimating postlarvae to salinities less than 15 ppt. The larvae should have branched gill filaments before any attempt is made to acclimate them to low salinities (see Chapter 8). The gills play an important roll in shrimp osmoregulation. The osmoregulatory capability of a postlarval shrimp is related to the amount of gill surface available for osmoregulation. Prior to PL10, the gills have very little branching and the shrimp have limited tolerance to low salinities. Branching is usually quite

evident by PL10. By the time they become PL12 the shrimp usually exhibit extensive branching of the gill filaments and can easily be acclimated to salinities as low as 0.5 ppt.

Calculating Water Exchange Requirements Changing the salinity can be accomplished either by lowering the water in the acclimation tank and replacing the water with water from the body of water where the postlarvae will be stocked, or by flowing the water through the tank at a slow rate and maintaining a constant tank volume. In either case, the actual fraction of water replaced to bring about a given change in salinity, or temperature, is determined by 1) the starting salinity (temperature) in the acclimation tank, 2) the salinity (temperature) of the water being added, and 3) the desired final salinity (temperature) at the end of the period. The fraction of new water that must be added to bring about a given salinity change can be calculated from the following formula:

Table 5-1: Recommended Salinity Acclimation Rates

Salinity Change Recommended Time Allowed Recommended PPT Per Hour

32 ppt to 16 ppt 8 hrs 2 ppt/hr 16 ppt to 8 ppt 8 hrs 1 ppt/hr 8 ppt to 4 ppt 8 hrs 0.5 ppt/hr 4 ppt to 2 ppt 8 hrs 0.25 ppt/hr 2 ppt to 1 ppt 8 hrs 0.125 ppt/hr 1 ppt to 0.5 ppt 8 hrs 0.063 ppt/hr


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where, Pnew = salinity (temperature) of the water added Pinitial = initial salinity (temperature) of tank water Pfinal = desired final salinity (temperature) Example 1: Initial Acclimation Tank Temperature = 18oC Water Temperature of New Water Added = 25oC Desired Temperature after addition of New Water = 20oC Fraction of New Water Needed = 1 − 25 − 20

25 − 18

= 1 - .71 = .29 The exchange is provided over the appropriate time period. Example 2: Initial Acclimation Tank Salinity = 30 ppt Salinity of New Water Added = 0 ppt Desired Salinity after addition of New Water = 15 ppt

= 1 – 0.5 = 0.5 In Example 2, a 50% water exchange is required to reduce the salinity by 50% (0.5), from 30 ppt to 15 ppt. This can be accomplished either by dropping the tank volume by 50% and slowly refilling the tank with freshwater or by performing a continuous water exchange at full volume. The continuous exchange procedure offers the advantage that the shrimp are maintained at a constant density. Dropping the volume of the tank by 50% effectively doubles the density of shrimp in the tank, which may increase the rate of cannibalism. Another advantage associated with the continuous exchange procedure is that is possible to adjust the flow rate of freshwater so that 50% of the water in the tank will be exchanged every 8 hours. The same flow rate will work whether one is decreasing the salinity from 32 ppt to 16 ppt, or from 2 ppt to 1 ppt. In other words, as long as the tank volume remains

Fraction of NewWaterNeeded = 1−

Pnew − Pfinal

Pnew − Pinitial

Fraction of New Water Needed = 1 –

0 – 150 – 30


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constant and a constant flow rate can be maintained into the tank, the flow rate only needs to be adjusted one time for the entire acclimation from full strength seawater to 0.5 ppt. An important point to note, however, is that a volume of water greater than 50% of the tank volume will be required to replace 50% of the water originally in the tank at the start of the exchange period. This is because a portion of the water draining out of the tank during the exchange will be new water. There is a very useful formula (Kraul, et al., 1985) that can be used to calculate the flow rate that is required to replace a given fraction of the tank volume during a set period of time: Q = -ln (1-F) x V/T where, Q = flow rate of freshwater (liters/hour) F = the fraction of water actually replaced by new water V = volume of the acclimation tank (liters) T = the time period over which the exchange takes place (hours) The following example illustrates how this formula can be used to calculate the required exchange rate for acclimating to near-freshwater conditions. Example 3: An acclimation tank has a working volume of 1,500 liters. What flow rate will be required to exchange 50% of the water in the tank in an eight hour period? Fraction of water to be exchanged (F) = 0.50 Tank volume (V) = 1500 liters Time required for exchange (T) = 8 hrs The required flow rate, Q (liters/hour), is calculated as follows: Q = – ln ( 1 – 0.5 ) x 1500 liters / 8 hours = 0.693 x 187.5 liters/hour = 130 liters/hr (i.e., 2.17 liters/min)


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Literature Cited

Kraul, S., J. Szyper, and B. Burke. (1985) Practical formulas for computing water exchange rates. Progressive Fish-Culturist 47(1): 69-70.

Lotz, J.M., C.L. Browdy, W.H. Carr, P.F. Frelier, and D.L. Lightner. (1995) USMSFP suggested procedures and guidelines for assuring the specific pathogen status of shrimp broodstock and seed. In: C.L. Browdy and J.S. Hopkins, editors. Swimming through troubled water, Proceedings of the special session on shrimp farming, Aquaculture ’95. World Aquaculture Society, Baton Rouge, Louisiana, USA.

Chapter 7 – Nutrition and Feeding of Litopenaeus vannamei

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Chapter 7 Nutrition and Feeding of Litopenaeus vannamei in

Intensive Culture Systems by

Peter Van Wyk

Elements of a good feeding program Feeding is one of the most critical aspects of shrimp husbandry. A good feeding program is necessary for shrimp to grow at their maximum potential. Feed represents one of the most significant operating expenses for most semi-intensive and intensive aquaculture operations. Often feed costs represent the single highest operating expense (50%) for an aquaculture enterprise. A well-managed feeding program insures that the feed is utilized efficiently. There are many things that a producer must do to guarantee a successful feeding program:

1) Feed a high quality diet that is formulated to meet the nutritional requirements of the shrimp and is manufactured from high quality, digestible ingredients;

2) Use only prepared feeds that are attractive, palatable and appropriate in size for the shrimp;

3) Maintain feed quality by utilizing proper feed storage and handling procedures;

4) Present the feed in quantities and frequencies that are appropriate for the number and size of the shrimp in the population being fed;

5) Distribute the feed evenly over the culture area to ensure that all the shrimp have equal access to the feed.

6) Make timely adjustments to the feeding regime based on water quality and the shrimp appetite.

Nutritional Requirements The nutrients required by cultured species can be broadly classified as proteins, carbohydrates, lipids, vitamins and minerals. The optimum levels of these nutrients vary from one species to the next.

Protein Requirements Protein makes up 65 to 70% of the dry weight of a shrimp, and is a major component of muscle. Protein in shrimp diet is the source of amino acids, which serve as building blocks for the shrimp’s own proteins. There are 20 different amino acids, but only 10 of these are considered to be essential in the diet. The rest can be synthesized by the shrimp from the


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10 essential amino acids. Strictly speaking, shrimp do not have a minimum protein requirement. Rather, they have minimum requirements for each of the ten essential amino acids (Table 7-1). Table 7-1: Recommended amino acid levels in commercial shrimp feeds, on an as-fed

basis (after Akiyama and Tan, 1991).

Percent of Feed Amino Acid

Percent of Protein (%) 36% Protein 38% Protein 40% Protein 45% Protein

Arginine 5.8 2.09 2.20 2.32 2.61 Histidine 2.1 0.76 0.80 0.84 0.95 Isoleucine 3.5 1.26 1.33 1.40 1.58 Leucine 5.4 1.94 2.05 2.16 2.43 Lysine 5.3 1.91 2.01 2.12 2.39 Methionine 2.4 0.86 0.91 0.96 1.08 Phenylalanine 4.0 1.44 1.52 1.60 1.80 Threonine 3.6 1.30 1.37 1.44 1.62 Tryptophan 0.8 0.29 0.30 0.32 0.36 Valine 4.0 1.44 1.52 1.60 1.80

The amino acid requirements for shrimp have not been well defined because shrimp do not efficiently utilize crystalline amino acids from the purified diets used to study amino acid requirements. As a general rule, however, the amino acid requirements of a species closely mirror the amino acid composition of their muscle tissue (Lim and Persyn, 1989). The amino acid composition of shrimp feeds is largely based on the amino acid composition of shrimp muscle (Akiyama, et al., 1991). Feed formulators mix and match different sources of protein, each with different amino acid profiles, so that the diet meets the minimum requirement for all 10 essential amino acids. The formulator must also take into account the digestibility of each of the feed ingredients and the availability of the amino acids. Fishmeal is generally considered to be the highest quality protein source because the amino acid composition of fishmeal closely matches that of shrimp. For commercial growout diets, krill and Artemia meal are better than fishmeal, but they are more expensive. However, they are used in larval and maturation diets. Most commercial shrimp feeds formulated for intensive culture systems contain between 35 and 50% protein. If the level of protein in the feed is too low, growth rates will be reduced. Severe protein deficiencies may actually lead to weight loss if the proteins in shrimp muscle tissue are used to maintain other vital functions. Excess protein in the diet may also inhibit growth (Lim and Persyn, 1989). The excess protein will be metabolized by the shrimp as a source of energy, and nitrogen will be excreted as ammonia. Protein requirements are fairly high for postlarvae and small juveniles, but decline as the shrimp grow larger. Table 7-2 gives the recommended protein levels for different sizes of shrimp in high-intensity culture systems.


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Table 7-2: Recommended protein levels for different sizes of shrimp in high-intensity culture systems.

Shrimp Size (g) Recommended Feed Protein Level

0.002 – 0.25 50 % 0.25 – 1.0 45% 1.0 – 3.0 40%

>3.0 35%

Lipids Lipids, or fats, are a group of organic compounds that include free fatty acids, phospholipids, triglycerides, oils, waxes and sterols. Lipids function as an important energy source for shrimp. In addition to their value as an energy source, lipids serve as a source for essential fatty acids. Fatty acids are chain-like organic molecules with many repeating units. Each “link” in the chain contains a carbon atom. Fatty acids differ in chain length and in the degree of saturation (number of double bonds and hydrogen atoms). A highly unsaturated fatty acid will have many double bonds, and few hydrogen atoms. These fatty acids appear to be important in the structure of cellular membranes. Four fatty acids are considered essential fatty acids in shrimp, because they are required in the diet and cannot be synthesized from other compounds. The essential fatty acids are: linoleic acid (18:2n6), linolenic (18:3n3), eicosapentaenoic acid (20:5n3), and decosahexaenoic acid (22:6n3) (Kanazawa an Teshima, 1981). Table 7-3 gives the recommended levels essential fatty acids in shrimp diets. Table 7-3: Recommended fatty acid levels in commercial shrimp feeds (after Akiyama, et

al. 1991)

Fatty Acid Percent of Feed Linoleic Acid (18:2n6) 0.4 Linolenic Acid (18:3n3) 0.3

Eicosapentaenoic Acid (20:5n3) 0.4 Decosahexaenoic Acid (22:6n3) 0.4

Phospholipids are compounds consisting of glycerol, fatty acids and phosphoric acid. They are important components of cell membranes and play an important role in lipid metabolism. Sterols are required by crustaceans as a precursor for maturation and molting. Lipids are often added to fish diets in the form of fish oil, soybean and sometimes squid oil. Table 7-4 gives the recommended lipid levels in shrimp diets for high-intensity culture


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systems as a function of shrimp size. The recommended total lipid level in the diet decreases with increasing shrimp size. Table 7.4: Recommended lipid levels for shrimp diets used in intensive culture.

Shrimp Size (g) Lipid Level (%) 0.002 – 0.2 15 % 0.2 – 1.0 9 % 1.0 – 3.0 7.5 %

>3.0 6.5 %

Carbohydrates Carbohydrates serve as an inexpensive energy source in shrimp diets. Starches, sugars and fiber are the main forms of carbohydrates. Organisms differ in their ability to use carbohydrates as an energy source. Carnivores, whose diets contain high levels of protein, tend to use protein as an energy source and often are unable to metabolize carbohydrates effectively. Omnivorous and herbivorous fish and shrimp utilize carbohydrates effectively. While no absolute carbohydrate requirement has been found for shrimp, carbohydrates in the diet can have a “protein sparing” effect for species that are able to utilize carbohydrates efficiently. That is, if carbohydrates are present in sufficient quantity in the diet, the protein requirement is reduced.

Vitamins Vitamins are organic compounds that are required in the diet in relatively small quantities for normal growth and development. Vitamins are classified as either water soluble or fat soluble. The B-complex vitamins are water soluble and are required in relatively small quantities. These vitamins function primarily as coenzymes in various metabolic processes. Three water-soluble vitamins are required in larger quantities and have functions other than coenzymes. These are Vitamin C, inositol, and choline. Vitamin C and choline are often added separately, as these vitamins are required in relatively large quantities. The fat-soluble vitamins are Vitamins A, D, E and K. Fish and shrimp diets usually are fortified with a vitamin premix that contains all of the 16 essential vitamins. The vitamin requirements for marine shrimp are affected by many different factors, including shrimp size, age, growth rates and environmental factors (Akiyama, et al., 1991). Young juvenile shrimp may require 50% higher vitamin levels in their diets than adult shrimp. Shrimp cultured in intensive culture systems typically require much higher vitamin concentrations than are required by shrimp grown at low densities. Vitamin deficiencies frequently result in symptoms, such as physical deformities, blindness, erratic swimming behavior, lethargy and poor growth. The physical symptoms displayed differ, depending on which vitamin is deficient in the diet. Vitamin C deficiency is associated with “Black


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Death” disease, characterized by melanized lesions of subcuticular tissues. Typically, feed manufacturers overfortify shrimp diets with vitamins. This is done for several reasons. Detailed information about shrimp vitamin requirements is lacking. Overfortification is cheap insurance against crop losses due vitamin deficiencies. In addition, many vitamins are unstable compounds that are easily destroyed during manufacture and feed storage. Vitamin C, in particular, has a very short half-life at room temperature. Stable forms of Vitamin C, such as Stay-C, have a much longer shelf life than the non-stabilized form of Vitamin C. Because shrimp are slow feeders, the feed may sit in the water for several hours before it is consumed. Significant quantities of water-soluble vitamins may leach into the surrounding water before the feed is eaten.

Minerals Minerals are inorganic elements required for various metabolic processes. Minerals required in large quantities are called major minerals. These include calcium, phosphorus, magnesium, sodium, potassium, chloride and sulfur. Calcium is required for exoskeleton formation, muscle contraction and osmoregulation. Shrimp are able to absorb calcium directly from the water, and shrimp living in seawater do not need calcium supplements in the diet (Davis, 1991). However, diets for shrimp cultured in near-freshwater systems should contain up to 2.5% calcium. Higher levels of calcium should be avoided because in high concentrations calcium appears to interfere with the bioavailability of phosphorus (Davis, 1990). Phosphorus is required for exoskeleton formation and is an essential component of phospholipids, nucleic acids, ATP, and many metabolic intermediates and coenzymes. Davis (1990) demonstrated that the phosphorus requirement for Litopenaeus vannamei was dependent upon the calcium content of the diet, and that in the absence of calcium, 0.34% phosphorus was sufficient for normal growth and development. Shrimp diets often contain up to 1% dietary phosphorus. Unlike calcium, phosphorus is not absorbed in significant quantities from the water and must be supplied in the feed (Davis, 1991). Calcium and phosphorus are often added to the diet in the form of dicalcium phosphate. Some minerals are required in minute quantities and are called trace minerals. Trace minerals include iron, iodine, manganese, copper, cobalt, zinc, selenium, molybdenum, fluorine, aluminum, nickel, vanadium, silicon, tin and chromium. The trace minerals are generally added to the diet in a mineral premix. Sometimes vitamins and minerals are combined into a single vitamin-mineral premix.

Shrimp Feeds

Formulated Diets There is a saying: “Man cannot live on bread alone.” The same is true of shrimp. A diet consisting of a single feed ingredient is not likely to be able to provide all of the nutrients required for normal growth and development. This is why aquaculturists usually feed their


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animals a formulated diet. Formulated diets are mixtures of different feed ingredients mixed in set proportions to provide the desired quantities of nutrients. A wide variety of different feed ingredients are used in commercial shrimp feed formulations. Common ingredients used in commercial shrimp feeds include soybean meal, fish meal, squid meal, shrimp head meal (cooked), wheat flour, wheat middlings, lecithin, cholesterol, starch, dicalcium phosphate, vitamin and mineral mixes, and binders. Formulated diets may be either supplemental or complete. Feeds that are applied to supplement natural food sources are called supplemental diets. Shrimp grown in ponds at very low densities may be able to survive and grow without any supplemental feed input to the pond. Under these conditions, naturally occurring plants and animals serve as the food source for the culture species. At higher densities, natural productivity is insufficient to support the nutritional requirements of the culture species, so prepared feeds must be included to supplement the nutrition obtained from natural food sources. Supplemental diets rarely meet the nutritional needs of the culture species, but are adequate when natural foods are available. Where natural foods are not available, such as in tank-based culture systems and high-density pond culture systems, nutritionally complete diets must be provided. Complete diets contain all of the essential nutrients in amounts sufficient for normal growth and development of the cultured organism. It is also necessary that these nutrients must be available in a form that is digestible. Complete diets typically have higher protein, vitamin, and mineral levels than supplemental diets. The majority of commercial shrimp feeds available today are considered to be supplemental feeds. Shrimp nutrition is very complex, and the current state of knowledge about shrimp nutritional requirements is incomplete. While some very good shrimp diets are available commercially, it is doubtful whether any of these can be considered a true complete feed. Growth rates of tank-reared shrimp that rely on prepared diets for 100% of their nutritional needs do not match the growth rates that are frequently observed in productive pond environments. However, the shrimp maintained on these diets develop normally and are generally healthy.

Feed Processing Shrimp are benthic feeders, so shrimp feeds must be processed into a sinking pellet. Most shrimp feeds are manufactured either using a steam pelleting process or an extrusion process. Steam pelleting uses a combination of moisture, heat and pressure to form finely ground feed ingredients into a dense, tightly bound pellet

Figure 7-1: Pelleted Shrimp Feed


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(Figure 7-1). The feed ingredients are finely ground and mixed together in the proper proportions. Moisture is added and the ingredients are thoroughly blended into a pasty mash. The mash is fed into a pelleting mill that uses an auger to compress the mash. Steam is introduced into the pelleting chamber, which causes the starch in the feed mixture to gelatinize and helps bind the ingredients together. Binders are often used to complement the binding provided by gelatinized starch. The feed mixture is then forced through holes in a die plate located at the end of the chamber. The diameter of the pellet is determined by the diameter of the holes in the die plate. Extruded feeds are formed using a similar process, but much higher temperature and pressure is generated within the barrel of the extruder. This results in more complete gelatinization of the starches contained in the feed ingredients, so additional binders are not required. Extruded feeds often are less dense than steam pelleted feeds because rapid release of the steam from the feed pellets after they pass through the die plate causes the pellets to expand. The extrusion process is often used to create floating pellets, which are popular for feeding fish. The extrusion process is typically carried out at a slightly lower temperature using formulations with less starch to obtain a sinking pellet.

Pellet Stability Good water stability is important in the preparation of shrimp feeds, regardless of pelleting process,. Shrimp are slow feeders and a pellet may sit in the water up to four or five hours before it is eaten. To evaluate the feed stability in water, place several pellets in a beaker of water. The pellets should remain largely intact for up to four hours. Periodic gentle swirling of the water in the beaker can help simulate the effects of water movement on pellet stability. Feeds with poor water stability are not efficiently utilized by the shrimp and will foul the water.

Pellet Diameter The required diameter of the feed pellets varies depending on shrimp size. Postlarvae and young juveniles are too small to eat a formed pellet. Feeds for these shrimp are made by grinding a pelleted feed and passing the ground feed through a series of sieves to obtain feed particles of a uniform diameter. Because pellet integrity is not as critical an issue for ground diets, postlarval and juvenile feeds are frequently manufactured using a cold pelleting process. Cold pelleting is less destructive to the vitamins in the feed. Shrimp that weigh less than one gram are typically fed ground feeds. Larger shrimp are able to eat pelleted diets. Table 7-5 lists recommended particle or pellet sizes for shrimp of different sizes.


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Table 7-5: Recommended pellet diameters for shrimp of different sizes.

Shrimp Size (g) Pellet Diameter 0.002 – 0.02 400 – 600 µm 0.02 – 0.08 600 – 850 µm 0.08 – 0.25 850 – 1200 µm 0.25 – 1.0 1200 – 1800 µm 1.0 – 2.5 3/32” pellet (2.4 mm)

>2.5 1/8” pellet (3.2 mm) When making the transition from one pellet size to the next, it is a good idea to feed a mixture of the two sizes of pellets for 5-7 days to allow the shrimp time to get used to the larger pellet size before discontinuing the smaller pellets.

Feed Application

Feeding Rates It is critical that feed be applied in the correct amounts and at the correct times throughout the culture period. Feed rates must be constantly adjusted to account for shrimp growth, mortality and appetite. If the feeding rate is too low, the shrimp will not grow well, and overall production will suffer. Underfeeding may also result in cannibalism, especially at high densities. Overfeeding also causes problems. Besides being wasteful, uneaten feed can contribute to deterioration of water quality in the culture system. The organic material in the feed becomes a substrate for heterotrophic bacteria, which metabolize the protein in the feed and give off ammonia. Elevated ammonia levels in the water suppress shrimp growth and increase the shrimp’s susceptibility to disease. The oxygen demand of these bacteria can lead to low dissolved oxygen levels in the system, inhibiting shrimp growth. Some heterotrophic bacteria release substances into the water, which can cause the shrimp to be off-flavor. Overfeeding causes overall feed conversion values to increase, since some of the leftover feed, and inefficient assimilation of the feed that is consumed. There are many factors that affect the amount of feed the shrimp will eat. Feed consumption varies with feed type, shrimp size, water temperature, stocking density, weather, water quality and health. Shrimp culturists must take all of these factors into account in order to maximize the efficiency of the feeding program. Temperature has an especially pronounced effect on feed consumption and growth. For L. vannamei, feed consumption is optimal when water temperatures are between 27°C and 31°C (81°F and 87°F). Feed consumption decreases both above and below these


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temperatures. Feed consumption may be reduced by 50% when the water temperature drops to 24°C (72°F), and ceases altogether when water temperature drops below 20°C (68°F).

Feed Tables Feed tables have been developed that give a recommended feed rate, expressed as percent of bodyweight per day (% BW/Day), for animals of different sizes. As a general rule, small animals are fed at a higher percentage of their bodyweight per day than are large animals. This is because small animals will generally have a higher metabolic rate than large animals. Table 7-6 shows a typical feed table for L. vannamei cultured in high-density tank systems.

To calculate the daily feed allowance for a population of shrimp, multiply the total biomass of the shrimp population by the recommended feed rate from the feed table:

Daily Feed Allowance = Total Biomass x % BW/Day

100 % (7.1)

The total biomass of the shrimp population is calculated by multiplying the estimated number of shrimp in the population by the average weight of the shrimp: Total Biomass = Total Number of Shrimp in the Population x Average Weight (7.2) Accurate information about the average weight and total number of shrimp in the population is required to correctly calculate the daily feed allowance. The shrimp population should be sampled at least every other week to determine the average shrimp

Table 7-6: Feed Table for High-Intensity Tank Production of Litopenaeus vannamei.

Average Shrimp Wt. (g)

Feed Rate (% BW/day)

<.1 35 – 25 0.1 - 0.24 25 – 20

0.25 – 0.49 20 – 15 0.5 – 0.9 15 – 11 1.0 – 1.9 11 - 8 2.0 – 2.9 8 – 7 3.0 – 3.9 7 – 6 4.0 – 4.9 6 – 5.5 5.0 – 5.9 5.5 – 5.0 6.0 – 6.9 5.0 – 4.5 7.0 – 7.9 4.5 – 4.25 8.0 – 8.9 4.25 – 4.0 9.0 – 9.9 4.0 – 3.75

10.0 – 10.9 3.75 – 3.5 11.0 – 11.9 3.5 – 3.0 12.0 – 12.9 3.25 – 3.0 13.0 – 13.9 3.0 – 2.75 14.0 – 14.9 2.75 – 2.5 15.0 – 15.9 2.5 – 2.3 16.0 – 16.9 2.3 – 2.1 17.0 – 17.9 2.1 – 2. 18.0 – 18.9 2.0 – 1.9 19.0 – 19.9 1.9 – 1.8 20.0 – 20.9 1.8 – 1.7


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weight. A minimum sample size of at least 30 shrimp should be used to calculate the average weight of the population. If there is large variation in size within the population, the sample size should be increased to 60 shrimp per sample. Best results are obtained by weighing each shrimp in the sample individually after blotting off excess water with a paper towel. Estimation of the total number of shrimp in the population is more difficult. The number of shrimp in the population at any given time is equal to the number of shrimp stocked multiplied by the fraction of shrimp still surviving at that time: No. shrimp at time t =Number of shrimp stocked x Fraction surviving to time t (7.3) Survival rates are very difficult to estimate. In tank culture systems, dead shrimp can often be removed from the tank and counted. Although observed mortality is very helpful in estimating survival in a tank, population estimates based on observed mortality rates are nearly always overestimates of the true number of shrimp. This is because it is very difficult to account for all of the mortality, especially for very small shrimp. Some of the shrimp may be consumed by other shrimp, while other mortalities may simply escape notice. Standardized survival curves based on historical average survival rates are often used to estimate shrimp numbers in a population. Survival curves may be linear (assuming a constant mortality rate), or may have varying slopes over different portions of the growout cycle. Often curves are constructed to reflect heavier mortality rates during the nursery phase than in subsequent phases of the growout. Even if standard survival curves are used to estimate the population of a culture tank, adjustments will be necessary in cases where survival is unusually high or low. It is important to note that feeding tables only provide a guide to the amount the shrimp will eat under optimal conditions of temperature, density, water quality, etc. Following these feed tables religiously will invariably lead to overfeeding when conditions are sub-optimal. As an example, following a low dissolved oxygen condition, feeding activity is typically depressed. If the feed rates recommended by the feed tables are followed, much of the feed will go uneaten. The uneaten feed may even exacerbate the dissolved oxygen problem. High ammonia levels will also suppress shrimp appetites, and overfeeding will contribute to even higher levels ammonia in the tank. The feed table should serve only as a guideline for determining the daily feed allowance.


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Demand-based Feeding Demand-based feeding is an alternative to using a feed table. With this method, the feed allowance is adjusted up or down depending on the feeding activity of the shrimp. At each feeding, the technician estimates the amount of feed that the shrimp can consume during the time interval between feedings. If a significant amount of feed remains from the previous feeding, the amount fed at the next feeding should be reduced by at least 10 percent. If all of the feed has been consumed between feedings, the feed amount can be increased by 10 percent. This approach to feeding ensures that the feed rates will be appropriate for the conditions in the tank. In clear water, it is easy to see how much of the feed is being eaten. However, if a dense algal bloom develops in a tank, it may be difficult to see uneaten feed on the bottom of the tank. One way to determine if the shrimp are eating all of the food is to place feed trays in the raceway. The trays can be lifted from the water to see if the feed has been eaten.

Feeding Frequency The number of feedings per day is determined by pellet stability and by the rate at which the feed is consumed, digested and metabolized by the shrimp. Dividing the daily feed ration into multiple feedings, spaced several hours apart, improves feed conversion ratios and growth rates. In addition, feeding only what the shrimp can consume in 3 or 4 hours reduces losses of nutrients due to leaching. It is not clear whether or not there is any benefit derived from feeding the shrimp throughout the 24-hour period. While L. vannamei are active at night, they may not be actively feeding during this time period. Robertson et al. (1993) reported that L. vannamei receiving four feedings a day during daylight hours performed as well as, or better than, shrimp fed around the clock. Small shrimp metabolize their food faster than large shrimp, and generally require more feedings per day. Postlarval shrimp require frequent feedings because they have very high metabolic rates, but are not able to store much feed in their guts. Ideally, postlarvae should be fed every 2-3 hours. Longer intervals between feedings may result in heavy losses due

Figure 7-2: Weighing out shrimp feed


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to cannibalism. Automatic feeders, which dispense small amounts of feed at programmed intervals or on a continuous basis, can be used to make sure the shrimp are fed in a timely manner. As the shrimp grow the feeding frequency can be decreased. Four feedings spaced three hours apart during daylight hours works well for juveniles larger than 1 gram in size.

Feed Distribution Feed may be distributed to shrimp either by hand or by automatic feeders (Figure 7-3). The feeding method used is a function of the culture system, requirements of the culture organism and the preferences of the aquaculturist. Hand feeding is frequently practiced when feeding animals held in tanks or raceways, especially when the animals are small. Hand feeding allows the technician to modify the feed distribution in accordance with the feeding response. Hand feeding may be impractical when a large number of tanks must be fed because it is very time-consuming. Automatic feeders dispense a given volume of feed on a timed basis. Automatic feeders operate by a wide variety of mechanisms. Automatic feeders for larvae, fry, or small juveniles often consist of a plate or belt onto which feed is loaded (Figure 7-3). The plate or belt is rotated in a manner that causes feed to fall off into the water at a steady rate throughout the day. Scatter feeders distribute feed from a hopper suspended over the water at timed intervals. Scatter feeders have a plate at the bottom of the hopper with vanes extending radially from the center of the plate. At timed intervals, feed is released from the hopper and the plate spins around, casting feed in a 360° arc around the feeder. Scatter feeders can be modified for raceways to throw the feed out in a single 45-degree direction. Large-scale operations with multiple raceways often use a conveyer system to load the hoppers. These consist of tubes through which feed is moved by a variety of means. Some use pneumatic blowers to move the feed, while others use augers or a similar mechanism.

Figure 7-3: Automatic Belt Feeder


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Feed Conversion Ratios An important measure of how well the feed is utilized by the animals in the culture system is the Feed Conversion Ratio or FCR. The Feed Conversion Ratio measures the number of pounds of feed required to produce a pound of shrimp. The following formula is used to calculate feed conversion:

Feed Conversion Ratio =

Total Weight of Feed AppliedTotalWeightGained

(7.4)

The lower the FCR value, the more efficiently the feed is being utilized. Generally speaking, FCR values less than 2.0 are considered good. High FCR values may result from nutritionally deficient feeds, overfeeding, poor water quality or crowding. Whenever high FCR values are obtained, it is important to take a critical look at the feeding program and production process to try to identify the causes.

Feed Storage Feed storage is an important and often neglected aspect of the feed management program. Aquaculture feeds are highly perishable. Inadequate storing and handling of feed can lead to nutrient losses, rancidity, mold growth and rodent infestations. Many of the vitamins in the feed are unstable at high temperatures and significant losses will occur if the feed is stored at high temperatures or exposed to ultraviolet light. Vitamin C (ascorbic acid) is particularly prone to degradation. At room temperatures, ascorbic acid has a half-life of less than a month. Two-month old feed will have a small fraction of the amount of ascorbic acid that was originally added to the feed. Stabilized Vitamin C (Stay C) is much more stable, but still is degraded over time. Feeds that are high in lipids often will become rancid when stored in warm, oxidative environments. Rancid feeds are unpalatable to the shrimp and are deficient in Vitamin E. Reduced growth rates are commonly seen in shrimp receiving rancid feeds. Rancid feeds have a very distinctive odor. The technicians should be sure to smell a handful of feed from each feed sack that is opened before using the feed to determine if the feed has gone rancid. Molds will often develop on feeds that are stored in humid or moist environments. Molds produce toxins that can be very damaging to the shrimp. Molds of certain species in the genus Aspergillus produce aflatoxins, which can cause severe liver damage to the shrimp. Given the correct environment, molds grow quickly on the feed. Feed does not have to be old to be moldy. Occasionally feed will already have mold growing on it when it arrives from the feed mill. This can happen when the feed is placed into the feed bag while it is still hot, or if the feed not been dried sufficiently. When hot feed cools, moisture condenses on the feed. The dark, humid environment inside the feed sac is a perfect incubator for


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mold growth. When receiving feed, inspect a few bags to make sure that the no mold is present. If mold is detected, the shipment should be rejected. Before using the feed from any feed bag, inspect the feed for mold. If even a small amount of feed in a bag appears to be moldy, discard the entire bag. Significant quantities of mold toxins will be present throughout the bag. Unless the feed is stored in a sealed room with narrow clearances around the door jam, the feed will soon be infested with rodents. Besides eating the feed, rodents will urinate and defecate in the feed sacks, ruining the feed. To avoid the kinds of problems described above, feed should be stored in a dry, cool, rodent-resistant storage shed. If possible the feed storage room should be air-conditioned. The air conditioner will help maintain a low humidity, as well as a cool storage environment. A “first-in, first-out” inventory management strategy should be employed to make sure that feed gets used before its expiration date. Ideally, feed should be used within one month of purchase. If storage conditions are ideal, feed can be used for up to three months, although the quality of the older feed will not match that of feed less than one month old.

Sources of Shrimp Feeds The following is a list of U.S. shrimp feed manufacturers:

Bonney, Laramore, and Hopkins, Inc. 5600 Highway U.S. 1 North Ft. Pierce, FL 34946 Tel: (561) 971-2925 Burris Mill and Feed , Inc. 1012 Pearl Street Franklinton, LA 70438 Tel. (504) 839-3400 Fax: (504) 839-3404 Cargill Nutrena Feeds 801 South Poplar Street Florence, AL 35630 Tel. (205) 764-1331 Ralston Purina International Checkerboard Square –11T St. Louis, MO 63164 Tel. (314) 982-2402 Fax. (314) 982-1613

Rangen Feeds 115 13th Ave. S. Buhl, ID 83316 Tel. (800) 657-6446 Fax (208) 543-4698 Rangen Feeds Angleton, TX Tel. (979) 849-6757 Star Milling Company PO Box 728 Perris, CA 92370 Tel. (909) 657-3143 Fax (909) 943-2400 Zeigler Brothers, Inc. PO Box 95 Gardners, PA 17324-0095 Tel. (800) 424-2033 Fax (717) 677-6826


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Literature Cited Akiyama, D. M., W.G. Dominy, and A.L. Lawrence. 1991. Penaeid shrimp nutrition for the

commercial feed industry: Revised. Pages 80-98 in D.M. Akiyama and R.K.H. Tan, editors. Proceedings of the Aquaculture and Feed Processing and Nutrition Workshop. Singapore, Republic of Singapore.

Davis, D.A. 1990. Dietary mineral requirements of Penaeus vannamei: evaluation of the

essentiality for thirteen minerals and the requirements for calcium, phosphorus, copper, iron, zinc, and selenium. Ph.D. Dissertation, Texas A&M University, College Station, TX, USA.

Davis, D.A. and D.M. Gatlin III. 1991. Dietary mineral requirements of fish and shrimp.

Pages 49-67 in D.M. Akiyama and R.K.H. Tan, editors. Proceedings of the Aquaculture and Feed Processing and Nutrition Workshop. Singapore, Republic of Singapore.

Kanazawa, A., and S. Teshima. 1981. Essential amino acids of the prawn. Bul. Jap. Soc.

Sci. Fish. 43(9): 1111-1114. Lim, C. and A. Persyn. 1989. Practical Feeding – Penaeid Shrimps. In, Editor, Tom

Lovell. Nutrition and Feeding of Fish. Van Nostrand Reinhold. New York. pp. 205-222.

Robertson, L., A.L. Lawrence, and F.L. Castille. 1993. Effect of feeding frequency and

feeding time on growth of Penaeus vannamei (Boone). Aquaculture and Fisheries Management 24: 1-6.


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Chapter 8 – Water Quality Requirements and Management

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Chapter 8 Water Quality Requirements and Management

by Peter Van Wyk and John Scarpa


Introduction Water used for aquaculture is more than just H2O. Water contains many ionic and non-ionic elements that make up what is termed "water quality". The concentration of dissolved inorganic ions, dissolved gases, suspended solids, dissolved organic compounds, and microorganisms determine the suitability of the water for aquaculture. Simply put, a supply of good water is essential in aquaculture and good water meets the specific environmental needs of the organism to be cultured. Why is water quality so important? Water is the environment in which aquatic organisms live. Their bodies and gills are in constant contact with what is dissolved and suspended in the water. Therefore, water quality directly affects the health and growth of the cultured organism. Poor water quality leads to stress, disease and, ultimately, death. Water quality is not a fixed characteristic of the water. The quality of the water is very dynamic, changing over time as a result of environmental factors, and biological processes. Initially, water quality is initially related to the source of the water. For example, if the water comes from a well, it may be low in dissolved oxygen and high in ammonia, iron, hydrogen sulfide, carbon dioxide, or a combination of these. Depending on mineral composition of the region, the source water may be hard and alkaline, or soft and acidic. After the water is in a culture system, its quality may be altered by biological processes such as photosynthesis, respiration, and excretion of metabolic wastes, as well as by physical processes such as temperature and wind. Water quality may even be altered by management strategies, such as overfeeding that leads to suspended solids and eutrophication of the system. To be successful, an aquaculturist must regularly monitor the water quality variables which are critical to the health of the organisms being cultured and understand the factors which affect these variables. Water quality requirements differ for different species and sometimes for different stages in the life cycle of the same species. For example, the salinity requirements of the Pacific white shrimp, Litopenaeus vannamei, are a function of developmental stage. Adult shrimp mature and spawn in seawater with a salinity of at least 28 ppt. The early larval stages also require seawater. However, postlarval shrimp migrate into estuaries, an environment which may experience extreme fluctuations in salinity. By the time the shrimp become a PL12 they can successfully acclimate to near freshwater conditions. Frequently early developmental stages of a culture organism are more susceptible to certain toxic compounds than are older animals. For example, the LC50 concentration of nitrite for postlarvae is about one-tenth of the LC50 concentration of nitrite for sub-adults. The aquaculturist must understand the water quality


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requirements of each stage in the lifecycle of the culture species, and make sure that the culture system provides water suitable for the stages being cultured.

Water Quality Testing During Site Selection The chemical characteristics of the source water is one of the most important considerations in choosing a site for an aquaculture enterprise. Some water quality parameters may be easily adjusted to bring it within acceptable limits. For example, concentrations of toxic dissolved gases, such as carbon dioxide and hydrogen sulfide, can be economically reduced to safe levels by passing the water through a degassing tower. High ammonia levels in the source water can usually be reduced to safe levels by passing the water through a biofilter. Other parameters can be modified, but at a significant cost. Salinity is a good example. While it is possible to add prepared mixes of ocean salts to freshwater to make seawater, it is not usually economical to do this on a large scale. However, if the salinity of the source water is not too far out of range, chemical additions may not be prohibitively expensive. Some water quality parameters cannot be economically modified, so the source water must be within acceptable limits. High concentrations of toxic compounds such as pesticides, herbicides, and heavy metals disqualify a site for aquaculture. A full range of water quality parameters should be tested before deciding to build an aquaculture facility on a given sit. Table 8-1 gives a list of water quality parameters that should be tested and the acceptable limits for shrimp culture. Table 8-1: Recommended Range of Water Quality Parameters for Shrimp Culture

Water Quality Parameter Recommended Range Temperature 28 - 32 ºC

Dissolved Oxygen 5.0 - 9.0 ppm Carbon Dioxide ≤ 20 ppm

pH 7.0 - 8.3 Salinity 0.5 - 35 ppt Chloride ≥ 300 ppm Sodium ≥ 200 ppm

Total Hardness (as CaCO3) ≥ 150 ppm Calcium Hardness (as CaCO3) ≥ 100 ppm

Magnesium Hardness (as CaCO3) ≥ 50 ppm Total Alkalinity (as CaCO3) ≥ 100 ppm Unionized Ammonia (NH3) ≤ 0.03 ppm

Nitrite (NO2–) ≤ 1 ppm

Nitrate (NO3=) ≤ 60 ppm

Total Iron ≤ 1.0 ppm Hydrogen Sulfide (H2S) ≤ 2 ppb

Chlorine ≤ 10 ppb Cadmium ≤ 10 ppb Chromium ≤ 100 ppb

Copper ≤ 25 ppb Lead ≤ 100 ppb

Mercury ≤ 0.1 ppb Zinc ≤ 100 ppb


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The water from a prospective site should also be analyzed for pesticides. Shrimp, like insects, are arthropods, and are very susceptible to insecticides. To the author’s knowledge, the lethal limits of many of these pesticides have not been determined for penaeid shrimp. However these pesticides are toxic to most aquatic organisms. Table 8-2 lists the range of LC50 levels for a variety of other aquatic organisms, as well as the safe levels recommended by the U.S. Environmental Protection Agency. Table 8-2: Toxicity of Pesticides to Aquatic Organisms and Safe Levels Recommended by

the U.S. Environmental Protection Agency

Pesticide Range of 96-hr LC50 (ppb)

Safe Level* (ppb)

Aldrin/Dieldrin 0.2 - 16 0.003 BHC 0.17 – 240 4

Chlordane 5 – 3000 0.01 DDT 0.24 - 22 0.001

Endrin 0.13 - 12 0.004 Heptachlor 0.10 - 230 0.001 Toxaphene 1 - 6 0.005

*Recommended safe levels by the U.S. Environmental Protection Agency In addition to testing the chemical composition of the water, bioassays may be used to determine if the culture organisms can live in the water. Bioassays are tests that use living organisms as indicators. To test the suitabilitity of water for aquaculture, the culture organism is placed in the water being tested to determine the percentage of animals that survive over a determined time period. Although many of the water quality requirements are known for cultured organisms, the use of water quality data alone, unless exhaustively done, may miss an essential parameter. Even after basic water quality parameters are identified, including pesticide and herbicide presence, the results of a bioassay with the intended culture organism is insurance that the site will be productive. The water quality parameters measured and bioassay results only give an indication of the potential for using a water source to culture marine shrimp. There are many variables (e.g., management, business plans, natural catastrophes) that will affect the success of such an operation that are beyond the scope of water quality and bioassay tests. Following is a discussion of individual water quality parameters for culturing the marine shrimp, Litopenaeus vannamei, in freshwater. The parameter levels indicated have not all been scientifically tested for culturing the Pacific white shrimp in freshwater, but are generally accepted for culturing aquatic organisms or the Pacific white shrimp.


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Salinity

(Requirement: >0.5 ppt or >300 ppm chloride) Salinity is a measure of the total concentration of dissolved inorganic ions, or salts, in the water. The salinity of seawater typically ranges from about 28 - 35 parts per thousand (ppt). One part per thousand is equal to 1 gram of inorganic salts per liter of water. Freshwater is usually defined as water that has less than 1 ppt of salinity. Estuarine water is a mixture of freshwater and seawater and therefore the salinity of estuarine water is intermediate between the salinity of seawater and freshwater. The salinity of estuarine water depends on the relative amounts of freshwater and seawater in the mixture. A mixture of salts contribute to the salinity of seawater. The most prevalent salt is typically sodium choride (table salt, NaCl). Salts are compounds that when dissolved in water dissociate into positively charged ions (called cations) and negatively charged ions (called anions). The most common cations in seawater are sodium (Na+), magnesium (Mg++), calcium (Ca++), and postassium (K+). The most common anions are chloride (Cl-), sulfate (SO4

=), bicarbonate (HCO3-), and bromide (Br-).

There are a variety of ways to measure the salinity of the water. Three methods for measuring salinity are based on the effect of salinity on the physical properties of the water. A refractometer is a device that measures salinity based upon the refractive index of the water. Light waves passing through a thin film of water are refracted, or bent, by the water. The amount of refraction, called the refractive index, is proportional to the concentration of dissolved salts in the water. The higher the salinity, the higher the refractive index. A refractometer consists of a prism positioned at the end of a viewing tube with a focusing ocular lens. A drop of water is placed on the surface of the prism. A transparent cover plate is placed over the water droplet and the prism pressing the water into a thin film. The salinity is read by holding the refractometer up to a light source and viewing the amount of refraction through the lens at the end of the viewing tube. The viewer will see a blue field over a lighted white field, with a sharp horizontal border between the two fields. The prism has a salinity scale etched into the glass that is visible when viewing through the viewing tube. The salinity is read by reading the salinity on the scale at the point of the border between the white and blue fields. A properly calibrated refractometer has an accuracy of ± 1 ppt of salinity. It is a very easy way to measure salinity and works well for measuring salinities from 2 or 3 ppt to full strength seawater. At lower salinities other measurement techniques are required for greater accuracy. Hydrometers are devices which measure salinity based upon the specific density of the water. The density of water increases in a near linear fashion with increasing concentrations of dissolved salts. The buoyancy of objects in the water is directly related to the density of the water. A hydrometer is a device in which an object is floated in the water and the degree of flotation is measured. How far the object sinks into the water is inversely proportional to the salinity. A calibrated scale is etched into the object which allows the salinity to be read. Hydrometers typically express salinity as specific density. Specific density is the ratio of the


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density of the water to the density of distilled water. Seawater with a salinity of 35 ppt has a specific density of 1.0282 at 4°C. Most hydrometers are not designed to measure small variations of the salinity of seawater and, therefore, are not useful for measuring low salinities. Conductivity meters measure the resistance of water to electrical flow, which is inversely proportional to the salinity. As salinity decreases, the resistance to electrical flow increases. The resistance is measured in µmhos cm-1 (pronounced micro moes per centimeter). A mho is the reciprocal of an ohm, which is the unit in which electrical currents are measured. A conductivity meter measures the electrical resistance to a current passing through the water between the anode and cathode of an immersed electrode. Tables can be used to convert specific conductivity measurements into salinity measurements. If sodium chloride is the dominant salt in the water, the salinity can be estimated by the concentration of chloride ions in the water. Chloride makes up approximately 55% of the weight of inorganic ions in seawater at 35 ppt. Using this relationship, one can calculate the approximate salinity in parts per thousand by dividing the measured chloride concentration by 550. For example, if the chloride concentration was found to be 300 ppm (1 ppm = 1 mg/L or 0.001 g/L) the corresponding salinity would be 0.545 ppt (= 300/550). This is an accurate and inexpensive method for determining the salinity for water with very low salinities in which the dominant salt is sodium chloride. Chloride can be measured using a titration method for which simple kits can be purchased from any aquaculture supply catalog. Aquatic organisms may be classified according to their tolerance to changes in salinity. Stenohaline organisms are adapted to a very narrow range of salinities, while euryhaline organisms tolerate a wide range of salinities. Organisms that normally live in environments with very stable salinities (e.g., freshwater lakes and open ocean) tend to be stenohaline, while organisms that live in environments with variable salinities (e.g., estuaries) tend to be euryhaline. The salinity requirements and tolerance to salinity variation may change throughout the life cycle of an organism. This is the case for most species of penaeid shrimp. Adult shrimp mature, mate and spawn in water with salinities between 28 and 35 ppt. Early larval stages also require full strength seawater. Juvenile Litopenaeus vannamei, however, are estuarine organisms and are extremely euryhaline, tolerating salinities ranging from less than 1 ppt to nearly 40 ppt. The physiological capability of penaeid shrimp to osmoregulate (regulate their internal salt and water balance) develops gradually while the shrimp are still in the postlarval stages. The gill filaments serve as the primary osmoregulatory organ in shrimp. In low salinity water the shrimp must selectively retain salts and excrete excess water. The gills serve as the primary site for water excretion. The effectiveness of the gills as an osmoregulatory organ is a function of the developmental stage of the shrimp. Shrimp in the early postlarval stages have insufficient gill surface are to be able to osmoregulate effectively at low salinities. The osmoregulatory capability of the shrimp improves dramatically once the gills begin to branch, which usually occurs after PL6. By the time the shrimp are a PL12 the gills have enough surface area to allow them to be gradually acclimated to near-freshwater conditions (see Chapter 6).


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Research at HBOI indicates that 0.5 ppt total salinity is probably the minimum level for the Pacific white shrimp to survive and grow to a marketable size. Chloride ion concentrations are the best predictor of the ability of shrimp to survive at least 24 hours in freshwater from a particular source. Twenty-four hour survival rates of shrimp postlarvae (PL15+) decline when chloride concentrations drop below 200 mg/L. Many other inorganic ions, including sodium, calcium, magnesium, potassium, and bicarbonate, are also required in some minimum amount for normal growth and development of the shrimp, but at this time the minimum requirements for these ions are not clear. It appears that total hardness (the combined concentration of calcium and magnesium ions) needs to be above 150 mg/L. Freshwater that has a low chloride concentration, or total hardness, may be made suitable for shrimp culture by adding the deficient salts to the water. If the water is found to contain insufficient chloride ion, the addition of solar or marine salt to your water may be the only modification necessary to make your water suitable for culture. Solar salt may be used to bring the salinity up to the levels required by the shrimp. Make sure that the salt that does not contain yellow prussiate of soda, a non-caking agent that is highly toxic to the shrimp. Marine salt is more expensive but contains other ions and trace elements that are necessary for the health of the shrimp. Other compounds can be added to the water to supplement hardness or alkalinity (see below). To calculate the amount of salt needed, first subtract the amount of salt in your water from 0.5 ppt (if you wish to use a higher level, e.g., 0.75 ppt, you may). For example, if your water was found to contain 38 ppm chloride you would first have to calculate your total salinity (38 ppm/550 = 0.069 ppt). Therefore, 0.5 ppt minus 0.069 ppt = 0.431 ppt, which corresponds to 0.431 g salt that will need to be added per liter of water. If your tank was 2500 gallons (9450 L), you would need to add just over 4 kilograms of salt to your system (0.431 g/L x 9450 L = 4073 grams of salt). Additional salt will need to be added when new water is added the system to replace water lost during backwash of filters. Perhaps the safest way to do this would be to adjust the salinity of the replacement water in a reservoir before adding the new water to the culture tank.

Temperature

(Optimal: 28-32oC/82-90oF) Litopenaeus vannamei, like all crustaceans, is poikilothermic (cold-blooded). This means that they are not able to regulate their body temperature. The shrimp's body temperature will normally be in equilibrium with the water temperature. This has profound consequences for the physiology of the shrimp because the rates of biochemical processes are temperature dependent. According to Van Hoff's Law, a 10ºC temperature increase will roughly double the rate of most biochemical reactions. This means that the temperature of the water directly affects the metabolism of the shrimp. As temperature increases, the metabolic rate will increase until a maximum rate is reached. As the temperature increases above that rate, the metabolic rate will decline rapidly until the temperature reaches an upper lethal limit. Many important processes are affected by the metabolic rate of the shrimp. The rates of feed


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consumption, oxygen consumption, ammonia excretion, and growth are all directly related to the metabolic rate of the shrimp. Shrimp can survive over a wide range of temperatures. The lower lethal limit is approximately 15ºC (59ºF), although shrimp may survive colder temperatures for a short period of time. The upper temperature limit for L. vannamei is about 35ºC (95ºF) for prolonged periods, or up to 40ºC for brief periods. The optimal temperature range is much narrower. While shrimp will survive temperatures below 24ºC (75ºF) and above 32oC (90oF), outside of this range the shrimp will be stressed and will not grow well. The temperature range for maximum growth is even narrower, ranging from 28-32ºC (82-90ºF). In tropical environments, temperatures are suitable for shrimp culture year round. The cool winters in Florida limit the growing season for shrimp in outdoor ponds to about 220 days/year. Greenhouses can significantly extend the growing system, but the water must still be heated to maintain optimal growing temperatures year round. Ideally, the water temperatures within the culture tanks should be maintained within the temperature range for maximum growth (28-32ºC). The daily variation in temperature should never exceed 4ºC. Fluctuating temperatures are stressful for shrimp, as well as for the bacteria in the biofilters. There are many different options for heating the culture tanks, including propane-powered heat exchangers, solar-powered heat exchangers or water heaters, electrical immersion heaters, and propane-powered space heaters. As a general principle, it is usually more cost-effective to heat the water directly than it is to maintain water temperatures by heating the air inside a greenhouse. Solar heating systems are very attractive because they are inexpensive to operate. However, the initial capital cost may be high and they may not be able to maintain temperatures during protracted periods of cold cloudy weather. Propane heat exchangers and electrical immersion heaters provide the highest degree of temperature control. If properly sized, these systems should be able to maintain temperatures to within ±1ºC. The operating costs of a propane heat exchanger system are likely to be cheaper than the electrical costs for immersion heaters. High water temperatures may become a problem during the summer months. In greenhouse systems, thermostatically controlled extractor fans and shuttered windows provide air exchange that helps cool the air temperatures within the greenhouse. Without air exchange the air temperture within a greenhouse may be as much as 11ºC (20ºF) above ambient outside temperaure. Air exchange alone, however, may not be sufficient to control water temperatures in the summertime. Covering the outside of the greenhouse with a 90-95% shade cloth will help maintain the water temperatures within an acceptable range even on hot sunny days. The shade cloth will also limit the growth of algae within the culture system. This may be considered a benefit from the standpoint that dissolved oxygen, pH, and unionized ammonia concentrations fluctuate widely in systems with high algal densities. Some producers, however, may want to maintain controlled blooms of algae within their systems. These producers might benefit from installing evaporative cooling panels on the opposite end of the greenhouse from the exhaust fans.


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Dissolved Oxygen

(Optimal: > 5 mg/L or ppm) Oxygen is required by shrimp for respiration, the physiological process in which cells oxidize carbohydrates and release the energy needed to metabolize nutrients from the feed. If oxygen is in short supply, the ability of the shrimp to metabolize feed will be limited, causing growth rates and feed conversion to suffer. Best growth and feed conversion ratios (FCRs) are obtained when dissolved oxygen (D.O.) levels are maintained at or above 80% of the saturation level (see Table 8-3). As a general rule, no stress will be placed upon aquatic organisms (including shrimp) if dissolved oxygen (D.O.) levels are maintained above 5 ppm. Prolonged periods of low oxygen concentrations (less than 1.5 ppm) are lethal, although shrimp can survive for short periods of time with as little as 1 ppm. If a level of 3 ppm or lower is found, measures should be taken to correct the problem. The solubility of oxygen in water is a function of temperature, salinity and altitude. As salinity, temperature, and altitude increase, the solubility of oxygen in water decrease (see Table 8-3). Freshwater at a temperature of 26oC (79oF) will have a D.O. of 8.1 ppm at saturation, but at 30oC (86oF) can only hold 7.6 ppm at saturation. The solubility of oxygen in seawater is significantly lower than in freshwater, but over a narrow range the influence of salinity is not pronounced. The effect of altitude on oxygen solubility will be negligible in Florida.

When the water is 100% saturated with dissolved oxygen, the rate of diffusion of oxygen from the water into the air is exactly balanced by the rate of diffusion of oxygen from the air into the water. Aquatic organisms and bacteria extract oxygen from the water for respiration, reducing the concentration of dissolved oxygen in the water. Water containing less than the 100% saturation concentration of oxygen is said to be undersaturated with oxygen. The difference between the concentration of dissolved oxygen in the water and the 100% saturation concentration for the existing conditions of temperature, salinity, and atmospheric

Table 8-3: Solubility of Oxygen in Water (mg/L) at Sea Level as a Function of Temperature and Salinity (after Stickney, 1979)

Salinity (ppt) Temperature

(ºC) 0 10 20 30 35 22 8.7 8.2 7.8 7.3 7.1 24 8.4 7.9 7.5 7.1 6.9 26 8.1 7.7 7.2 6.8 6.6 28 7.8 7.4 7.0 6.6 6.4 30 7.6 7.1 6.8 6.4 6.2 32 7.3 6.9 6.5 6.2 6.0 34 7.0 6.7 6.2 6.0 5.8


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pressure is called the oxygen deficit. The rate of net transfer of air into the water is dependent upon the magnitude of the oxygen deficit. Water can can also become supersaturated (>100%) with oxygen if air is injected into the water under pressure or from photosynthesis in plants. Bubbling pure oxygen gas into the water can also supersaturate the water with respect to oxygen. Supersaturation of water with nitrogen gas from air through pipe leaks may lead to gas bubble disease and shrimp mortality. The two biological factors that affect the D.O. level are respiration and photosynthesis. Respiration removes oxygen from the water, while photosynthesis adds oxygen to the water. The rate of oxygen consumption by respiration is dependent upon water temperature and the total biomass of animals, plants, and aerobic bacteria in the system. Accumulation of solid wastes within the system will dramatically increase the biomass of heterotrophic bacteria, resulting in a very large oxygen demand. Careful attention should be paid to solids removal (see Chapter 4) when designing your system to avoid this problem. Phytoplankton (microalgae) carry out both respiration and photosynthesis. During the day, the rate of oxygen production by photosynthesis generally exceeds the rate of oxygen consumption by the phytoplankton. At night, photosynthesis does not occur so oxygen levels will decline. In systems with heavy phytoplankton blooms oxygen concentrations fluctuate widely on a diurnal basis (Figure 8-1). The oxygen demand of the algae may deplete the oxygen in the culture tank during the early morning hours, especially after a period of warm, overcast days. If the bloom crashes, bacterial decomposition of the dead algae cells will result in a very high oxygen demand. Additional aeration, water exchange, or both may be required to prevent loss of shrimp due to oxygen depletion.

0

2

4

6

8

10

12

14

6 A.M. 12 A.M. 6 P.M. 12 P.M. 6 A.M.

Time of Day

Light Bloom

Dense Bloom

Figure 8-1: Relationship Between Algal Density and Diurnal Dissolved Oxygen Fluctuations


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There are several things that can be done to prevent excessive algal growth within the culture system. The most effective method for controlling algal growth is to reduce the light levels inside the greenhouse. Covering the outside of the greenhouse with a 90-95% shade cloth will prevent heavy blooms from developing and will also prevent overheating of the greenhouse during summer months. If a heavy bloom does develop in spite of these precautions, then a heavy water exchange may be required to bring the bloom under control. Dissolved oxygen concentrations should be closely monitored. Minimally, dissolved oxygen concentrations should be measured twice a day: once early in the morning (before or soon after sunrise) when oxygen levels are likely to be at their lowest point, and again in the late afternoon when they are likely to be at their highest point. The frequency of monitoring should be increased if D.O. levels drop below 4.0 mg/L. A good dissolved oxygen meter is an essential piece of equipment since oxygen is such a critical water quality parameter. Digital D.O. meters that automatically compensate for altitude, temperature, and salinity work best. D.O. meters should be recalibrated each time they are turned on and between measurements made at different salinities. D.O. meters are calibrated with the probe in the air because the concentration of oxygen in the air is a non-varying function of altitude and air temperature. Continuous oxygen monitoring and alarm systems are a good investment. Aeration equipment can fail at any time. Rapid detection of these failures may prevent total crop loss. Most problems with dissolved oxygen can be avoided by providing for adequate aeration when designing your production facility. The aeration requirements should be calculated based on the maximum possible shrimp biomass anticipated for each system (see Chapter 4). Remember that oxygen will be consumed not only by the shrimp, but also by the autotrophic and heterotrophic bacteria and algae living in the culture system. As a rule of thumb, you will need to transfer at least one kilogram of oxygen to the water for each kilogram of feed that you give to the shrimp. At loading rates in excess of 4.0 kg shrimp/m3 (0.033 lbs shrimp/gallon) it will be difficult to maintain dissolved oxygen concentrations above 5.0 mg/liter using airstones and a blower. Pure oxygen should be considered if higher loading rates are anticipated. Pure oxygen can be supplied either by an oxygen generator or by a liquid oxygen system.

pH

(Acceptable range: 7.0-9.0, optimal: 7.4-7.8) pH is defined as the negative log of the hydrogen ion concentration. Because pH is the negative log of the hydrogen concentration, low pH values indicate high hydrogen ion concentrations, while high pH values indicate low hydrogen ion concentrations. The pH scale ranges from 0-14. Each pH unit represents a ten-fold difference in hydrogen ion concentration. Water with a pH of 7 has a hydrogen ion concentration of 10-7 moles/L, while water with a pH of 8 has a hydrogen ion concentration of 10-8 moles/L. Aqueous solutions with pH values less than 7.0 are considered to be acidic, while those with pH values greater


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than 7.0 are considered to be basic. A solution with a pH of 7.0 is considered to be neutral. Freshwater usually has a pH near 7 whereas the pH of seawater is usually near 8.3. Shrimp can tolerate a pH range from 7.0 to 9.0. Very acidic water (pH less than 6.5) or very basic water (pH greater than 10.0) is harmful to the gills of the shrimp and growth rates will be suppressed. Although the shrimp are not harmed by pH values in the range from 7.0-9.0, in a recirculating culture system it is better to maintain the pH in the range from 7.2 - 7.8. This is due to the relationship between pH and the concentration of ammonia within the culture tank. Most species of nitrifying bacteria are adapted to a pH range of 7.2-7.8 so it is within this range that biofilters function most effectively. Also, the fraction of total ammonia nitrogen that is in the toxic, unionized form (NH3) is less than 5% when pH is less than 7.8. At a pH of 9.0 approximately 50% of the total ammonia nitrogen is in the form of NH3. The pH of water is strongly influenced by both respiration and photosynthesis. As a result of respiration carbon dioxide (CO2) is released into the water. Dissolved carbon dioxide (CO2) combines with water to form carbonic acid (H2CO3). A series of reversible equilibrium reactions occur which result in the formation of hydrogen ions, bicarbonate ions (HCO3

–) and carbonate ions (CO3

= ): CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3

– ↔ H+ + CO3= Eq. (8.1)

When carbon dioxide is removed from the water as a result of photosynthesis by aquatic plants, the reactions described in Equation 8.1 occur in reverse (from right to left). Free hydrogen ions in the water will react with carbonate and bicarbonate ions, reducing the overall hydrogen concentration and raising the pH of the water. Thus, photosynthesis has the effect of raising pH, while respiration has the effect of lowering pH. In systems with heavy phytoplankton blooms pH fluctuates on a diurnal basis. During the day photosynthesis consumes CO2 causing the pH to rise. In systems where phytoplankton blooms are particularly heavy, or which have a low alkalinity, pH may rise above 9.0 in the afternoon. During the night, respiration releases CO2 into the water causing the pH to fall. pH swings of 2 pH units are possible between early morning and late afternoon. pH swings of this magnitude are stressful for both shrimp and the nitrifying bacteria in the biofilter. Wide swings in pH can be minimized by maintaining adequate buffering capacity in the water. Certain compounds, called buffers, are capable of releasing hydrogen ions into the water at high pH levels and withdrawing hydrogen ions from the water at low pH levels. The effect of these buffers is to dampen the fluctuations in pH that would otherwise result from photosynthetic and respiratory processes. Alkalinity is a measure of the buffering capacity of the water (see section on alkalinity for a more detailed discussion). Alkalinity should be maintained above 100 mg/L as CaCO3 to minimize fluctuations in pH. The nitrification process generates hydrogen ions and consumes bicarbonate ion (a source of alkalinity). Over time, the net effect is a reduction in alkalinity and pH. The alkalinity consumed by the biofilter should be replaced by the addition of liming compounds or makeup water with a higher alkalinity.


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A variety of techniques may be used to measure pH. The simplest method is to dip a pH indicator strip in the water and compare the resulting color with the color scale. This method is suitable for quick analyses but is not as accurate as some other methods. Another simple method that is only slightly more accurate, is a colorimetric test in which a few drops of a pH indicator solution are added to a water sample and the resulting color change is matched to a standardized color wheel. When a higher degree of accuracy is desired a pH meter should be used. A pH meter measures the transmission of an electrical current through an aqueous solution using a pair of glass electrodes. The electrical potential measured is related directly to the hydrogen ion activity. pH meters must be calibrated frequently using buffer solutions of known pH that bracket the pH of the solution being measured. Two point calibrations are usually performed using pH 4.01 and 10.0 buffer solutions. The frequency of measurement of system pH is a function of stocking intensity and pH variability. High density systems and systems with a high degree of pH variability should be monitored daily and occasionally twice a day. The daily measurement should be made at the time of day when the pH is likely to be most critical. In systems with algae blooms, pH should be measured late in the afternoon to determine the maximum daily pH, and occasionally early in the morning to determine minimum daily pH. Usually the maximum daily pH is the more critical measurement because ammonia toxicity is highest when the pH is at its highest point. Ideally, pH and temperature should be measured whenever total ammonia nitrogen is measured so that the concentration of unionized ammonia can be calculated.

Dissolved Carbon Dioxide

(Acceptable: <20 ppm, Optimal <5ppm) Respiration is the source of most dissolved carbon dioxide (CO2) in the system water. Dissolved carbon dioxide concentrations often bear an inverse relation to dissolved oxygen concentrations. High concentrations of carbon dioxide interfere with the ability of shrimp to extract oxygen from the water, reducing the tolerance of the shrimp to low oxygen conditions. In extreme cases, shrimp may die from asphyxiation even when there is adequate oxygen present in the water. The carbon dioxide produced by shrimp respiration must be unloaded across the gills from the shrimp blood to the water. Unloading of CO2 at the gills can only take place when the concentration of CO2 in the blood exceeds the concentration of CO2 in the water. High CO2 concentrations in the water cause a buildup of CO2 in the blood of the shrimp. High CO2 levels in the blood lower the blood pH which interferes with the ability of the blood to unload oxygen at the tissues. Carbon dioxide has no detrimental effects on shrimp at concentrations of less than 20 ppm. Concentrations in the range of 20-60 ppm are not generally lethal, but do interfere with CO2 exchange across the gills and with the tolerance of shrimp to low dissolved oxygen conditions. Carbon dioxide concentrations above 60 ppm may be life threatening. Carbon dioxide exerts an important influence on the pH of the water, as discussed in the previous section. In systems with dense phytoplankton blooms carbon dioxide levels decrease during the day and rise during the night. Maintaining a high alkalinity (>100 mg/L


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as CaCO3) limits the pH shift resulting from the addition of CO2 to the water. In heavily loaded systems the high rate carbon dioxide production can lead to low system pH levels. The most effective way to prevent carbon dioxide levels from becoming dangerously high is to "degas" the excess CO2. Degassing is accomplished by expanding the amount of water surface area that is in direct contact with the air. Aeration of the water with airstones and spraybars will usually provide sufficient degassing to control carbon dioxide levels in shrimp culture systems. However, if very high stocking rates are used (≥ 300 shrimp/m2), a degassing column may be required (see Chapter 4). In an emergency, carbon dioxide may be removed by the addition of quicklime (CaO): CaO + CO2 → CaCO3 ↓ Eq. (8.2) However, the situation that led to the carbon dioxide level must be corrected or it will reoccur. The quicklime should be added slowly to the water or the pH may rise too rapidly, stressing the shrimp. Quicklime is very caustic, so care should be exercised when handling this material. Well water is typically low in oxygen and high in carbon dioxide. Untreated well water should not be added directly to the culture tank. A degassing column is the most effective way of eliminating excess CO2 and aerating the water. In a degassing column, water is sprayed over a bed of plastic media with a large amount of void space and air from a fan or blower is introduced at the bottom of the column. The idea is to create a very high air:water ratio to maximize the rate of diffusion of gases between the air and the water. Alternatively, heavy aeration or spraying of the water into the air may be sufficient to aerate the water and drive off the CO2.

Ammonia

(Optimal: unionized form <0.03 ppm, chronic effects/lethality >0.1 ppm) Ammonia is the principle nitrogenous waste-product excreted by shrimp and most other aquatic organisms. Much of the nitrogen from protein in the feed that is added to a culture tank is converted into ammonia. Most of the feed that is ingested by the shrimp is assimilated and the proteins are metabolized by the shrimp. Ammonia, a major by-product of protein metabolism, is excreted across the gills of the shrimp. Heterotrophic bacteria utilize uneaten feed, fecal wastes or other decaying organic material as a protein source and convert the protein nitrogen into inorganic ammonia, which is excreted. This process, in which organic nitrogen from proteins is converted into inorganic nitrogen (NH3), is called mineralization. Nearly 85% of the nitrogen in the feed fed to shrimp in the culture tanks will end up as ammonia. Ammonia levels in the culture tank must be carefully managed because ammonia can be highly toxic to the shrimp. Ammonia exists in two different forms in the water: as unionized ammonia (NH3) and as ammonium ions (NH4

+). These two forms are usually present


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simultaneously in the water and are transformed from one form to another in an equilibrium reaction: NH3 + H20 ↔ NH4

+ + OH- Eq. (8.3) Only the unionized form of ammonia is toxic to the shrimp. The toxicity of ammonia is partly a function of shrimp age. Postlarvae and small juveniles are are less tolerant of high concentrations of unionized ammonia levels than are larger juveniles and adult. The 96-hr LC50 concentration of unionized ammonia is about 0.2 ppm for postlarvae (Chen and Chin, 1988) and about 0.95 ppm for 4.87 gram adolescents (Chen and Lei, 1990). Shrimp health and growth rates are not affected when unionized ammonia levels are maintained below 0.03 ppm. However, chronic exposure to elevated sublethal concentrations may have a number of detrimental effects on the shrimp. Growth rates decrease and feed conversion rates increase. High ammonia concentrations irritate the gills of the shrimp and may lead to gill hyperplasia (i.e., swollen gill filaments) reducing the ability of the shrimp to extract oxygen from the water. In additon, high ammonia levels in the water lead to an increase in the ammonia concentrations in the blood. High ammonia concentrations in the blood reduce the affinity of the blood pigment (hemocyanin) for oxygen. Together, these latter two effects reduce the tolerance of the shrimp to low oxygen conditions. Chronic exposure to high ammonia concentrations may also reduce the resistance of the shrimp to disease. Ammonia is usually measured as total ammonia nitrogen (TAN). TAN is a measure of the combined concentrations of unionized ammonia and ammonium ion. The fraction of TAN that is in the unionized form is a positive function of both pH and temperature (Table 8-4). The relationship between the unionized fraction of ammonia and temperature is nearly linear, while the relationship with pH is logarithmic. The fraction of unionized ammonia at 30ºC at pH values of 7.0, 8.0, and 9.0 increases from 0.008 to 0.075 to 0.449, respectively (Table 8-4). This illustrates the importance of maintaining system pH below 8.0. This example also

Table 8-4: Proportion of Total Ammonia Nitrogen in the Unionized Form as a Function of Temperature and pH

24 26 28 30 327.0 0.005 0.006 0.007 0.008 0.0097.2 0.008 0.010 0.011 0.013 0.0157.4 0.013 0.015 0.018 0.020 0.0237.6 0.021 0.024 0.028 0.031 0.0367.8 0.033 0.038 0.043 0.049 0.0568.0 0.051 0.058 0.066 0.075 0.0858.2 0.078 0.089 0.101 0.114 0.1298.4 0.119 0.134 0.151 0.170 0.1908.6 0.176 0.197 0.220 0.245 0.2718.8 0.253 0.281 0.309 0.340 0.3719.0 0.349 0.382 0.415 0.449 0.4839.2 0.460 0.495 0.530 0.564 0.5979.4 0.574 0.608 0.641 0.672 0.7019.6 0.681 0.711 0.739 0.764 0.788

10.0 0.843 0.861 0.877 0.891 0.903

TemperaturepH


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illustrates the importance of measuring pH and temperature whenever TAN is measured. Without these values, the concentration of unionized ammonia cannot be calculated. The following example explains how unionized ammonia is calculated from measurements of TAN, pH, and temperature. Example: You determine that the total ammonia nitrogen (TAN) in your system is 0.8 ppm. The temperature is 28oC and the pH is 7.6. What is the unionized ammonia concentration? On the table locate the column for 28oC and the row for pH of 7.6. Where these two cross is the number 0.028. This number will be multiplied by your TAN (0.028 x 0.8 ppm) to yield the unionized ammonia concentration (= 0.0224 ppm). This level is lower than the acceptable level (<0.03 ppm) and no corrective action would need to be taken. However, if the pH of the system were only 0.2 units higher (i.e., at 7.8) then the unionized ammonia level would be 0.0344 and corrective measures may have to be taken. Almost all ammonia testing methods yield the total ammonia nitrogen level. There are two different analytical procedures that are commonly used to test for ammonia. The salicylate method can be used for both fresh and saltwater samples. The Nessler method works best with freshwater samples, but the procedure can be modified for saltwater use. Nevertheless, we prefer the salicylate method because it is accurate and the exact same procedure is used for both fresh and saltwater. There are three basic mechanisms by which ammonia can be removed from a recirculation system: water exchange, plant uptake, and nitrification. Water exchange is an effective way to lower ammonia levels rapidly in an emergency, but should not be counted on as the primary strategy for controlling ammonia levels. For water exchange to be effective, 50-100% of the water would need to be exchanged per day. Such a high volume of effluent would require enormous effluent treatment systems. However, when primary ammonia removal systems fail, heavy water exchange may be the only way to save the shrimp. Phytoplankton and other aquatic plants remove ammonia from the water and use it as a nitrogen source for protein synthesis. Systems with high exposure to sunlight will develop dense blooms of phytoplankton. As long as the phytoplankton population is increasing, the phytoplankton will constitute an effective nitrogen sink. However, if the bloom crashes, a large amount of the nitrogen tied up in the phytoplankton will be converted back into ammonia as the dead algae cells are decomposed by heterotrophic bacteria. Dangerously high ammonia levels typically follow crashes of phytoplankton blooms. Algal blooms can be controlled by water exchange to prevent "overblooms" from developing. Some recirculating systems depend upon aquatic macrophytes to control ammonia. In these systems water from the culture tank is recirculated through a separate water treatment tank or pond containing a large number macrophytes such as water hyacinths, water lilies, bullrushes, etc. Ammonia is removed by the macrophytes and water low in ammonia is returned to the culture tank. Hydroponic systems are sometimes combined with aquaculture systems. In these systems the roots of vegetables or herbs absorb ammonia and other nutrients from the water coming from the aquaculture system.


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The more traditional approach to ammonia control in recirculating systems is to promote the process of nitrification within a biofilter. Nitrification is the sequential oxidation of ammonium ion to nitrite and then to nitrate by autotrophic bacteria. Nitrosomonas spp. convert ammonium ions to nitrite, while Nitrobacter spp. convert nitrite to nitrate. In addition to oxygen, the nitrifying bacteria require bicarbonate ions, which they utilize as a carbon source for cell growth. The nitrification process is represented by the following equations (EPA, 1975): 55NH4

+ + 76O2 + 109HCO3– → 54NO2

– + 57H2O + 104H2CO3 + C5H7NO2 Eq. (8.4) 400NO2

– + NH4+ + O2 + 4H2CO3 + HCO3

– + 195O2 → 400NO3= + 3 H2O + C5H7NO2

Eq. (8.5) A biofilter is simply a device that provides a large amount of surface area for the nitrifying bacteria to grow (see Chapter 4 for a more complete discussion of biofilter design and operation). When a new system is started up, the biofilter will not be active. Before stocking animals into the system, the biofilter will need to be conditioned. During the conditioning period an inorganic source of ammonia, such as ammonium chloride, is added to the system. The rate of addition of inorganic ammonia should equal or exceed the rate at which ammonia will be generated by the quantity of feed that the animals receive on a daily basis immediately after they are stocked. A source of inorganic nitrite, such as sodium nitrite, can also be added to the system water to accelerate the conditioning process, but the Nitrobacter

Nitrosomonas

Nitrobacter

0 5 10 15 20 25 30 35Days

NH3-N

NO3-N

NO2-N

Figure 8-2 : Changes in Concentration of Total Ammonia (NH3-N ), Nitrite (NO2-N) andNitrate over (NO3-N) Time During the Conditioning of a Biofilter.


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bacteria will colonize the biofilter even without the addition of nitrite. What is typically seen during the conditioning period (Figure 8-2) is a gradual decline in the ammonia concentration as the Nitrosomonas population becomes established. As the ammonia concentration falls, the nitrite concentration rises. The nitrite levels will peak and then begin to fall as the Nitrobacter population becomes established. The biofilter conditioning is complete when both ammonia and nitrite levels can be maintained within acceptable limits even with daily ammonia input equal to the amount that will be produced at the initial feeding rates. Even with well-established biofilters, high ammonia levels may occasionally develop in a culture tank. When this occurs, the cause of the problem must be quickly determined and an appropriate response must be decided upon. While water exchange may provide immediate relief from high ammonia conditions, corrective measures should be taken that address the cause of the problem. Otherwise, ammonia levels will quickly return to high levels. There are a number of reasons why the ammonia concentrations may be high. Chronic overfeeding may lead to a buildup of uneaten feed in the culture tank and in sumps and filters. In addition to causing high ammonia levels, decomposing feed in a tank can serve as a substrate for Vibrio bacteria. These bacteria may infect and kill shrimp, especially if high ammonia levels have weakened the disease resistance of the shrimp. If accumulations of uneaten feed are observed in the tank, feeding rates should be reduced and excess feed should be siphoned or vacuumed from the culture tank. If the shrimp are very small it may not be possible to siphon out the feed without siphoning up some shrimp. An alternative way of removing the accumulated solid wastes is to increase the flow rate through the tank and brush the bottom of the tank to suspend the solid wastes so that they will be carried in the flow to the drain. This may need to be done several times. Accumulated solid wastes in sumps may also be source of ammonia production and biological oxygen demand (BOD). Sumps should be siphoned or flushed out to remove these accumulations. High ammonia levels may be indicative of a problem with the biofilter. The effectiveness of the biofilter can be determined by measuring the efficiency of the biofilter. Biofilter efficiency is a measure of the percentage of ammonia (or nitrite) removed by the biofilter in a single pass:

Biofilter Efficiency = T.A.N. in – T.A.N. out

T.A.N. in

x 100% Eq. (8.6)

Biofilter efficiency should be monitored on a regular basis (at least weekly) so that changes in efficiency can be detected. Historical data allows changes in biofilter nitrification rates to be detected. A number of conditions must be present for a biofilter to provide reliable ammonia and nitrite control:

1. The water should be filtered to remove the majority of suspended solids before it enters the biofilter. Solid wastes will smother the autotrophic bacteria and provide a substrate for heterotrophic bacteria which will compete for space and oxygen.

2. The oxygen level within the biofilter should be maintained above 2 ppm.


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3. The alkalinity of the water should be maintained above 100 ppm to minimize pH fluctuations and to provide a source of bicarbonate ions for the nitrifying bacteria.

4. Stable temperatures should be maintained, ideally between 28ºC and 32ºC. 5. Salinity should be kept as stable as possible. 6. Adequate shearing forces should be present within the biofilter to slough off

accumulations of dead bacteria and other organic material. 7. Avoid exposing a biofilter to antibiotics or other potentially toxic chemicals.

If biofilter efficiency has declined, the reason for the decline must be determined and the problem must be corrected. Even after the condition causing a decline in nitrification rates has been corrected, it may take awhile for the population of nitrifying bacteria to recover. In the meantime it may be necessary to reduce feeding rates and increase the rate of water exchange. If unionized ammonia levels are dangerously high, it may be necessary to reduce the pH of the system by adding muriatic acid. This will reduce the percentage of the total ammonia in the toxic unionized form. Acid should be added very slowly to the system to avoid causing a pH shock to the shrimp or the biofilter bacteria. This would only make the problem worse.

Nitrite

(Acceptable: <1 ppm) Nitrite is a product of the first step of nitrification, in which ammonium ion is oxidized by Nitrosomonas bacteria to form nitrite. Nitrite can accumulate in the system if the second step in the nitrification process, in which nitrite is oxidized by Nitrobacter bacteria to form nitrate, occurs at a much slower rate than the first nitrification step. Nitrite is toxic to penaeid shrimp. The toxicity of nitrite is influenced by shrimp age and the salinity of the water. The 96-hour LC50 concentration of nitrite to Penaeus monodon postlarvae was reported by Chen and Chin (1988) to be 13.6 ppm. The 96-hour LC50 for adolescent P. monodon (5 grams) was reported by Chen and Lei (1990) to be 171 ppm. The LC50 concentration for adolescent L. vannamei appears to be much lower than for P. monodon. At Harbor Branch we have observed mortalities approaching 50% in tanks stocked with 10 gram L. vannamei at nitrite concentrations less than 20 mg/L. Nitrite is more toxic at low salinities and low pH values than it is at higher values. To be safe, nitrite concentrations should be maintained below 1 mg/L. In fish, nitrite in the blood binds with hemoglobin to form methemoglobin. Methemoglobin is unable to transport oxygen to the tissues so fish die from asphyxiation. If chloride concentrations in the water are at least six times the concentration of nitrite, then nitrite is not transported across the gill membranes and the toxic effects of nitrite are avoided. The mechanism of nitrite toxicity is not well understood in shrimp, which have a different blood pigment (hemocyanin) than fish. The mechanism may be similar, since high nitrite levels reduce the tolerance of shrimp to low oxygen levels. Although a high chloride concentration provides some protection against nitrite toxicity it does not provide complete protection.


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A well-conditioned biofilter with adequate surface area is the best protection against high nitrite levels. Before stocking a new system, make sure that the nitrite peak has passed (see above). Some types of biofilters do not seem to support good populations of Nitrobacter and nitrite levels will tend to run high. This is especially true of bead filters and some submerged, moving bed biofilters. Compared to Nitosomonas, Nitrobacter is not very "sticky". When the biofilter media is agitated, Nitrobacter are dislodged and washed out of the filter. Bead filter manufacturers are addressing this problem by using media with recesses to protect the bacteria. Nevertheless, frequent backwashing reduces the Nitrobacter population significantly. Reducing the frequency of backwashing can help minimize this problem. Whenever high nitrite levels are encountered, the cause of the problem must be determined. Often the causes of high nitrite levels are the same as the causes of high ammonia levels (see above). Often the two problems occur simultaneously. Water exchange and reduced feeding rates can provide short term relief, but unless the root causes are remedied the problems will return.

Nitrate

(Acceptable: <60 ppm) The biofilter contains another nitrifying bacteria, Nitrobacter, that oxidizes nitrite to nitrate (NO3). Nitrate is virtually non-toxic. Shrimp can survive nitrate levels as high as 200 ppm, but it is not known if levels this high affect growth or disease resistance. Ideally nitrate levels should be maintained less than 60 ppm. In general, your system should be balanced in that nitrate levels may increase over the course of grow-out but not to a point that should cause worry. Although most biofilters are oxidative reactors, there are portions of the biofilter and elsewhere in a system where oxygen levels are virtually zero and anaerobic bacteria thrive. Anaerobic bacteria utilize nitrate and convert it to nitrogen gas (N2) in a process known as denitrification. During this reaction, nitrite may also be formed.

Hardness

(Acceptable: >150 ppm as CaCO3) Hardness is the measurement of all divalent cations (i.e., those ions carrying a plus two charge) of which calcium and magnesium (Ca++, Mg++) are the predominant species in water. These two ions may be absorbed by shrimp through their gills and thus are important not only in water quality but in the nutrition of the animal. Hardness is generally measured by titration, although color strip indicators are available, and is expressed in terms of mg/L as calcium carbonate. This expression helps in equalizing different water compositions (e.g., one with only calcium) for comparisons. Therefore, "total" hardness does not divulge the ionic makeup of the hardness. For that, calcium hardness may be determined and the difference assumed to be magnesium, although the only way to verify that would be through expensive analytical chemistry procedures.


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You may be familiar with hardness since many water sources have to be treated to reduce hardness so as not to build up scale in pipes and boilers. Water with a total hardness of 0-75 ppm is considered soft, 75-150 is moderately hard, 150-300 is hard and greater than 300 is very hard. Seawater, in which shrimp normally grow, has a total hardness of approximately 6600. Yet, marine shrimp can be grown in water with moderate hardness (150) and may be able to be grown in waters with even lower hardness levels, although no research has been done to verify the possibility. Hardness is often confused with alkalinity (describe later) because both are expressed in similar terms (mg/L as CaCO3) and often hardness and alkalinity values are similar. However, if the alkalinity is from sodium carbonate instead of calcium or magnesium carbonates it is possible to have low hardness and high alkalinity. High hardness and low alkalinity may occur in acidic well or surface waters. Low hardness can be increased with agricultural limestone (calcium carbonate), agricultural gypsum (calcium sulfate), or food grade calcium or magnesium chloride.

Alkalinity

(Acceptable: >100 ppm as CaCO3) Alkalinity is defined as the sum of exchangeable bases reacting to neutralize acid when an acid is added to water. In other words, alkalinity is the buffering capacity of water. This buffering capacity is primarily due to bicarbonates (HCO3

-), carbonates (CO3--), hydroxides

(OH-) or a mixture of these. As mentioned earlier in the section on pH, sufficient alkalinity will help moderate pH swings from photosynthesis and respiration. Since little water is exchanged in most high-density shrimp recirculating systems, alkalinity should be maintained at relatively high levels (>100 ppm). Seawater has an average alkalinity of 116 ppm (as CaCO3 ) and alkalinities in freshwater fish ponds typically average about 40 ppm. Alkalinity in freshwater can range from 20-300 ppm as CaCO3. Alkalinity can also be expressed as milliequivalents (1 meq/L = 50 ppm as CaCO3 = 2.92 grain/gallon CaCO3). As with hardness, alkalinity may be increased with agricultural limestone (calcium carbonate). Sodium bicarbonate may be used to increase alkalinity without increasing hardness.

Hydrogen Sulfide

(Acceptable: None) Hydrogen sulfide (H2S) is a colorless, toxic gas with a distinctive odor similar to rotten eggs. It is primarily derived from anaerobic decomposition of organic matter. It may be found in well water or in pond bottoms composed of mud and other organic matter. Hydrogen sulfide is highly toxic in the unionized form (similar to ammonia), however, the unionized form is predominant at low pH (<8) and high temperature. At pH 7.5 approximately 14% of the sulfide is in the toxic H2S form, at 7.2 about 24%, at pH 6.5 about 61%, and at pH 6 about 83% of total sulfide is in the toxic unionized form. Therefore, sulfide concentrations should


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be less than 0.002 ppm. If you can smell it, then there is too much present and it will slow growth and eventually kill most of your shrimp. Well water that contains hydrogen sulfide must be vigorously aerated ("degassed") before use. There are a number of kits available to test for hydrogen sulfide in your water.

Iron

(Acceptable: <1.0 ppm, none preferred) Iron is found in two forms, soluble (ferrous, Fe++) and insoluble (ferric, Fe+++) in well water. Iron in and of itself is not toxic, but the oxidation of the soluble form to the insoluble leads to the formation of precipitates that can irritate and clog the gills of shrimp, ultimately leading to a reduced oxygen supply, asphyxiation and death. Soluble iron can be removed from water by aeration and letting it oxidize to form a precipitate that can be removed by filtration or settling before use in your system.

Chlorine

(Acceptable: None) A common mistake in talking about chloride is the use of the word chlorine. Chlorine and chloride are two forms of elemental chloride, however, their effect on the health of your shrimp is totally opposite. Chlorine is used for disinfection of systems and is toxic at extremely low levels (<0.05 mg/L). Municipal water sources contain at least 10-fold higher levels if not higher (0.5 - 2.0 mg/L). The addition of untreated municipal water to your system or the accidental introduction of chlorine into your system can have devastating results. Chlorine can be removed by using sodium thiosulfate at a rate of 7 mg/L for each 1 mg/L of chlorine. Chloride is the ion discussed at the beginning of this section that is the predominant ion in the composition of salinity and functions in osmoregulation.

Selected Literature Ammonia by Ruth Francis-Floyd and Craig Watson. IFAS Fact Sheet FA-16 by IFAS, University of Florida, Gainesville, Florida, 4 pp. Aquaculture Engineering by Frederick W. Wheaton. 1977. John Wiley & Sons, Inc., New York, NY, 693 pp. Crustacean Farming by Daniel O'C. Lee and John F. Wickins. 1992. Halsted Press of John Wiley & Sons, Inc., New York, NY, 392 pp. Fundamentals of Aquaculture by James W. Avault, Jr. 1996. AVA Publishing Co., Baton Rouge, Louisiana, 889 pp.


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Interactions of pH, Carbon Dioxide, Alkalinity and Hardness in Fish Ponds by William A. Wurts and Robert M. Durborow. 1992. SRAC Publ. No. 464, Southern Regional Aquaculture Center, IFAS, University of Florida, Gainesville, Florida, 4 pp. Joint Action of Ammonia and Nitrite on Tiger Prawn Penaeus monodon Postlarvae by Jiann-Chu Chen and Tzong-Shean Chin. 1988. Journal of the World Aquaculture Society 19: 143-148. Marine Shrimp Culture: Principles and Practices by Arlo W. Fast and L. James Lester. 1992. Elsevier Science Publ. Co., Inc., New York, NY, 862 pp. Practical Manual for Semi-intensive Commercial Production of Marine Shrimp by Jose R. Villalon. 1991. TAMU-SG-91-501 by Texas A&M University Sea Grant College Program, Texas A&M University, College Station, Texas, 104 pp. Principles of Warmwater Aquaculture by Robert R. Stickney. 1979. John Wiley and Sons, Inc., New York, NY, 375 pp. Toxicity of Ammonia and Nitrite to Penaeus monodon Juveniles. by Jiann-Chu Chen and Shun-Chiang Lei. 1990. Journal of the World Aquaculture Society 21: 300-306. Understanding and Interpreting Water Quality by Michael McGee. IFAS Fact Sheet FA-2 by IFAS, University of Florida, Gainesville, Florida, 4 pp. Water Quality in Ponds for Aquaculture by Claude E. Boyd. 1990. Auburn University, Auburn, Alabama, 482 pp.

Chapter 9 – Shrimp Health Management

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Chapter 9 Shrimp Health Management

by Kevan L. Main and Rolland Laramore


Introduction One of the most important criteria for a successful operation is to insure that the health of the shrimp is maintained. The goal of this chapter is to provide an overview of the shrimp health issues that farmers will encounter during the nursery and growout culture phases. We will highlight some of the stress producing factors that promote disease in shrimp, discuss the more common disease symptoms, and explain how to evaluate the health of your shrimp. We will not discuss broodstock or larval health issues. In addition, we will not address the issues that affect traditional pond culture in saltwater systems. Those topics are already addressed in other publications on shrimp diseases (Brock and Main 1994; Fulks and Main 1992; Lightner 1988, 1993, 1996; Wyban and Sweeney 1991). Disease and production problems vary during the different phases of shrimp culture. Production shortages resulting from shrimp mortality, slow growth and high food conversion ratios occur and affect the economics of L. vannamei farms. Substantial losses have occurred in some farming areas due to diseases, such as Taura syndrome virus, runt-deformity syndrome, vibriosis and necrotizing hepatopancreatitis. Effective strategies to control the occurrence and spread of disease are primarily related to proper management of the production system. When shrimp are cultured in a poorly managed environment, both growth and survival rates will decrease. This is true whether farming is conducted in extensively stocked saline ponds or in intensively stocked freshwater raceways. However, as stocking densities are intensified and the animals are moved into a controlled production environment, there is a greater need to reduce stress factors. To insure that the animals are under a limited amount of stress, it is necessary to establish and implement good management practices. Seasonal changes are also known to influence shrimp health and disease problems are often associated with the high summer temperatures. This chapter is intended for field use by culturists, farmers, students and extension personnel who are not formally trained in microbiology or veterinary medicine. The information presented in this chapter is drawn from a number of sources, but the primary resource was Brock and Main (1994).


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Variables to Consider in Determining the Health of Your Shrimp Farmers commonly consider a number of variables when they evaluate the health of the shrimp in the culture system. These variables include survival rate, mortality rate, growth rate, size variation, food conversion ratio and appearance of the shrimp (Brock and Main, 1994).

Survival Rates The survival rate is an estimate of the change in number of shrimp in a tank over a period of time. The validity of a survival rate value depends on the accuracy of the data. Mistakes may result from inappropriate counting techniques, human error or inexperience and variability in the counting method. Often the shrimp are miscounted when they are first added to the tank - this can lead to survival rates that exceed 100%.

Mortality Rates Mortality rate is the number of dead shrimp that are counted over a period of time. Daily, weekly or monthly counts of dead shrimp can be used to show trends in disease progression, transmission, response to treatment or other management manipulations. Occasionally survival rates are the opposite of mortality rates in shrimp production. However, you cannot assume this is the case because reduced survival does not always mean the shrimp have died. Escapement, predation, miscounting and theft can also result in lower survival.

Growth Rates Growth rates are a good indicator of the health of the shrimp in your system. Growth rates are influenced by environmental, genetic, biological and nutritional factors. Water temperature is a major factor that affects growth rates and will affect weekly gains during warmer and cooler months of the year. During the first month of growout, growth rates can be measured as the percent increase in body weight. By the 6th week in growout, weekly weight gains are estimated to determine the growth rates. Growth rates in L. vannamei can vary from 0-2.5 grams per week. Growth rates that are less than of < 0.5 grams per week are considered to be a poor rate of growth.

Size Variation Some disease or health problems can be identified by looking at the size distribution of shrimp in a raceway system. The size distribution of the shrimp does not need to be determined unless you see that there is a problem with growth rates or mortality. For routine management of healthy shrimp, we do not recommend calculating size variations for the shrimp in the production system.

Feed Conversion Ratio The feed conversion ratio (FCR) is a measure of the efficiency of feed utilization by shrimp. FCR values, together with other information, can be used to understand a problem with a production run. In general, the lower the FCR, the more efficient the feed utilization. Poor


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feed quality can reduce growth and increase FCR. Experimental growth trials can indicate if the reduced growth rate is diet related (Brock 1992).

Appearance of Shrimp Shrimp should be visually examined on a daily basis for changes in behavior and color. In order to identify problems, you must first be able to recognize normal appearance and behavior of cultured L. vannamei. Unusual behaviors (i.e., lethargy, disorientation) can be indications of a disease problem. Changes in appearance and color of organs (i.e., gills, appendages, cuticle, abdominal muscle) should also be noted.

The Effect of the Environment on Shrimp Health The environment can have a significant impact on shrimp health, growth and production. A change in one environmental parameter can affect the other variables. It is important to look at the direction, rate and amount of change in evaluating the impact of environmental conditions on shrimp health. The types of environmental variables that have been associated with shrimp disease include temperature extremes, pH extremes, low dissolved oxygen, abrupt changes in salinity and gas supersaturation. Some environmental parameters are associated with a site (i.e., water chemistry parameters), while others are associated with management and production strategies. Both site selection and the levels of specific environmental are important factors in determining the success or failure of an operation. The levels of specific environmental parameters are often associated with changes in behavior and eventually growth and survival. Health Evaluation Tests Stress Tests Stress tests are widely used by shrimp farmers as a quality control measure for postlarval L. vannamei. The survival of a shrimp sample following a thermal or pH shock is used to assess population health, with survival after a defined time period used as a measure of postlarval quality.

Gill Examination The physical condition or structural integrity of the gill provides an indirect assessment of the functional status of the respiratory system of the shrimp. Gill exams do not require sophisticated laboratory equipment beyond glass slides and a microscope. The methods are described in Brock and Main (1994).

Gut Content Examination Gut content exams are only done when a growth or survival problem is encountered. This evaluation can be done by an assessment of the relative degree of fullness of the abdominal intestine. Healthy shrimp feed continuously, so their gut contents should be relatively full, unless a suitable feed is not available or there is a health problem.


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Detecting Diseases and Diagnostic Techniques There are a variety of different diagnostic tests that are used to identify shrimp diseases. Examples of the diagnostic tests used for shrimp diseases include visual examination, microscopic and histological examination, tissue culture and serum neutralization, specific gene probes (DNA probes), ELISA (Enzyme Linked Imunosorbent Assay) and PCR (Polymerase Chain Reaction). Visual and microscopic examinations can often be done by the farm manager; however, the majority of these diagnostic techniques need to be done by a shrimp health professional or veterinarian.

Factors Leading to Losses and Disease Outbreaks During Growout The factors leading to losses and disease outbreaks include poor quality postlarvae, postlarval acclimation procedures, management strategies, human factors and environmental factors.

Poor Quality Postlarvae Farmers may have problems with weak or dying postlarvae when they are received at the farm. This can happen because the postlarvae were either incubating a disease before shipment or they were in a weakened condition prior to shipment. You can also see these problems when the larvae are exposed to damaging or toxic conditions during packing and transport. When there is a problem with poor quality postlarvae, the PLs should be examined using a microscope to determine the prevalence and severity of bacterial fouling, the prevalence of larval vibriosis and black spot disease, the hepatopancreas lipid level and gut-to-muscle ratio, the prevalence and severity of BP infection, and the prevalence of postlarvae with missing appendages or body parts, abdomen or appendage deformities. In addition to the microscopic evaluation, the larvae should be exposed to a stress test to assess the quality of the postlarvae. If disease symptoms are not present, the postlarvae may have been exposed to unsuitable environmental conditions. Selected physical and chemical water quality analyses (e.g., temperature, dissolved oxygen, total ammonia, pH, nitrite and carbon dioxide) should be measured and recorded on the water in five or ten shipment bags.

Postlarval Acclimation Procedures Poor postlarval performance can also result when the shrimp are exposed to inappropriate conditions during acclimation. You need to be sure and slowly acclimate the animals to the new environment in order to avoid stressing the animals (see Chapter 6). A careful review of the acclimation procedures and monitoring records is needed and appropriate corrective actions must be taken.

Management Strategies that Lead to Disease Problems Management strategies can also lead to disease problems. Overstocking or excessive densities can stress the animals and make them more susceptible to disease. High densities can also lead to injuries that develop into bacterial infections. In recirculating systems, the


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biofilter has to adequately filter out the ammonia that is being produced by the shrimp. When shrimp biomass exceeds the capacity of the biofilter, you will see a buildup in nitrite levels, which are highly toxic to the shrimp. Overfeeding can result in a heavy accumulation of organic detritus and hydrogen sulfide in the culture system, which can become toxic to the shrimp. Underfeeding will result in a considerable size variation in the shrimp population in a raceway.

Dietary Issues Several shrimp diseases are known to be caused by nutritional deficiencies. In recirculating raceway systems, it is critical to provide a nutritionally complete feed because there are no other organisms living in the system for the shrimp to consume. When there is a nutritional problem, it will likely result in a decrease in growth of all the shrimp in the system at once. Dietary deficiencies can also result in abnormal pigmentation, subcutical melanized lesions that are symmetrically distributed, muscle cramp, chronic soft cuticle, and slow growth in spite of a good appetite and weakness (Brock and Main 1994).

Human Factors Well-trained personnel are a critical element to the successful operation of a shrimp farm. The number of individuals employed on a farm varies depending on economic factors, farm size and production intensity. Worker failure to perform tasks according to directions or accidental errors can result in decreased shrimp production, animal mortality and disease outbreaks (Brock 1996). Employee training workshops and feedback discussions about improving procedures and the importance of daily tasks, are potentially effective means of reducing worker-related production losses.

Environmental Factors Changes in environmental conditions during growout will be detected by routine water quality analyses. The likely candidates are low dissolved oxygen during the night and elevated pH during the late afternoon. Low temperatures can stop or slow growth for extended periods of time.

Factors to Consider in Disease Prevention The factors that need to be considered in preventing diseases in shrimp farming include site selection and environmental condition, feed quality, biosecurity in relation to the source of seedstock, probiotics, transfer and handling procedures, accurate record keeping and training of personnel.

Site Selection & Environmental Conditions Site selection is one of the most important factors in determining the success or failure of an operation. Suitable environmental conditions must be considered as a part of site selection. Information on temperature range and suitable levels of various water chemistry parameters (see Chapter 8) need to be evaluated.


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Feed Quality A consistent source of high-quality feeds is needed to insure the shrimp health (see Chapter 7). Unfortunately, problems with feed ingredients are usually difficult to diagnose because of complicating infectious diseases or differences in densities or biomass loading within and between raceways or ponds. These differences may mask a problem with the feed. Feeds that are old or improperly stored can cause disease problems and may also have decreased levels of the essential lipid or water-soluble vitamins.

Biosecurity Maintaining biosecurity and preventing introduction of disease is the goal of all farm managers. Biosecurity can be defined as procedures that protect shrimp from contracting, carrying and spreading diseases and other health problems (Moss et. al. 1998). The best way to prevent introduction of disease is to obtain your seedstock from a supplier that has specific pathogen free (SPF) or high-health seedstock. When you bring in a group of postlarvae from a new source, those stocks should be quarantined and tested for pathogens at time of purchase or stocking. Employees must wash their hands before work if they have handled shrimp from another source. Something as simple as handling a box that contains frozen shrimp in a grocery store can result in the transfer of viruses from the frozen product to the shrimp on your farm. Human traffic can spread disease from one farm to another (Moss et. al 1998). If you visit another farm, be sure to shower and change clothing and shoes before coming to work.

Probiotics In aquaculture, probiotics is a term used to signify the addition of live, beneficial bacteria to the water or feed used in the cultivation of aquatic animals. Studies conducted using fish, oysters and shrimp have added to a growing body of evidence that the use of probiotics in hatchery and growout facilities is beneficial in helping to maintain a healthy environment (Browdy 1998; Moriarty 1998). In some instances, culture conditions have been adjusted to encourage the growth of fermenting bacteria, to the detriment of the nonfermenting bacteria. Shrimp culturists have added sugar or molasses to the water to promote the growth of fermenting Vibrio sp. over the more pathogenic nonfermenting species. Several commercial probiotic products are currently marketed for use to treat the culture water prior to and during stocking and cultivation of fish and shrimp. Most of these preparations are stabilized forms of various strains of Bacillus subtilus. Trials conducted at HBOI have suggested that probiotics can reduce bacterial septicemia in raceway cultures of shrimp. Inoculums of one million to one hundred million cells per milliliter of water have been recommended. The lower number may be used to inoculate newly stocked systems, whereas higher numbers are needed for the probiotic bacterium to gain dominance over older systems that have developed heterotrophic bacterial populations. It is recommended that the water be reinoculated every two to three weeks to maintain the desired bacterial population.


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Aside from their roll in displacing pathogenic bacteria, probiotic bacteria produce high levels of proteolytic enzymes that are useful in reducing the organic loading of the water introduced by the feed and fecal mater. This can reduce the loading of the biofiltration system.

Transfers and Handling Care must be taken during transfer and handling of shrimp to avoid stressing the shrimp and lowering their resistance to disease. In a multi-phase production system, the shrimp must be transferred from one section of the raceway to another several times during the growout period. To avoid losses during transfer from one section to the next, the staff need to be well trained. The shrimp must be carefully moved to avoid physical trauma and environmental extremes (e.g., high temperature, low dissolved oxygen).

Record Keeping Careful record keeping is a must. Monitoring production variables and environmental factors will provide information on how well the system and shrimp are doing. Changes in conditions can be detected and steps taken to alleviate problems.

Personnel Well-trained personnel is the key to operating a successful farm and disease prevention. Worker failure to perform tasks according to directions or accidental errors are common causes for decreased shrimp production. Regular evaluation by managers provides an effective, positive means for keeping workers on track.

Practical Approaches to Disease Control Once you discover that you have a disease problem there are a number of approaches that can be used to control the disease. The general tendency is for farmers and veterinarians to focus on identifying the “bug” or disease identification. Keep in mind that diseases are caused by a number of factors and that you need to carefully review all the factors in determining the cause of the problem. We do however, need to work closely with a veterinarian in order to identify and control disease problems. Good management practices can often be used to keep a disease under control and reduce the losses. In order to determine if the management strategies are effective, you need to carefully monitor the culture system as well as the health of the shrimp. Quarantine infected populations, as much as possible, to avoid introduction to clean shrimp growout systems. Careful monitoring is needed to make sure that sick populations do not infect healthy shrimp. When a disease problem occurs, you may be able to reduce the stress and complete the growout cycle by decreasing the density within a raceway. A number of diseases result from problems with the feed. Improvements in the diet can improve the health of the shrimp. In some cases, farmers have found that if they withhold feed for a period of time, the system will get in balance and the health will improve. Be sure to remove sick and dead individuals so that they are not consumed by the remaining stocks.


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Another approach to a disease outbreak is to shorten the production cycle and harvest the crop early. That will allow you to reduce your losses and get some income before the disease problem becomes more severe. Depending on the disease problem, it may be necessary to destroy the infected stocks. Remember to recognize and address the potential for human error. People influence both animal health and production levels. Successful operations depend on reliable decisions and appropriate activities. Production management skills and technical implementation are critical.

Eradication of Viral Diseases Once a viral disease is present, the only proven method of control is total cleanup or complete eradication of the viral disease. The clean-up process is a two step procedure involving eradication of the infected and potentially infected shrimp stocks, followed by the clean up of the facility itself. This procedure is time consuming and expensive. But when it is done correctly, it has been effective in preventing future disease problems. The procedures, disinfectants and equipment required to eradicate a viral disease are described in Bell and Lightner (1992).

Important Shrimp Pathogens

Overview Shrimp pathogens fall into seven major categories: viruses, bacteria, fungi, protozoans, rickettsia, nutritional, toxic and environmental diseases and diseases of unknown etiology. Viral disease outbreaks often result from stress factors, such as overcrowding, abnormal temperatures or low dissolved oxygen. The only way to prevent transfer of the virus is through strict quarantine procedures. There are NO known treatments for viral diseases. In order to diagnose a viral disease you need to consult with a veterinarian or expert in shrimp pathology. A variety of diagnostic techniques have been developed for the principal shrimp viruses, but they require extensive laboratory testing, which is expensive and time-consuming. Most bacterial infections result from extreme stress. The most common bacterial infection in marine shrimp is Vibrio. Vibrio infections often occur following environmental stresses or viral diseases and are not the primary disease problem. They have been successfully treated with chemicals and antibiotics; however, these treatments are only available with a veterinarians prescription in the U.S. Protozoans are found naturally in the culture environment and can cause mortalities, but they only become a problem for shrimp farmers when the environmental conditions are poor.

Common Disease Concerns During Growout


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Shrimp are exposed to a variety of opportunistic pathogens during growout and the common disease concerns during growout are listed in Table 9.1. Many of these pathogens are present in the environment and only become a problem when the shrimp are exposed to an environmental or biological stress. The remainder of this section will include a brief synopsis of the diseases or syndromes listed in Table 9.1, methods of diagnosis and control strategies. The diseases that are addressed are those diseases that will impact shrimp in raceway growout systems. This material is primarily adapted from Brock and Main (1994).

Infectious Hypodermal and Hematopoietic Necrosis Virus Infectious Hypodermal and Hematopoietic Necrosis Virus (IHHNV) is a parvo-like virus that is commonly found in cultured and wild L. vannamei stocks. Shrimp can be affected by this virus during the larval and adult stages. If shrimp are infected after the postlarval stage, the most obvious sign of the disease is cuticle deformities. Shrimp will become infected with IHHNV if they consume infected shrimp and can potentially be infected by contact with IHHNV contaminated water. Diagnosis of this disease must be done using laboratory test procedures. The most reliable means of identification is with specific gene probes that have been developed for IHHNV. These tests are usually done by a shrimp health professional. There are no treatments available for shrimp viruses such as IHHNV. The best control strategy is prevention by use of specific pathogen free (SPF) postlarvae.

Runt-Deformity Syndrome Runt-deformity syndrome (RDS) is believed to be caused by IHHNV (Wyban and Sweeney, 1991). RDS occurs during both nursery and growout stages in L. vannamei and primarily impacts farm yields and consequently affects production economics. Harvests contain large numbers of small shrimp. The major symptoms are variations in size distribution (e.g., coefficient of variation = 30-60%), cuticle deformities or mottled pigmentation on the majority of small shrimp, and slightly lower survival rates. The most reliable means of

Table 9-1: Common disease concerns associated with growout of L. vannamei.

Infectious Hypodermal and Hematopoietic Necrosis Virus (IHHNV) Runt-deformity syndrome Taura Syndrome Virus (TSV) White Spot Syndrome Virus (WSSV) Yellowhead Virus (YHV) Vibriosis Necrotizing hepatopancreatitis Mycobacteriosis Epicommensal fouling disease Black spot disease Gas bubble disease


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identification is with specific gene probes that have been developed for IHHNV. These tests are usually done by a shrimp health professional. There are no treatments available for shrimp viruses such as IHHNV. The best control strategy is prevention through the use of SPF postlarvae.

Taura Syndrome Virus TSV has had a major impact on the L. vannamei culture industry in Central and South America (Lightner 1995; Brock et. al. 1977). Shrimp are primarily affected during the juvenile stage by this noda-virus. When SPF or hatchery reared juvenile L. vannamei develop TSV, it is often fatal because the infection has a rapid onset and short course. TSV should be suspected in cases where rapid mortality of juvenile L. vannamei is associated with sickly animals that have soft exoskeletons and expanded chromatophores. Shrimp dying in the acute infection stage will be characteristically weak and disoriented, have a soft cuticle and expanded chromatophores. You can recognize the chronic form of TSV because the cuticle degenerates and has black spots or areas of melanization. Adult animals will either die from the infection or display the black spots and cuticle degeneration. TSV is diagnosed using histopathology, bioassay or with specific gene probes that have been developed for TSV. These tests are usually done by a shrimp health professional. There are no treatments available for shrimp viruses such as TSV. The best prevention strategy is to use SPF postlarvae. There are facilities that have ongoing breeding programs for resistance to TSV.

White Spot Syndrome Virus White Spot Syndrome Virus (WSSV) has had a significant impact on shrimp farming in Asia since 1993 (Flegel et. al. 1997). It was identified in cultured shrimp in Texas in 1995 and in South Carolina in 1997. The Texas and South Carolina populations were destroyed in order to prevent the spread of this serious disease to cultured and wild stocks. Research in South Carolina has suggested that WSSV may have come from wild populations. Over 40 species have been found to act as reservoirs for this virus, including lobsters and crabs. The symptoms of this disease vary between the eastern and western hemisphere (Lightner 1996). L. vannamei in the western hemisphere have few white spots, which is the primary symptom of the disease in the eastern hemisphere. In western shrimp, we see a reduction in feeding, a loosening of the cuticle and a pink to reddish coloration. Diagnosis is done by a health professional using histology or a specific gene probes that has been developed for WSSV. A new rapid detection method has been recently been developed for WSSV (Lightner 1999) that can be used with acutely infected shrimp. There are no treatments available for shrimp viruses such as WSSV. The best strategy is prevention by use of SPF postlarvae. If WSSV is suspected, all water exchange should be stopped and if the tests are positive, the stocks should be destroyed.

Yellowhead Virus Yellowhead virus (YHV) has caused major losses in P. monodon farms in southeast Asia since 1992 (Flegel et. al., 1997). In 1995, YHV was identified at a farm in Texas where P. setiferus were being cultured (Lightner 1996). The close proximity of shrimp processing


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plants that were handling shrimp from Asia to the farm with the disease problem, may have resulted in the infection diagnosed in the Texas shrimp. ). To our knowledge, there have been no other known cases of YHV in the U.S. Research studies have shown that YHV can infect and cause mortalities in many of the penaeid species, including L. vannamei. Symptoms include an initial increase in feeding rates, followed by cessation of feeding and death. The shrimp have a reddish tail, the cephalothorax is light yellow and the gills are white to pale yellow or brown in color. This virus must be diagnosed by a health professional using histopathology. Like other viruses, there is no treatment for YHV. The best strategy is prevention by use of SPF postlarvae.

Vibriosis Vibriosis has caused significant disease problems during growout of L. vannamei. However, these types of bacterial infections can often been controlled or prevented by reducing stressful conditions during growout. Vibriosis has been associated with stress factors such as handling, high densities, nutritional deficiencies, extremes in temperature, cuticle injuries, and elevated levels of ammonia, salinity or nitrogen. Environmental conditions that increase the density of a particular Vibrio spp. can often lead to vibriosis. It is commonly found concurrently with other viruses or microbes. The impact of vibriosis will vary depending on the severity of infection, but mortalities can exceed 70%. When infections are severe, dead and dying shrimp will be plentiful and animals will not be cannibalized. In less severe cases, shrimp may be eaten by the unaffected shrimp. The symptoms include extreme weakness (shrimp may lay on the bottom), disoriented swimming, increased pigmentation, partial cramping of the tail, diffuse muscle opacity of the abdominal musculature, or brown or black wounds on the cuticle. To prevent vibriosis in growout, you need to provide balanced culture conditions. Be sure that the shrimp are maintained in appropriate water quality conditions and that they are getting a good quality feed. Antibiotic-medicated feeds have been used to control vibriosis, however, this can only be done when the antibiotics are prescribed by a veterinarian.

Necrotizing hepatopancreatitis Necrotizing hepatopancreatitis (NHP), previously known as Texas pond mortality syndrome, is caused by a Gram-negative, bacteria that attacks the cells of the hepatopancreas. NHP occurs in juvenile and subadult L. vannamei and causes the shrimp to stop feeding and growing. Other symptoms include a soft exoskeleton, generalized surface fouling, weakness and slow death. NHP is diagnosed by histopathology or with a specific gene probe. Losses have been reduced through early detection and rapid application of oxytetracycline medicated feed. This feed must be administered under the guidance of a veterinarian. NHP does not seem to occur when salinities are below 10 ppt and therefore, may not be a problem in freshwater systems.

Mycobacteriosis Mycobacteriosis has been occasionally identified in L. vannamei broodstock populations and can occur in juvenile and subadult shrimp. Mycobacterium spp. are slow growing, acid-fast bacteria that are present in the natural environment. Like other bacterial infections, it is


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probably associated with poor conditions (i.e., high numbers of bacteria) in the culture environment. This bacteria can infect human cuts or abrasions. Mycobacteriosis can only be diagnosed using specialized laboratory methods. There are no outward behavioral or physical signs; however, chronic infections can result in mortalities. Although the mode of transmission is unknown, it is probably transferred by ingestion or wound contamination. There are no known treatments for mycobacteriosis.

Epicommensal fouling disease Epicommensal fouling disease is a condition caused by a variety of agents and affects all life stages of L. vannamei. It occurs when respiratory, feeding or locomotory functions are impaired by excessive colonization of the cuticle surface by bacteria, protozoans, diatoms or blue-green algae. The infestation usually involves a mixed population of organisms with one dominant species. In juvenile and subadult shrimp the gills are commonly affected, which can inhibit respiration. Shrimp appear outwardly normal, but die rapid during or immediately following exercise, handling or exposure to low oxygen conditions. The symptoms of epicommensal fouling disease include, reduced growth, reduced feed consumption, gill discoloration, abnormal swimming behavior, intolerance to exercise or low dissolved oxygen. The agents of this condition are commonly found in the culture environment, but they will increase in numbers and cause disease problems when the environmental conditions are not suitable. High densities and high concentrations of nutrients have been associated with the occurrence of this condition. Epicommensal fouling disease can be controlled by increasing the frequency and amount of water turnover, improving water circulation, decreasing density, biomass or organic loading and providing a nutritionally balanced feed.

Black spot disease Black spot disease occurs in all life stages of L. vannamei and is a condition where there are one or more brown or black spots on the cuticle. It can occur on any cuticle-covered body part and the size, shape and number of lesions or spots can vary. The spots occur as a defensive response to a break in the cuticle surface. The location of the spots on the body can provide a clue about the initial cause of the disease. In tank raised shrimp, a high frequency of black spots on the dorsal surface of the third abdominal segment suggests trauma to the cuticle from contact with the tank surface during the rapid backward escape. Multiple lesions randomly distributed over the body may indicate TSV or it can result from prolonged exposure to anoxic conditions or otherwise unhealthy water quality conditions. A few black spots mainly on the lateral body surface of the shrimp indicates that the shrimp have been transferred or held in crowded conditions and have damaged each other. Diagnosis and understanding of Black Spot disease requires observing the distribution pattern of the lesions and assessing the number of individuals affected. The best control is prevention of black spot disease. However, if the problem is a result of overcrowding, it can be reduced by lowering the densities. Lesions associated with poor water quality conditions may respond once environmental quality is improved.


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Gas Bubble Disease Gas bubble disease is a potentially lethal condition caused by the supersaturation of the water with air. The condition generally occurs when air is forced into the water under pressure. While gas bubble disease has not been much of a problem in pond culture and flow through systems it can become a problem in recirculating systems. The source of the problem is often traced to faulty plumbing of the filtration system. Often people neglect to glue the PVC fittings on the suction side of the pump. It is reasoned that since it is not under pressure, it will not separate, and not gluing makes it easier to reroute the piping if needed. However, air may be sucked in around loose fitting joints, mixing with the water, where it goes into solution under pressure. This is especially true if the water is pumped through mechanical filters, which can produce considerable backpressure on the pump. Another source of air entering the water line is through a venturi setup by low water levels at the entrance to the standpipes. The saturated water is then returned to the tank or raceway. Rapid warming of the water may also produce gas bubble disease. If the temperature increases, the blood of a shrimp will hold less nitrogen in solution allowing gas bubbles to form. This, however, is a rare occurrence that most often happens when a pump overheats. The saturated air is taken up by the shrimp through their gills. The less soluble nitrogen fraction of the air is released from the blood where it is free to form small bubbles, generally in the gill region. There it blocks the flow of blood and thus the exchange of oxygen and carbon dioxide. Gas bubble disease is generally detected by erratic, disoriented swimming and by the appearance of white discolored gill tissue. Looking at gill tissue under a microscope, using low-resolution magnification, will reveal numerous gas bubbles in the gill filaments. Gas bubble disease is controlled through the prevention of exposure to conditions of nitrogen gas supersaturation. For a recirculating system, weekly saturometer readings for total gas pressure at several points in the system is a preventative tactic for early detection of nitrogen gas supersaturation. If a problem is detected, the source must be identified and corrected.

Dissolved Oxygen Crisis Dissolved oxygen crisis occurs when the oxygen demand exceeds the tank system inputs. This often occurs late in the growout cycle when you are feeding large quantities of feed and have high biomass of shrimp in the system. It can also occur in recirculating systems when there is a power failure that interferes with supplemental aeration and water exchange. When shrimp are first exposed to low dissolved oxygen conditions, they will display increased activity in the form of slow surface swimming. Affected shrimp swim aimlessly, have a depressed escape response and have diffuse opacity (milky loss of transparency) of the abdomen. This problem is controlled by having backup for power failures and a monitoring system to determine dissolved oxygen levels. Once the oxygen crisis is corrected, the surviving shrimp will usually improve their condition.


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Literature Cited Bell, T.A. and D.V. Lightner 1992. Shrimp facility clean-up and restocking procedures.

Cooperative Extension, College of Agriculture, University of Arizona, Tucson, Arizona, USA.

Brock, J.A. 1992. Current diagnostic methods for agents and diseases of farmed marine

shrimp. In: Fulks, W. and K.L. Main (Eds.) Diseases of cultured penaeid shrimp in Asia and the United States. The Oceanic Institute, Honolulu, Hawaii. pp. 209-231.

Brock, J.A. 1996. Some consideration of human factors as variables in disease events in

marine aquaculture. In: Main, K.L. and C. Rosenfeld (Eds.) Aquaculture health management strategies for marine fishes. The Oceanic Institute, Honolulu, Hawaii. Pp. 157-162.

Brock, J.A. and K.L. Main 1994. A guide to the common problems and diseases of cultured

Penaeus vannamei. World Aquaculture Society, Baton Rouge, Louisiana, USA. Brock, J.A., R.B. Remedios, D.V. Lightner and K. Hasson 1997. Recent developments and

an overview of Taura Syndrome of farmed shrimp in the Americas. In: Flegel, T.W. and I.H. MacRae (Eds.). Diseases in Asian Aquaculture III. Fish Health Section, Asian Fisheries Society, Manila, Philippines. Pp. 275-283.

Browdy, C.L. 1998. Recent developments in penaeid broodstock and seed production

technologies: Improving the outlook for superior captive stocks. Aquaculture 164 (1-4): p. 3-21.

Flegel, T.W., S. Boonyaratpalin and B. Withyachumnarnkul 1997. Progress in research on

yellow-head virus and white-spot virus in Thailand. In: Flegel, T.W. and I.H. MacRae (Eds.). Diseases in Asian Aquaculture III. Fish Health Section, Asian Fisheries Society, Manila, Philippines. Pp. 285-295.

Fulks, W. and K.L. Main 1992. Diseases of cultured penaeid shrimp in Asia and the United

States. The Oceanic Institute, Honolulu, Hawaii. 293 pp. Lightner, D.V. 1988 Diseases of cultured penaeid shrimp in the Americas In: Sindermann,

C.J. and D.V. Lightner (Eds.). Disease diagnosis and control in North American Marine Aquaculture. Second Edition. Elsevier Scientific Publishing Co., Amsterdam. Pp. 8-113.

Lightner, D.V. 1993. Diseases of cultured penaeid shrimp. In: McVey, J.P. (Ed.) CRC

Handbook of Mariculture Crustacean Aquaculture Vol. 1, Second Edition. CRC Press, Boca Raton, Florida. Pp. 393-486.


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Lightner, D.V. 1995. Taura Syndrome: an economically important viral disease impacting the shrimp farming industries of the Americas including the United States. In: Proceedings of the Ninety-ninth Annual Meeting, USAHA, Reno, NV, USA. Pp. 36-52.

Lightner, D.V. 1996. A Handbook of Shrimp Pathology and Diagnostic Procedures for

Diseases of Cultured Penaeid Shrimp. World Aquaculture Society, Baton Rouge, Louisiana, USA.

Lightner, D.V. 1999. Rapid method for diagnosis of WSSV in whole tissue mounts.

Addendum to: A Handbook of Shrimp Pathology and Diagnostic Procedures for Diseases of Cultured Penaeid Shrimp. World Aquaculture Society, Baton Rouge, Louisiana, USA.

Moriarty, D.J.W. 1998. Control of luminous Vibrio species in penaeid aquaculture ponds.

Aquaculture 164 (1-4): 351-358. Moss, S.M., W.J. Reynolds and L.E. Mahler 1998. Design and economic analysis of a

prototype biosecure shrimp growout facility. In: Moss, S.M. (Ed.) Proceedings of the U.S. Marine Shrimp Farming Program Biosecurity Workshop February 14, 1998. The Oceanic Institute. Pp. 5-17.

Wyban, J.A. and J.N. Sweeney 1991. Intensive shrimp production technology – The

Oceanic Institute Shrimp Manual. Oceanic Institute, Honolulu, Hawaii, USA. 158 pp.

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Chapter 10 Economics of Shrimp Culture in

Recirculating Aquaculture Systems

by Peter Van Wyk


Introduction Demonstration by Harbor Branch Oceanographic Institution of the technological feasibility of culturing the marine shrimp, Litopenaeus vannamei (also known as Penaeus vannamei), in high density, freshwater recirculating aquaculture systems has generated a tremendous amount of interest. Many people are looking at this new technology as a potential investment opportunity. The purpose of this analysis was to determine the economic feasibility of culturing marine shrimp in freshwater recirculating aquaculture systems using the data gathered by Harbor Branch during a one-year demonstration study sponsored by the Florida Department of Agriculture and Consumer Services (FDACS Contract No. 4520). In addition, an analysis was made of the sensitivity of the enterprise profitability to selected economic and production variables.

Baseline Assumptions A spreadsheet model was developed to evaluate the economic feasibility of producing Litopenaeus vannamei in freshwater recirculating production systems. The model is based on the Harbor Branch Oceanographic Institution (HBOI) shrimp production facilities. We have incorporated into the model actual construction and production costs and system productivity data in an effort to make the model as realistic as possible.

Facility Description The economic model describes a hypothetical enterprise consisting of twelve greenhouses, instead of the three that are currently in operation at HBOI. This was done to take advantage of the economies of scale associated with a larger production facility. The two most important economies of scale are related to the manager’s salary and the cost of feed. In a small operation the manager’s salary is divided over a relatively small amount of production. As a result, the cost per pound of shrimp produced goes up significantly. In a larger operation the manager’s salary is divided over a much larger amount of production, and therefore has a much smaller impact on the cost per pound produced. The second important economy of scale a large facility can take advantage of is the ability to buy feed in truckload quantities. Freight costs on less than truckload quantities of feed may add as much as $0.15 per pound to the cost of the feed. The feed itself is often cheaper when purchased in

Chapter 10 - Economics of Shrimp Culture in Recirculating Systems

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truckload quantities. Most manufacturers have a sliding price scale, according the volume of feed purchased. Feed purchases in excess of 20 tons may be as much as $0.05/lb cheaper than feed purchased in 1 ton lots. The combined savings associated with purchasing feed by the truckload may be as much as $0.20 per pound of feed. This may reduce the overall cost of production by $0.40/lb, assuming a feed conversion rate of 2.0. The economic model is based on a production system housed in twelve 30 ft x 150 ft greenhouses. The greenhouses are built from a commercial greenhouse kit consisting of a frame made from 14-gauge galvanized pipe, and a roof covering made from a double layer of 6-mil polyethylene plastic sheeting (see Chapter 3). A small inflator fan fills the space between the two layers of plastic with air, creating an insulating air space between them. Each greenhouse is covered with a 40 ft x 150 ft 95% shade cloth. The gable ends consist of a treated lumber frame covered with 6-mil plastic sheeting. The greenhouses are ventilated using two 54-inch 1.5-hp exhaust fans located on one gable end and two 57-inch air inlet shutters located on the other gable end. The fans and shutters are thermostatically controlled. Each greenhouse is provided with a 300,000 BTU propane gas heater and air circulation fan to maintain suitable temperatures during the winter months. This system is sized to be able to maintain inside temperatures up to 25°C higher than outside temperatures. The greenhouses used in the HBOI/FDACS demonstration project were not heated. However, other greenhouses at HBOI are equipped with this type of heating system, so reliable data was available relative to the cost and performance of propane space heaters. Other types of heating systems could be used, and may be more economical. Generally it is more cost-effective to heat the water directly, rather than to heat the air. The model assumes each greenhouse houses two three-phase production systems similar to the System B design described in Chapter 4. The size of the tanks, filters, pumps, blowers, and ventilation system have been scaled up in the model to make sure that they give the same performance as the corresponding units in the 96-foot greenhouses. The cost of the system in the model was determined from prices actually paid for equipment items and quoted prices for equipment items that differed from those actually used in the demonstration project. Each culture tank in the model system measures 14.5 ft x 140 ft divided into three sections, each section corresponding to a different phase in the production process. The first phase is carried out in a 14 ft x 14.5 ft section at the upper end of the raceway. This section occupies approximately 10% of the culture area. Phase II is carried out in the middle section of the raceway, which measures 14.5 ft x 43 ft (30.7% of the culture area). The shrimp are grown up to market size in Phase III, which measures 14.5 ft x 83 ft (59.3% of the culture area). The culture tanks consist of a wooden frame supporting a black 30-mil high-density polyethylene liner (Chapter 5). The wooden frame is one board width high and is built using 2”x12” pressure-treated lumber boards supported by galvanized pipe set vertically in a concrete anchor. The vertical pipe supports are set on 6-ft centers. The ground inside of the wooden frame is excavated to a depth of 1.5 foot with a berm (1:1 slope) extending from the


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bottom of the wooden frame to the floor of the tank. When filled, the culture tanks are 22’ deep. The solids filter consists of a 6-ft diameter cylindro-conical tank filled with small plastic filter beads. The beads are hollow polyethylene cylinders with short, radiating fins to enhance the surface area of the bead. The water enters the filter near the bottom and flows up through the beads and exits by gravity through a slotted pipe near the water surface. The filtered water flows into an adjacent sump that functions as the biofilter. Solid wastes trapped in the bead bed are flushed twice a week by emptying the tank. The biofilter consists of an aerated, submerged bed of biofilter media contained in a cage constructed of PVC pipe and 1/4” extruded plastic screen material. The biofilter media consists of the same polyethylene cylindrical beads as was used in the solids filter. This biofilter media has a specific surface area of 259 ft2/ft3. The biofilter media and cage are contained in a rectangular 6 ft x 8 ft x 4 ft polyethylene tank. The water is circulated using a 3/4-hp, low head, high volume centrifugal pump that delivers 130 gpm of flow at 10 feet of head. At this flow rate the water is recirculated through the water treatment system approximately every 200 minutes. Air is supplied to the system by a 3.5-hp regenerative blower, which delivers 150 cfm at a pressure of 50 inches of water. One blower is required for every two greenhouses. A 16-hp gasoline powered blower serves as a backup for each of the electrical blowers. These blowers are equipped with a pressure switch that starts up the backup blower in the event of a power loss. The air is delivered into the culture tanks through 54 3”x1” medium pore fused silica diffusers. An additional 10 diffusers are positioned in a grid located in the bottom of the biofilter tank to aerate and tumble the biofilter media. The source of water for the shrimp production facility is a freshwater well with two 3-hp centrifugal well pumps. A well water pretreatment system is required for every six greenhouses. The pretreatment system consists of a degasser, a biofilter tank, a 5,500-gallon water storage tank, and a 1.25 hp centrifugal water supply pump. The raw well water is pumped to the top of a degassing column filled with plastic media. Air is pumped through the degassing tower by a high volume 0.25 hp air blower. Supersaturated gases such as hydrogen sulfide and carbon dioxide are removed in the degassing column, and dissolved oxygen levels are increased. The water leaving the degassing column flows into an aerated submerged bed biofilter consisting of a 12-ft diameter fiberglass tank filled with barrels of crushed oyster shell. Water is circulated through the oyster shell beds powered by airlift pumps inserted into the beds. The bed is designed to reduce Total Ammonia Nitrogen concentrations in the well water from 1.0 mg TAN/liter to less than 0.05 mg TAN/liter. The water flows from the biofilter to the water storage reservoir. The water in the storage reservoirs is recirculated through a sand filter and is available on demand to the shrimp culture tanks.


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The effluent from the solids filter drains into a concrete sump/lift station where a 1/2-hp trash pump is located. The trash pump is fitted with a mercury float switch that activates the pump when the sump begins to fill. The wastewater is pumped from the solids sump to a 1-acre retention pond. Chlorine tablets are placed in this sump to kill any escaping shrimp. There is one concrete drain/chlorination sump for every four greenhouses.

Investment Requirements

Land The land requirement for twelve greenhouses is approximately two-and-a-half acres. An additional 1 to 1.5 acres are needed for a retention pond, and perhaps another acre for water treatment and storage tanks, feed sheds, office, and parking. A minimum of five acres would be required to house a 12-greenhouse production facility. The land should be zoned either for agriculture or for industry. The most important requirement is the quality of wellwater that is available (see Chapter 8). Minimally, the water should have a chloride concentration of at least 300 mg/l, and a Total Hardness of 200 mg/l. The water should be free from pesticides. The cost of land varies widely, depending on zoning, proximity to population centers and surrounding land use. For the purposes of this model I have assumed a land price of $2,500 per acre. The overall investment in land is $12,500. This model assumes access roads and electrical power are already available at the location. The land cost would be significantly higher if it were necessary to build an access road or bring power lines in to the site.

Buildings and Improvements The shrimp culture tanks and recirculating water treatment systems are housed in 30-foot x 150-foot quonset-style greenhouses. These greenhouses are built using commercially available kits. The kit comes complete with all the necessary frame hardware, two 54" 11/2-hp exhaust fans, two 57" air inlet shutters, 2 NEMA 4X thermostats, inflator fans, double-polyethylene roof and sidewall covering, a man-door and an 8'x8' sliding door. Additional lumber is needed to build the frame for the gable ends. The cost for the kit is $7,095/greenhouse ($1.58/square foot). Site preparation and construction costs add another $5,000 to the cost of a greenhouse. Each greenhouse must be equipped with a 95% shade cloth (40’ x 150’), at a price of $1,500/shadecloth. Installation of the electrical wiring costs approximately $5,000/greenhouse. Plumbing costs are placed at $2,500 per greenhouse. A 20’ x 15’ metal building with a slab floor and harvest sump is required for acclimation and quarantine of newly purchased postlarvae. The acclimation/quarantine building houses two 1,500 liter fiberglass, U-shaped acclimation tanks, each supplied with air from a 1/3 hp blower. The freshwater for acclimation is fed through an elevated head tank. The water level in this tank is maintained at a constant depth by means of a float valve. This building will cost approximately $3000, excluding the tanks and blower.


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A rodent-proof, air-conditioned feed storage building capable of storing up to 30 tons of feed is a necessity. A 1,000 square foot metal frame building built on a 6-inch concrete slab can be built for $3,000, not including the air conditioner. Well-installation is assumed to cost $1,000, but this can vary depending on the depth of the well and the type of material the well digger must drill through. Three one-quarter acre retention ponds, connected in series by 6-inch overflow pipes, can be built for about $1,500. The overall cost of buildings and improvements for the wells, retention ponds, greenhouses and support buildings comes to $261,640. An additional $60,000 must be spent in Years 6-10 to replace polyethylene greenhouse covers and shade cloth. Table 10-1 summarizes the investment requirements for buildings and improvements.

Tanks & Sumps Table 10-2 summarizes the investment requirements for tanks and filtration equipment, which are described in detail in Chapter 4. Each greenhouse houses two culture tanks (raceways) which measure 14.5’ x 140’. Each of these raceways is partitioned into a nursery section, an intermediate growout section, and a final growout section. The lumber, galvanized pipe, and hardware for the raceway frame costs $1,200/raceway. A 23-ft wide roll of 30-mil HDPE liner material sells for $0.23/ft2 (delivered price). The total cost for lining one three-phase raceway is $900, or $1,800/greenhouse. Combining the cost of the frame and the liner, a single three-phase raceway costs $2,100. The polyethylene solids filter and biofilter tanks each cost $500. Each of these tanks is loaded with a polyethylene biofilter media called Kaldnes media. The delivered price of Kaldnes media is $28.55/ft3, or about $1,000/m3 of media. Each system requires 1.5 cubic meters of media. The cost of the biofilter media is $1,500 per system.

Table 10-1: Summary of the capital costs for initial purchase and installation of buildings. Four greenhouses are built in each of the first three years of the project.

Buildings and Improvements

Item Units Price Value Year 1

Value Year 2

Value Year 3

Retention Ponds acre $1,500 $1,500 $ 0 $ 0 Well Installation ea $1,000 $1,000 $ 0 $ 0 Feed & Storage Shed ea $3,000 $3,000 $ 0 $ 0 Acclimation Greenhouse ea $3,000 $3,000 $0 $0 Greenhouse Kit, complete ea $7,095 $28,380 $28,380 $28,380 Site Preparation greenhouse $1,000 $4,000 $4,000 $4,000 Greenhouse Construction Costs greenhouse $4,000 $16,000 $16,000 $16,000 Shade Cloth greenhouse $1,500 $6,000 $6,000 $6,000 Electrical Installation greenhouse $5,000 $20,000 $20,000 $20,000 Plumbing greenhouse $2,500 $10,000 $10,000 $10,000

$92,880 $84,380 $84,380


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Two 1,500-liter fiberglass U-bottom acclimation tanks supported in a frame constructed of 2"x 4" lumber will cost $1,500 per tank. A 300-liter polypropylene head tank elevated above the acclimation tanks will cost about $500, including the lumber for the support tower, and a float valve to maintain a constant level of freshwater in the tank. The water pretreatment system consisting of a degassing column, biofilter, and water storage reservoir tank represents a significant expense, totaling $8,500 per system. One system is needed for every six greenhouses. One water pretreatment system is built in each of the first two years of the project. It is conceivable that some sites will have adequate water quality straight from the well. This might allow the pretreatment degassing column and biofilter to be eliminated. The water storage reservoir would still be a required piece of equipment. The total cost of tanks and sumps for the facility comes to $135,400. An additional $21,600 will be required during Years 6-8 for replacement of the raceway tank liners.

Machinery & Equipment Machinery and equipment capital costs are summarized in Table 10-3. The gas heating systems for the greenhouses represent a significant expense. One gas heater is required for each of the twelve greenhouses. The cost for a single heater is $3,275, so the total initial investment in heaters would be $39,300. This cost is spread evenly over the first three years of the project. The economic life of the heaters is expected to be 7 years. An additional $13,100 will be spent in each of Years 8, 9, and 10 of the project to replace heaters. Other options for heating the greenhouses are also available, including propane powered plate heat exchangers, electric immersion heaters, and solar water heating systems. A separate analysis

Table 10-2: Summary of the initial capital investment requirements for tanks and filtration equipment.

Tanks and Filtration Equipment


Value Year 2

Value Year 3

Degassing Column + Media ea $ 1,500 $1,500 $1,500 $ 0 Pretreatment Biofilter Tank ea $ 1,500 $1,500 $1,500 $ 0 Reservoir Tank ea $ 5,500 $5,500 $5,500 $ 0 Drain/Chlorination Sump ea $ 1,500 $1,500 $1,500 $1,500 Growout Tank Lumber & Hardware raceway $ 1,200 $9,600 $9,600 $9,600 Growout Tank Liners ea $ 900 $7,200 $7,200 $7,200 Acclimation Tanks (1500 liters) ea $ 3,000 $3,000 $ 0 $ 0 Acclimation Head Tank ea $ 500 $ 500 $ 0 $ 0 Solids Filter Tank ea $ 500 $4,000 $4,000 $4,000 Biofilter Tank ea $ 500 $4,000 $4,000 $4,000 Biofilter Media cu.m. $ 1,000 $12,000 $12,000 $12,000

$50,300 $46,800 $38,300


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would be required to determine which of these options would best balance between performance and overall cost (capital cost + operating cost). The low head, high volume 3/4-hp centrifugal pumps ($800) cost more than twice as much as high head, low volume 2-hp centrifugal pumps ($375) that deliver roughly the same volume at the required 10 feet of head pressure. However, over the three-year life of the pump, a low head pump will save over $3,000 in electrical costs. Nevertheless, the capital investment for the pumps is high. With two pumps required per greenhouse, a total of 24 pumps are needed. The economic life of the pump is 5 years, so 8 new pumps will need to be purchased in Years 6, 7, and 8 at a cost of $6,400/year. One 3.5-hp regenerative blower is required for every 2 greenhouses and one 16-hp gasoline powered backup blower is required for every 4 greenhouses. Each of the 3.5-hp blowers costs $972, so the total investment for blowers is just under $4,000. Three gasoline-powered blowers will be required at a cost of $3,269 each. The 1/4-hp regenerative blower for the acclimation tanks will cost $350. The initial investment for machinery and equipment $135,400 spread over the first three years of the project. Over the remaining 7 years of the 10 year planning horizon, an additional $127,520 will be required for replacement of old equipment.

Table 10-3: Summary of the capital costs for initial purchase of machinery and equipment. Machinery & Equipment


Value Year 2

Value Year 3

Pickup Truck, 1/2 ton ea $ 20,000 $ 20,000 $ 0 $ 0 Gasoline Powered Blower, 11 hp ea $ 3,269 $ 3,269 $ 3,269 $ 3,269 2 HP Well Pump ea $ 783 $ 783 $ 783 $ 0 2 hp Sump Pump for Lift Station ea $ 400 $ 400 $ 400 $ 400 0.25 HP Air Blower for Degasser ea $ 200 $ 200 $ 200 $ 0 1.25 HP Centrifugal H2O Supply Pump ea $ 800 $ 1,600 $ 1,600 $ 0 1/4 HP Acclimation Blower ea $ 350 $ 350 $ 0 $ 0 3.5 HP Blower ea $ 972 $ 1,944 $ 1,944 $ 1,944 1.5 HP Centrifugal Pump ea $ 800 $ 6,400 $ 6,400 $ 6,400 Gas Heating System ea $ 3,275 $ 13,100 $ 13,100 $ 13,100 Air Conditioner ea $ 600 $ 1,200 $ 0 $ 0 D.O. Meter ea $ 650 $ 1,300 $ 650 $ 650 Water Quality Test Kit ea $ 450 $ 450 $ 0 $ 0 Insulated Harvest Tote ea $ 500 $ 500 $ 500 $ 500

$ 51,496 $ 28,846 $ 26,263


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Office Equipment In Year 1 a computer, printer, facsimile machine, copying machine, telephone, and office furniture are purchased for the office at a total cost of $4,850 dollars. The office equipment is replaced after 5 years, while the furniture is replaced after 10 years. The costs for all office equipment are summarized in Table 10-4.

Total Investment Requirements The total capital investment requirements for the entire 10-year planning horizon are summarized in Table 10-5. Initial purchase of the required capital items during the 3-year construction phase of the project will cost $517,507. An additional $212,470 will be spent in Years 4-10 to replace worn out equipment.

Table 10-4: Capital costs for office equipment. Office Equipment


Value Year 2

Value Year 3

Personal Computer ea $ 1,200 $1,200 $ 0 $ 0 Printer ea $ 250 $250 $ 0 $ 0 Fax Machine ea $ 300 $300 $ 0 $ 0 Copying Machine ea $ 1,500 $1,500 $ 0 $ 0 Office Furniture office $ 1,500 $1,500 $ 0 $ 0 Telephone ea $ 100 $100 $ 0 $ 0

$4,850 $ 0 $ 0

Table 10-5: Capital cost summary for a 12-greenhouse facility for producing marine shrimp in recirculating aquaculture systems.

Item Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year10 New Investment Agricultural Land $12,500 $0 $0 $0 $0 $0 $0 $0 $0 $0 Buildings & Improvements $92,880 $84,380 $84,380 $0 $0 $0 $0 $0 $0 $0 Tanks and Filtration Equipment $50,300 $46,800 $38,300 $0 $0 $0 $0 $0 $0 $0 Machinery & Equipment $51,496 $28,846 $26,263 $0 $0 $0 $0 $0 $0 $0 Office & Office Equipment $4,850 $0 $0 $0 $0 $0 $0 $0 $0 $0

Total Investment $212,026 $160,026 $148,943 $0 $0 $0 $0 $0 $0 $0

Capital Replacement Buildings & Improvements $0 $0 $0 $6,000 $6,000 $6,000 $6,000 $12,000 $12,000 $12,000 Tanks & Filtration Equipment $0 $0 $0 $0 $0 $7,200 $7,200 $7,200 $0 $0 Machinery & Equipment $0 $0 $0 $450 $4,783 $40,377 $17,744 $28,600 $17,883 $17,683 Office Equipment $0 $0 $0 $0 $0 $3,350 $0 $0 $0 $0

Total Investment $0 $0 $0 $6,450 $10,783 $56,927 $30,944 $47,800 $29,883 $29,683

Total Capital Investment $212,026 $160,026 $148,943 $6,450 $10,783 $56,927 $30,944 $47,800 $29,883 $29,683


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Process Description and Production Assumptions The economic model assumes shrimp will be raised using a three-phase production strategy. In a three-phase system, the growout is divided into three periods, each period lasting for approximately one third of the total growout time period. In this case the nursery phase will last 58 days, while the intermediate and final growout phases will each last 61 days, giving a total growout time of 180 days. The nursery phase will lie fallow an average of 3 days between harvest and subsequent restocking. With this approach, market size shrimp will be harvested from the final growout section every 61 days. In one year a single three-phase system will produce 6 crops of shrimp. The three-phase system allows for higher production levels than a single-phase system because in a three-phase system the biomass of each raceway at the time of stocking is much closer to the final harvest biomass. Raceway space is used more efficiently than in a single-phase system in which raceways are maintained at low densities throughout the early part of the growout cycle. Another advantage of the three-phase system is that it provides more frequent harvests of marketable shrimp. By staggering the stocking schedule, the 24 raceways in this production system allow for harvests of market-size shrimp every 2 to 3 days. Table 10-6: Key production assumptions. Production Assumptions

Item Nursery Intermediate Final

Number of Production Units/Greenhouse 2 2 2 Rearing Unit Length (ft) 14.0 43.0 83.0 Rearing Unit Width (ft) 14.5 14.5 14.5 Number of PLs Purchased per Crop 28,800 Acclimation Survival 85% Number of Acclimated PLs Stocked in Nursery 24,480 Percent Survival - Phase (%) 80% 85% 90% Number of Days to Transfer or Harvest 58 61 61 Number of Fallow Days 3 Number of Shrimp Surviving to End of Phase 19,584 16,646 14,982 Percent Survival - Overall (%) 80% 68% 61% Density at Beginning of Phase (shrimp/m2) 1,351 348 150 Density at End of Phase (shrimp/m2) 1,081 296 135 Density at End of Phase (kg/m3) 1.68 2.43 2.46 Size of Shrimp at End of Phase 1.5 8.2 18.1 Total Weight of Shrimp at End of Phase (kg) 29 137 271 Cumulative Feed Conversion Ratio (end of phase) 1.72 1.77 1.76


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For each crop, 28,800 postlarval shrimp (PL8) are purchased and held for one week in the acclimation/quarantine tanks. Assuming a survival of 85% in the acclimation system, a total of 24,480 freshwater acclimated postlarval shrimp (PL15+) will be stocked into the nursery section for each crop. The stocking density in the nursery section will be 1,351 shrimp/m2. The shrimp spend 58 days in the nursery section. Growout temperatures will be maintained throughout the year between 28-32°C. Based on the growth rates observed in the HBOI demonstration systems, the shrimp will weigh approximately 1.5 grams at the end of the 8-week nursery period. Survival to the end of the nursery phase is expected to be about 80%. The intermediate and final growout phases last 61 days apiece. Survival is expected to be 85% during the intermediate phase of the production cycle, and 90% during the final growout phase, giving an overall survival (PL to final harvest) of 61%. The shrimp are expected to average 8.2 grams at the end of the intermediate phase. The final harvest weight of the shrimp after 180 days of growout is expected to be 18.1 grams, which corresponds to a 36-40 tail count per pound. The final harvest density of shrimp is expected to be 135 shrimp/m2, or 2.45 kg/m2 (4.8 kg/m3). Table 10-6 provides a summary of the important assumptions regarding the growout process.

Production Schedule The cash flow model assumes the first three years are building years. Four greenhouses are built in each of the first three years. Greenhouse construction takes place during the first quarter of the year. One greenhouse per month is completed and stocked in March, April, May, and June of each of the first three years of the project. Table 10-7 summarizes the production schedule for the first four years of the project. The production cycle for any given phase of the production process lasts 61 days, so each phase completes 6 complete production cycles in a one year period. The production schedules for the post-construction years of the project (Years 4 -10) are nearly identical. In these years, 144 complete production cycles are completed per year over the whole facility (12 greenhouses x 2 raceways/greenhouse x 6 cycles/ raceway/year = 144 cycles/year). Table 10-7: Production Schedule for the first four years of the project. The production

schedule for subsequent years is the same as that for Year 4. Production Schedule

Year Year 1 Year 2 Year 3 Year 4-10 Raceways Stocked 38 86 134 144 Raceways Harvested 14 64 112 144 Phase I Production Cycles 34 83 120 144 Phase II Production Cycles 26 75 124 144 Phase III Production Cycles 18 67 104 144


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Table 10-8: Summary of production inputs and outputs. Farm Production and Inputs Used

Farm Production Quantity Year 1

Quantity Year 2

Quantity Year 3

Quantity Years 4-

10

Production Cycles Completed Phase I 33.6 83.0 132.5 143.6 Phase II 25.6 75.0 124.5 143.6 Phase III 17.6 67.0 116.5 143.6 Stocking No. of Raceways Stocked 38 86 134 144 No. of PLs Stocked Per Raceway/Crop 28,800 28,800 28,800 28,800 Total No. of PLs Stocked Per Year 1,094,400 2,476,800 3,859,200 4,147,200 Harvest No. of Raceways Harvested During Year 14 64 112 144 Ave. No. of Shrimp Harvested/Raceway/Crop 14,982 14,982 14,982 14,982 Ave. Wt. of Shrimp at Harvest (g) 18.1 18.1 18.1 18.1 Total Wt. of Shrimp Harvested/Raceway/Crop (kg) 271 271 271 271 Total Wt. of Shrimp Harvested During Year (kg) 3,787 17,314 30,299 38,956 Total Wt. of Shrimp Harvested During Year (lbs) 8,342 38,136 66,738 85,806 Feed Postlarval Diet - 400 micron Juvenile 600-850 Required/Year (kg) 85 211 337 365 Juvenile 850-1200 Required/Year (kg) 164 405 646 700 Juvenile No. 3 Crumble Required/Year (kg) 961 2,376 3,791 4,110 40% Protein 3/32" Grower Required/Year (kg) 835 2,066 3,296 3,573 35% Protein 3/32" Grower Required/Year (kg) 10,543 33,272 57,820 71,298 Labor Hourly Employees 1 2 3 3 Other Inputs Electricity (kw-hrs) 158,155 290,030 413,510 413,510 Propane (gallons) 4,068 16,068 28,068 36,000 Fuel - Diesel (gallons) 120 120 120 120 Fuel - Gasoline (gallons) 315 1,228 2,116 2,638 Office Rental (months) 0 0 0 12 Maintenance & Repairs 179,526 359,052 538,578 538,578 Operating Supplies (1 unit/crop) 26 75 124 144 Ice & Packing (1 unit/kg harvested) 3,787 17,314 30,299 38,956 Marketing (1 unit/kg shrimp harvested) 3,787 17,314 30,299 38,956 Shipping & Sales (1 unit/kg shrimp harvested) 3,787 17,314 30,299 38,956 Accounting Fees (1 unit/kg shrimp harvested) 3,787 17,314 30,299 38,956 Legal Fees (1 unit/kg shrimp harvested) 3,787 17,314 30,299 38,956 Insurance (1 Unit/Dollar Capital Investment) 212,026 160,026 148,943 0


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Expected Production At the end of the 180-day growout period the shrimp are expected to average 18.1 grams per shrimp. The growout period was selected to allow the shrimp to reach an average harvest size 18 grams per shrimp. This improves the chance of selling the shrimp for an acceptable price. The shrimp in our study grew at an average rate of 0.7 grams per week when temperatures averaged between 26-28ºC. At this growth rate, the shrimp reach the size of 18 grams in 26 weeks. Assuming an overall survival of 61%, and 24,480 freshwater acclimated postlarvae stocked per crop, a total of 14,982 shrimp will be harvested each crop. The total weight harvested per crop will be approximately 271 kg (597 lbs). With twelve greenhouses and 24 growout tanks in production, a total of 144 crops would be harvested per year, yielding an expected 38,956 kg (85,806 lbs) of shrimp per year. This corresponds to an average weekly harvest total of 1,650 lbs of shrimp per week. Predicted annual production is summarized in Table 10-8. If these shrimp bring an average of $5.24/lb, the annual revenue from shrimp sales (after Year 4) would be $449,623.

Production Inputs and Operating Costs Production inputs and operating costs increase as a function of the number of production cycles completed. Production units in operation for a full year complete six production cycles per year. When the faciltiy is operating at full capacity (Years 4-10), 144 production cycles are completed per year. Operating costs during these years total just over $300,000 per year. The most important production inputs are seed, feed, labor, and energy. Table 10-9 itemizes the key production inputs and their unit costs.

Seed All postlarvae stocked must be Specific Pathogen Free (SPF). SPF postlarvae are guaranteed to be free from the known viral diseases, including Baculovirus, Infectious Hypodermal Hematopoetic Necrosis Virus (IHHN), Taura Syndrome Virus (TSV), and White Spot Syndrome Virus (WSSV) (see Chapter 9). SPF postlarvae are available from only a few hatcheries and are more expensive than non-SPF postlarvae. Commercial hatcheries do not sell freshwater acclimated postlarvae, preferring instead to sell younger animals. The postlarvae most likely would be purchased as a PL8. Animals this young are not yet capable of being acclimated to freshwater. These PLs must be held in seawater until they reach the age of PL12, when they have developed the physiological capability of adapting to near freshwater conditions. At that time they can be acclimated to freshwater over a period of three days (Chapter 6). The price of commercially available SPF postlarvae ranges from $7.00 - $10.00 per thousand, depending on the hatchery and the age of the animals purchased. Sea salt, feed, and labor to acclimate the PLs adds up to about $2.61/1000 acclimated PLs. Assuming an initial cost of $10.00 per thousand and a survival of 85% through the acclimation period, the net cost of the acclimated postlarvae stocked in the nursery phase would be about $14.85 per thousand. However, postlarvae are highly cannibalistic during this period in their lives, and at high densities losses of up to 3% or more of the population per day are possible. Excessive mortality will significantly drive up the cost


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of acclimated postlarvae. The use of artificial substrates may help reduce mortality during the acclimation period. Assuming a survival from initial stocking to final harvest of 61%, 24,480 freshwater acclimated postlarvae will be required per crop to yield a harvest density of 135 shrimp/m2. A total of 28,800 postlarvae (PL8) will need to be purchased per crop, assuming 85% survival of the postlarvae through the acclimation process. After Year 4, when all 12 greenhouses are in operation, the annual requirement for postlarvae (PL8) is estimated at just over 4.1 million postlarvae (Table 10-8). Assuming a seed cost of $10/1000 postlarvae, and an annual demand of 4.1 million postlarvae, the annual seed cost would be approximately $41,000, or $52,300 if the feed, salt and labor costs for acclimating the postlarvae are included. This represents 17.4% of the total operating cost for the facility.

Table 10-9: Key production inputs and unit costs.

Operating Input Unit Costs Item Units Unit Cost

Shrimp Postlarvae 1000 PLs $10.00 Cost of Acclimating PLs (Labor, Feed, Energy) 1000 PLs $2.61 Juvenile Feed - 600-850 microns kg $4.06 Juvenile Feed - 850-1200 microns (kg) kg $3.04 Juvenile Feed - No. 3 Crumble (kg) kg $0.58 40% Protein Grower - 3/32 " kg $0.32 35% Protein Grower - 1/8" kg $0.29 Labor (Hourly Wage + Fringe) man-hour $8.22 Manager's salary (including fringe) $/year $47,950 Electricity kw-hrs $0.08 Propane gallon $0.62 Diesel Fuel gal $1.00 Gasoline gal $1.10 Office Rental month $500 Maintenance & Repairs $/$ Capital $0.05 Operating Supplies ($/crop) $/Crop $25 Ice & Packing ($/kg shrimp harvested) kg $0.10 Marketing ($/kg shrimp harvested) kg $0.10 Shipping & Sales ($/kg shrimp harvested) kg $0.10 Accounting Fees kg $0.05 Legal Fees kg $0.05 Insurance kg $0.01 Contingency Rate % of Operating Costs 10%


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Feed Table 10-10 summarizes the feed requirements for producing a crop of shrimp. A variety of feeds are required to raise the shrimp from postlarvae to harvest size (Chapter 7). During the nursery phase, three different feeds are required. The shrimp are initially fed a 50% protein postlarval diet (600-850 microns) for a period of approximately 5 days. The feed rate declines from an initial 35% of bodyweight/day to 18% bodyweight/day. The shrimp are then transitioned to a similar diet but with a larger particle size (850-1200 microns). The shrimp receive this diet for 6-7 days. When the shrimp reach a size of approximately 0.2 g/shrimp they are transitioned to a No. 3 crumble (1200-1800 microns). The crumble feed typically has a protein content of about 40%. The shrimp remain on the crumble for about 21/2 weeks until they reach a size of about 0.8 grams/shrimp. At this point they are transitioned to a 3/32" 40% protein pelleted feed. The shrimp remain on this diet until the end of Phase I. In Phase II and Phase III the shrimp are fed a 35% protein grower diet at a rate beginning at 6.5% of their bodyweight per day, and declining to 1.5% of their bodyweight per day by the end of the growout period. The cumulative feed conversion ratio for the entire growout process is expected to be about 1.76. Table 10-10 summarizes the feeding regimes used in the production process. The 600-850 micron ($8.94/kg) and 850-1200 micron ($6.70/kg) juvenile diets are quite expensive, but only small amounts of these diets are required. The No. 3 crumble ($1.28/kg) is also used sparingly. The feeds that are used in the greatest quantity are the 40% protein 3/32" pellets ($0.70/kg) and the 35% protein 1/8" pellets ($0.65/kg). All prices given are delivered prices. Annual feed costs are expected to total $61,265 dollars for 12 greenhouses during Years 4-10. This represents 20.4% of total operating expenditures. Feed costs tend to be volatile, with prices fluctuating according to the prices of raw ingredients. Feed costs could go up if the price of fishmeal, soybean meal, and other key ingredients go up. The percentage of operating expenses represented by feed will also be sensitive to feed conversion rates. Feed

Table 10-10: Feed Table for Intensive Shrimp Production in Raceways

Feed Type Initial Size (g) Final Size (g) Production Phase

Feed Rate (%BW/day) Kg Feed/Crop

600-850 0.005 0.08 Nursery 35%-18% 2.5 850-1200 0.08 0.20 Nursery 18%-11% 5

No. 3 Crumble 0.2 0.9 Nursery 10%-7.5% 29 3/32 - 40% 0.9 1.5 Nursery 7.5%-6.5% 25 3/32 - 35% 1.5 8.2 Intermediate 6.5%-2.5% 227 3/32 - 35% 8.2 18.1 Final 2.5%-1.5% 269


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conversion ratios are assumed to average 1.76. Feed conversion averaged 1.74 in the 1999 HBOI/FDACS intensive shrimp growout demonstration tanks.

Labor It is assumed that one laborer is required for every four greenhouses. Hourly workers are paid a wage of $6.00/hour plus 37% fringe. The cost to the business is $8.22/hour. One new hourly worker is hired in each of the building years (Years 1-3). The model assumes that the owner of the facility will also operate and manage the facility. The owner-operator will not draw a salary but will be paid out of the proceeds from shrimp sales after all other expenses are paid. After Year 4, labor costs account for about 17.1% of all operating costs.

Energy Annual electrical consumption for a 12-greenhouse production facility is summarized by type of equipment item in Table 10-11. Total annual energy cost for the 12-greenhouse facility is approximately $55,400, or $4,617 per greenhouse. There are three categories of energy consumption in the production process. Electricity is used to power pumps, blowers, fans and lights. Propane is used to heat the greenhouses. Gasoline is used by the company pickup truck and to power the generator for the backup blower. The facility will use approximately 413,500 kw-hrs of electrical energy for twelve greenhouses, resulting in an annual electric bill of about $33,000 (assuming electricity costs $0.08/kw-hr). The electrical consumption by the 3/4-hp raceway pumps, which run 24 hours/day, 365 days per year accounts for 44% of the total usage. The blowers, which also run continuously, account for another 38% of the electrical consumption. Electricity accounts for 11% of the total annual operating budget. Propane-powered gas heaters are used to heat the greenhouses during 6 months out of the year, with almost 95% of the consumption-taking place in the months of December, January, February and March. In similar 30’ x 152’ greenhouses at HBOI equipped with propane

Table 10-11: Expected electrical consumption for a 12-greenhouse shrimp production facility.

Equipment Items HP Watts Hours/ Day

No. of Units

Days in Operation

Kw-hrs/Year

Annual Cost

Well Pump 3 3000 4 2 365 8,760 $701 Degasser Blower 0.25 250 4 2 365 730 $58 Water Supply Pump 2 2000 4 2 365 5,840 $467 Sump Pump 0.5 500 4 2 365 1,460 $117 Regenerative Blower 3.5 3500 24 6 365 183,960 $14,717 Raceway Pump 0.75 750 24 24 365 157,680 $12,614 Extractor Fan 1.5 1500 4 24 200 28,800 $2,304 Air Conditioner 3 3000 24 1 365 26,280 $2,102


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heaters, about 3,000 gallons of propane are required per greenhouse per year to maintain system water temperatures of above 26°C. Based on this figure, the total annual propane use for a twelve-greenhouse facility would be about 36,000 gallons of propane. Assuming that propane costs $0.62/gallon, the annual cost for heating the facility is about $22,300. This corresponds to 7.4% of the total annual operating cost for the facility. Gasoline is primarily required for a multi-purpose pickup truck. The pickup is used to transport shrimp to markets, as well as for general usage. It is expected that the pickup truck will travel approximately 40,000 miles per year, and will get about 15 miles per gallon. Annually, the pickup truck will use more than 2,600 gallons of gasoline. At $1.10/gallon of gasoline, this corresponds to almost $3,000 dollars per year. A small amount of gasoline is used by the gasoline-powered backup blower.

Maintenance It is assumed that repairs and maintenance of equipment and facilities will be 4% of the replacement value of all items in the capital inventory (excluding land). In a typical year the cost of repairs and maintenance is expected to be $26,959, or 9% of the annual operating budget.

Marketing Assumptions The model assumes that the shrimp produced will be direct-marketed as a whole, fresh product. The product will be sold directly to area seafood brokers, seafood markets, supermarkets, and/or restaurants. Preliminary analysis indicated that the cost of production of shrimp in tank-based recirculation systems is too high to allow the shrimp to be profitably sold on the wholesale frozen tail market. While little hard data exists on the potential for direct-marketing fresh, whole shrimp, entrepreneurs investigating this kind of enterprise have

Table 10-12: Current shrimp prices (June 12, 1999) for wholesale and direct-marketed fresh shrimp, assuming a 150% mark-up factor for the fresh, direct-marketed product.

Average

Shrimp Wt (Min)

Average Shrimp Wt.

(Max) Category Wholesale

Price/lb (Tails)Direct Price/lb

(Tails)

Wholesale Price/lb

(Heads-on)

Direct Price/lb (Heads-on)

10.3 11.9 61-70 $3.30 $4.95 $2.08 $3.12 12.0 14.2 51-60 $3.50 $5.25 $2.21 $3.31 14.3 17.9 41-50 $4.55 $6.83 $2.87 $4.30 18.0 20.5 36-40 $5.55 $8.33 $3.50 $5.24 20.6 23.9 31-35 $6.25 $9.38 $3.94 $5.91 24.0 28.7 26-30 $7.15 $10.73 $4.50 $6.76 28.8 35.9 21-25 $8.15 $12.23 $5.13 $7.70 36.0 46.4 16-20 $9.95 $14.93 $6.27 $9.40 46.5 72.1 U-15 $10.85 $16.28 $6.84 $10.25


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reported that they would be able to consistently sell heads-on 18 gram shrimp (36-40 tail count per pound) for $6.00 a pound. The current wholesale market price for frozen, heads-off, farm-raised Litopenaeus vannamei in the 36-40 count size category is $5.55/lb. Assuming the tail is 63% of the whole shrimp, this corresponds to a price of $3.50/lb for the whole shrimp (see Table 10-12). A price of $6.00/lb represents a 71% markup, relative to the wholesale price. For this analysis it is assumed that the direct market price for whole, fresh, shrimp is 50% above the wholesale price for frozen shrimp of the same size.

Revenues Once all twelve greenhouses are in production, a total of 144 crops per year will be harvested, yielding 85,800 pounds of whole shrimp. Assuming these shrimp are sold directly as a whole, fresh product, and the average price received is $5.24/lb, annual revenues are expected to be approximately $449,600 per year.

Cash Flow A cash flow analysis was performed to assess the future cash inflows and outflows over the 10-year planning horizon (Table 10-13). All revenues are assumed to come from shrimp sales. Old equipment that is replaced is assumed to have zero salvage value. Total Operating Expenses are calculated by adding the various categories of operating expenses, including seed costs, feed costs, labor costs, fuel and electrical costs, operating supplies, maintenance, marketing, packing, shipping, accounting and legal fees. A contingency cost is included to cover miscellaneous items not accounted for in the other categories. The contingency cost is 10% of the other budgeted operating costs. Total operating expenses increase in Years 1-4 as new greenhouses are put into production. After Year 3 Total Operating Expenses level out at approximately $300,400 per year, based on the assumptions of the model. The breakeven price required to cover operating costs in the typical year is $3.50/lb (heads-on). Year 1 is the only year of the project for which Income – Operating Expenses is negative (–$33,208). Year 1 is a building year and shrimp sales do not begin until August in that year. Years 2 and 3 are also building years, but revenues from the production from existing greenhouses is more than enough to cover all operating expenses in these years. Income – Operating Expenses is $26,343 in Year 2, and $78,927 in Year 3. In subsequent years, Income- Operating Expenses averages $149,246 per year. Total Cash Outflow is calculated by adding together Total Operating Expenses, Total Capital Investment, and Total Taxes (Income + Social Security). Cash Available at the end of each year is calculated by adding the Total Cash Inflow to the Beginning Cash Balance and subtracting from that total the Total Cash Inflow. At the end of each year a minimum of$30,000 must be carried over to the next year as a Beginning Cash Balance. If the calculated amount of cash available is less than $30,000, then the investor(s) will need to pay the difference.

Table 10-13: Cash flow analysis for 10-year planning horizon.

Item Value Year 1

Value Year 2

Value Year 3

Value Year 4

Value Year 5

Value Year 6

Value Year 7

Value Year 8

Value Year 9

Value Year 10

Beginning Cash Balance $0 $30,000 $30,000 $30,000 $132,968 $229,135 $280,024 $356,896 $416,912 $494,845 Receipts: Pounds of Shrimp Sold (heads-on) 8,342 38,136 66,738 85,806 85,806 85,806 85,806 85,806 85,806 85,806 Ave Price Received ($/Lb Whole Shrimp) $5.24 $5.24 $5.24 $5.24 $5.24 $5.24 $5.24 $5.24 $5.24 $5.24 Shrimp $43,712 $199,833 $349,707 $449,623 $449,623 $449,623 $449,623 $449,623 $449,623 $449,623 Total Cash Inflow $43,712 $199,833 $349,707 $449,623 $449,623 $449,623 $449,623 $449,623 $449,623 $449,623 Operating Expenses: Shrimp Postlarvae $13,799 $31,230 $48,660 $52,292 $52,292 $52,292 $52,292 $52,292 $52,292 $52,292 Feed $10,411 $30,342 $51,435 $61,265 $61,265 $61,265 $61,265 $61,265 $61,265 $61,265 Labor $17,098 $34,195 $51,293 $51,293 $51,293 $51,293 $51,293 $51,293 $51,293 $51,293 Electricity $12,652 $23,202 $33,081 $33,081 $33,081 $33,081 $33,081 $33,081 $33,081 $33,081 Fuel $2,988 $11,433 $19,849 $25,342 $25,342 $25,342 $25,342 $25,342 $25,342 $25,342 Office Rental $0 $0 $0 $6,000 $6,000 $6,000 $6,000 $6,000 $6,000 $6,000 Maintenance & Repairs $8,976 $17,953 $26,929 $26,929 $26,929 $26,929 $26,929 $26,929 $26,929 $26,929 Operating Supplies ($/crop) $639 $1,875 $3,111 $3,590 $3,590 $3,590 $3,590 $3,590 $3,590 $3,590 Ice & Packing ($/kg shrimp harvested) $379 $1,731 $3,030 $3,896 $3,896 $3,896 $3,896 $3,896 $3,896 $3,896 Marketing ($/kg shrimp harvested) $379 $1,731 $3,030 $3,896 $3,896 $3,896 $3,896 $3,896 $3,896 $3,896 Shipping & Sales ($/kg shrimp harvested) $379 $1,731 $3,030 $3,896 $3,896 $3,896 $3,896 $3,896 $3,896 $3,896 Accounting Fees $189 $866 $1,515 $1,948 $1,948 $1,948 $1,948 $1,948 $1,948 $1,948 Legal Fees $189 $866 $1,515 $1,948 $1,948 $1,948 $1,948 $1,948 $1,948 $1,948 Insurance $2,120 $1,600 $1,489 $0 $0 $0 $0 $0 $0 $0 Contingency Rate (% of Operating Costs) $6,721 $14,732 $22,812 $25,003 $25,003 $25,003 $25,003 $25,003 $25,003 $25,003 Total Operating Expenses $76,920 $173,489 $270,780 $300,377 $300,377 $300,377 $300,377 $300,377 $300,377 $300,377 Capital Expenditures New Capital Investment $212,026 $160,026 $148,943 $0 $0 $0 $0 $0 $0 $0 Capital Replacement $0 $0 $0 $6,450 $10,783 $56,927 $30,944 $47,800 $29,883 $29,683 Taxes Income Tax $0 $0 $1,933 $17,942 $17,942 $17,942 $17,942 $17,942 $17,942 $17,942 Social Security $0 $0 $1,598 $6,882 $6,882 $6,882 $6,882 $6,882 $6,882 $6,882 Medicare $0 $0 $374 $2,413 $2,413 $2,413 $2,413 $2,413 $2,413 $2,413 Total Cash Outflow $288,946 $333,515 $423,629 $334,065 $338,398 $384,542 $358,559 $375,415 $357,498 $357,298 Cash Available ($245,234) ($103,683) ($43,922) $145,559 $256,785 $321,867 $412,932 $487,141 $579,266 $671,592 Investors Paid in Capital $275,234 $133,683 $73,922 $0 $0 $0 $0 $0 $0 $0 ENDING CASH BALANCE $30,000 $30,000 $30,000 $145,559 $256,785 $321,867 $412,932 $487,141 $579,266 $671,592


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The Ending Cash Balance is calculated by adding the Investor Paid-In Capital to the Cash Available. For the purposes of this analysis, it is assumed that bank financing is not available due to the newness of the technology and the inherent high risk associated with recirculating aquaculture systems. Investor capital will have to be paid into the enterprise in each of the first three years, but after Year 3 all cash expenses can be paid out of revenues from shrimp sales. Investor Paid-in Capital totals $275,234 in Year 1, $133,683 in Year 2, and $73,922 in Year 3. In the last seven years of the project the average net income after operating expenses and capital replacement costs have been taken out is $118,893 per year. After income taxes have been taken out, the owner-operator will net an average of $91,656 per year. The cash flow analysis shows available cash increasing from year to year by this amount. It is important to note that this amount does not include any cash withdrawals for owner salary and living expenses. The cash available to the operation after family living withdrawals would be significantly lower than the amount indicated in the cash flow analysis. If the owner operator takes a family living withdrawal of $40,000/year, investor paid-in capital would have to be increased by this amount in each of the first three years of the project. In Years 4-10 the available cash for the operation would increase by an average of $51,656/year, rather than $91,656 per year.

Income Statement A pro forma income statement was developed (Table 10-14) to examine the impact of non-cash items such as depreciation on net farm income. The straight-line method was used to calculate annual depreciation. The replacement values of all capital assets with the same economic life were summed and the total value was divided by the number of years of useful life for the group. Salvage value was assumed to be zero for all capital items. The annual depreciation value is subtracted from the Net Cash Income (Total Income – Total Cash Expenses). The result is the Net Income Before Taxes. Net Income Before Taxes averages $83,209. This is the basis on which the income tax, social security, and Medicare payments are figured. The Net Income After Taxes shows the profit or loss in a given year. It is a measure of the amount available to the owner-operator for unpaid labor, management, and equity capital used to produce that net farm income. The project operates at a loss the first two years, but records profits thereafter. Once production stabilizes in Year 4 the annual profit (Net Income After Taxes) averages about $55,972 per year. Note that this value is lower than the average annual net revenue (Net Revenue = Cash Inflow – Cash Outflow) . This is due the difference between annual depreciation and annual replacement costs incurred during the time period examined.

Table 10-14: Pro forma income statement. Item Value

Year 1 Value Year 2

Value Year 3

Value Year 4

Value Year 5

Value Year 6

Value Year 7

Value Year 8

Value Year 9

Value Year 10

Income: Shrimp $43,712 $199,833 $349,707 $449,623 $449,623 $449,623 $449,623 $449,623 $449,623 $449,623 Sale of Capital Equipment $0 $0 $0 $0 $0 $0 $0 $0 $0 $0

Total Income $43,712 $199,833 $349,707 $449,623 $449,623 $449,623 $449,623 $449,623 $449,623 $449,623 Cash Expense:

Operating $76,920 $173,489 $270,780 $300,377 $300,377 $300,377 $300,377 $300,377 $300,377 $300,377 Interest $0 $0 $0 $0 $0 $0 $0 $0 $0 $0

Total Cash Expense $76,920 $173,489 $270,780 $300,377 $300,377 $300,377 $300,377 $300,377 $300,377 $300,377 Net Cash Income ($33,208) $26,343 $78,927 $149,246 $149,246 $149,246 $149,246 $149,246 $149,246 $149,246 Non Cash Adjustments

Depreciation $25,402 $44,362 $66,037 $66,037 $66,037 $66,037 $66,037 $66,037 $66,037 $66,037 Net Income Before Tax ($58,610) ($18,019) $12,890 $83,209 $83,209 $83,209 $83,209 $83,209 $83,209 $83,209 Taxes

Income Tax $0 $0 $1,933 $17,942 $17,942 $17,942 $17,942 $17,942 $17,942 $17,942 Social Security $0 $0 $1,598 $6,882 $6,882 $6,882 $6,882 $6,882 $6,882 $6,882 Medicare $0 $0 $374 $2,413 $2,413 $2,413 $2,413 $2,413 $2,413 $2,413

Total taxes $0 $0 $3,906 $27,237 $27,237 $27,237 $27,237 $27,237 $27,237 $27,237

Net Income Above Taxes ($58,610) ($18,019) $8,984 $55,972 $55,972 $55,972 $55,972 $55,972 $55,972 $55,972

Table 10-15: Breakeven analysis.

Item Value Year 1

Value Year 2

Value Year 3

Value Year 4

Value Year 5

Value Year 6

Value Year 7

Value Year 8

Value Year 9

Value Year 10

Breakeven Production (lbs, whole shrimp) To Cover Cash Costs 14,679 33,109 51,676 57,324 57,324 57,324 57,324 57,324 57,324 57,324 To Cover All Costs Before Taxes 19,527 41,575 64,278 69,926 69,926 69,926 69,926 69,926 69,926 69,926 To Cover All Costs Including Taxes 19,527 41,575 65,023 75,124 75,124 75,124 75,124 75,124 75,124 75,124

Breakeven Price ($/lb) To Cover Cash Costs $9.22 $4.55 $4.06 $3.50 $3.50 $3.50 $3.50 $3.50 $3.50 $3.50 To Cover All Costs Before Taxes $12.27 $5.71 $5.05 $4.27 $4.27 $4.27 $4.27 $4.27 $4.27 $4.27 To Cover All Costs Including Taxes $12.27 $5.71 $5.11 $4.59 $4.59 $4.59 $4.59 $4.59 $4.59 $4.59


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Breakeven Analysis A breakeven analysis was conducted to determine the minimum levels the venture must achieve to cover costs (Table 10-15). There are two types of breakeven analyses. Breakeven production analysis identifies the level of production that achieves zero profit when price and other factors are held constant. Breakeven price analysis identifies the price that must be received to achieve zero profit when production and other factors are held constant. The breakeven price is a measure of the production cost per pound of shrimp produced. The level of production required to cover all cash costs, assuming shrimp are sold for $5.24/lb whole, and all costs are held constant, is about 57,300 lbs of shrimp, once production levels off in Year 4. About 69,900 lbs must be produced to cover cash costs plus annual depreciation. Just over 75,000 lbs of shrimp must be produced per year to cover all costs, including taxes. The profit that is made is based on the production in excess of this final figure. In the baseline scenario, the profit margin is based on about 10,700 pounds of production over and above the breakeven production level. The breakeven price of $3.50 is required to cover all cash costs, assuming production of 85,800 pounds of whole shrimp, with costs as indicated in Table 10-13. A price of about $4.28/pound must be received to cover both cash and non-cash costs (depreciation). The breakeven price reveals the high risk of this venture. The breakeven price per pound ($4.28/lb) is for a whole shrimp, which equates to a price of about $6.79/lb for tails. This is 22% above the current wholesale market price (New York, June, 1999) for 36-40 count frozen tails. Wholesale shrimp prices are volatile and may vary by plus or minus 30-40%. The current price for wholesale shrimp is near a ten-year high. It wouldn't take a very large drop in the price received for the whole, fresh shrimp to make this a money-losing venture. The baseline assumption is that the producer can get a price that is 50% above the market price for frozen tails by selling a whole, fresh shrimp direct. However, at this time formal market studies have not been carried out to establish the relationship between wholesale prices for frozen shrimp and direct market prices for fresh shrimp. It is entirely possible that the products will be seen as substitutes, and the price differential may not be as large as 50%. If there is substitutibility betwen the two products, the price of the fresh shrimp will probably float with the wholesale price of frozen shrimp.

Investment Analysis Another way to examine the profitability of the venture is to look at the Net Present Value (NPV). The NPV of an investment is calculated by summing the present values of the net cash flows that result from an investment, minus the amount of the original investment. The present values of the net cash flows are calculated using a discount rate that corresponds to that which might be earned in the best alternative use of the capital invested in the project. This is the opportunity cost of capital. Six percent, a typical interest rate earned on secure investments such as certificates of deposit, is a figure commonly used to account for the opportunity cost of capital. Because farming shrimp in high intensity, recirculating


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aquaculture systems is a risky venture, the discount rate used to calculate the NPV must be increased to compensate the investor for the risk factor. Each investor has their own risk factor depending on their willingness to take on risk. In this analysis the investor's risk factor is set at 20%. The combined discount rate necessary to compensate the investor for the opportunity cost of capital and the risk factor is 26%. If the NPV is greater than zero, then the venture is considered to be an economic success. If the NPV equals zero the investor will be indifferent, because the investment will be equivalent to the best alternative use of the money. If the NPV is less than zero, the investor should not invest, because the earnings from the project will not provide adequate compensation for the opportunity cost and risk factor. The internal rate of return (IRR) is another financial measure that is often used to evaluate investments. The internal rate of return is the discount rate that makes the net present value of the annual cash flows from an investment equal to zero. Taken together, the NPV and IRR give a good picture of the economic value of an investment. The NPV and IRR were calculated for this project on the net cash flows (Total Operating Expenses - Total Capital Expenditures). Under the baseline set of assumptions, the NPV is –$124,311. The IRR was calculated to be 13%. These values indicate that the earnings from the project of the 10-year planning horizon are not sufficient to compensate the investor for the risk associated with the project. The investor needs to receive an IRR greater than 26% to make it worth the risk of taking the money out of a more secure investment, but in fact only earns 13%.

Sensitivity Analysis Sensitivity analyses were performed to analyze the effects of selected variables on the profitability of the enterprise. These analyses provide information about the values of operating parameters that are required for the enterprise to become profitable. Sensitivity analyses were performed to examine the effects of improving survival and growth. One of the biggest uncertainties facing the producer is the price of shrimp. This is especially true in this situation, where new markets must be developed. An analysis was conducted to determine the minimum price the producer can receive and still make an acceptable profit.

Survival The survival rate used in the baseline analysis (61%) was selected based on the average survival rates observed in the HBOI/FDACS demonstration tanks. However, higher average survival rates may be possible. In fact, half of the crops harvested in the study had survivals above 75%. Two different production systems (System A and System B) were tested during the demonstration project (see Chapter 4 for a full description of these systems). The survival rates observed in the System A three-phase tanks were consistently low, averaging only 49%, compared to an average survival of 82% in the System A single-phase tanks. The majority of the mortality occurred in the nursery section where the tank depth was only 10 inches. The high density in the nursery sections (5,400 shrimp/m3) led to increased rates of


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cannibalism. In the System B three-phase study the water depth was nearly double, and water velocities were maintained much higher. These differences in system design appeared to reduce the frequency of cannibalism. The survival in this study was 82% after 135 days. The above discussion of the survival rates in the study is included to show that higher survival rates are possible in the system modeled, and that survival rates can be improved with better tank design. Deeper tanks with better water circulation appear to have fewer problems with cannibalism. Although not tested in this study, the use of artificial substrates in the raceways may also increase survival rates. Artificial substrates provide refuges for the shrimp, which may cut down on cannibalism. In addition, shrimp graze on the periphyton growing on artificial substrates, which may significantly improve their growth rates (Jeff Peterson, personal communication). Because there appears to be potential for improved survival rates in these systems, a sensitivity analysis was performed to determine what the economic implications would be of different survival rates in the range between 60% and 75%, holding all other variables constant (Table 10-16). Table 10-16: Sensitivity of net present value and internal rate of return to survival rate,

assuming a growout time of 180 days.

Survival Rate Financial Measure 60% 65% 70% 75%

Net Present Value ($142,281) ($71,424) $2,833 $76,138 Internal Rate of Return 11% 19% 26% 33%

Average survivals of 60% and 65% resulted in negative net present values, assuming all other variables are held constant. However, improving the survival to 70% would result in a 26.3% internal rate of return, marginally higher than the 26% rate of return required for investment in the project. If average survival rates can be improved to 75%, the internal rate of return increases to 33%, a very acceptable figure. However, investing in a shrimp culture enterprise based on anticipated survival rates as high as this would be risky. In any aquacultural enterprise, equipment failures, human error, or disease occasionally lead to very low survivals in given crops. These unanticipated events should be taken into account in the analysis.

Growth Rates The growth rates observed in the Harbor Branch demonstration system were significantly slower than are commonly recorded for Litopenaeus vannamei. In pond production systems it typically takes the shrimp 30 days to attain an average weight of 1 gram/shrimp, and thereafter the shrimp increase in size by approximately 1 gram/week. At these growth rates 18 gram shrimp may be harvested after 22 weeks, or 154 days. There are probably several reasons for the relatively slow growth that was observed in the Harbor Branch systems. The


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raceways were not heated, so temperatures were sub-optimal for at least part of each study. Another factor that may have impacted the growth rates was a general lack of algal and detrital food sources for the shrimp in the culture tanks. It is well known that in ponds, L. vannamei grows best in ponds with high levels of natural productivity (Scura, 1995). Phytoplankton and organic detritus are both important components of the shrimp’s diet (Moss, 1992). L. vannamei has a very inefficient digestive system consisting of short, straight gut. Evidence is accumulating that L.vannamei does not utilize prepared diets efficiently, especially if their feces are rapidly filtered from the system. However, if their feces are allowed to remain in the system, heterotrophic bacteria will colonize the fecal material and convert feed protein into bacterial protein. Shrimp consume the decaying fecal material and the associated bacteria. The shrimp derive significant nutritional benefits from the bacterial proteins and partially digested feed proteins during this second pass. The importance of the detrital food chain to shrimp growth was not fully appreciated until this study was nearly over. The culture tanks were shaded to control algae growth, and solid wastes were quickly removed from the system in the interest of maintaining optimal conditions for biofiltration. As a result, the shrimp were almost completely dependent upon the nutrition they could absorb from the prepared feeds in a single pass through the gut. Recent unpublished work at Harbor Branch has demonstrated that the growth rates of shrimp grown in tanks managed for optimization of the detrital food chain have been up to 50% faster than the growth rates observed in the systems modeled in this report. Similarly, Moss (1999) reported growth rates of L.vannamei cultured in a high density tank-based culture system at the Oceanic Institute (OI) in Hawaii that were double the growth rates observed in the HBOI system. The primary difference between the OI system and the HBOI system was that the OI system was a “greenwater” system, while the HBOI system was a “clearwater” system. The presence of algae and organic detritus in the tanks was credited by Moss for the rapid growth rates that were observed in the OI system. These results suggest that the mediocre growth rates observed in this study were not strictly a function of the tank environment, or the high densities that were used. Rather, the slow growth may be related to the scarcity of detritus in the system. Better growth rates might be realized with alternative management strategies. A sensitivity analysis was carried out to determine what the impact would be of faster growth rates and shorter production cycles on the profitability of the hypothetical enterprise of the model. The net present value and internal rate of return were calculated for growout to an average market size of 18 grams in 150, 159, 168, and 177 days. All other variables were held constant. The results are summarized in Table 10-17. As expected, the profitability of the enterprise is sensitive to shrimp growth rates. The enterprise becomes marginally profitable (NPV = $1,371) if the time required to reach market size is reduced by 21 days (from 180 days to 159 days). If the shrimp can be grown to market size in 150 days, the norm for pond culture, then the NPV is strongly positive ($60,314), and the IRR is healthy 32%. These results indicate that even modest improvements in growth rates can significantly alter the profit potential for this type of enterprise.


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Improved nutrition for the shrimp could conceivably result in both improved growth rates and better survival, especially if well-nourished shrimp engage less frequently in cannibalism. The sensitivity of the NPV and IRR to growth rates was examined at a 70% survival rate (Table 10-17). The combination of improved growth and higher survival dramatically improves the profit potential of the enterprise. Even with a minimal reduction in the growout time from 180 days to 177 days, the enterprise is marginally profitable if survival averages 70%. Further reductions in the growout time dramatically improve the profit potential of the enterprise. If the shrimp can be grown to an average size of 18 grams in 150 days with a 70% survival rate, the net present value improves to a very attractive $216,911, which corresponds to an IRR of 48%.

Seed Costs Seed costs comprise 17.4% of the total operating cost, following slightly behind feed and energy among the most costly inputs for the enterprise. Seed costs are influenced heavily by two factors: 1) the price charged by hatcheries for the postlarvae, and 2) the survival of the postlarvae through the acclimation/quarantine period. Like most goods, the price of SPF postlarvae will be determined by supply and demand. Currently the supply of SPF postlarvae is extremely limited, with limited numbers being produced by just a few hatcheries. However, the demand for SPF postlarvae is not very great at the moment. At present there is only one commercial SPF hatchery in the state of Florida. The market price (FOB hatchery) is approximately $7.00 per thousand postlarvae. Postlarval shipments can be delivered to most points in Florida for less than $3.00 per thousand postlarvae. If the demand for postlarvae were to increase without a corresponding increase in hatchery production capacity, the price for seed would almost certainly increase significantly. However, if hatchery production increases faster than the demand for SPF postlarvae, seed prices could fall to as low as $5.00 per thousand.

Table 10-17: Sensitivity analysis of the effects of the time required for shrimp to reach a market size of 18 grams on the net present value and internal rate of return of a hypothetical 12-greenhouse shrimp production facility.

Days Required to Reach 18 grams Item

150 159 168 177 Production Cycle/Year/Tank 7.16 6.76 6.4 6.08 Production Cycles/Year 171.8 162.2 153.7 146 Baseline Scenario: Survival = 61% Net Present Value (r=26%) $ 60,314 $ 1,371 ($ 59,413) ($ 105,761) Internal Rate of Return 32 % 26 % 20 % 15 % Alternative Scenario: Survival = 70% Net Present Value (r=26%) $ 216,911 $ 148,134 $ 78,262 $ 24,494 Internal Rate of Return 48 % 41 % 34 % 28 %


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Another important factor affecting the net price paid for seed is the survival of the postlarvae through the acclimation/quarantine period. Cannibalism can be significant in the late postlarval stages. Survival will be a function of density and management. Inadequate frequency of feeding and/or inappropriate feeding rates can result in high rates of cannibalism. Excessive feeding rates and/or inadequate water exchange can result in fouled water and low survivals. A well-managed acclimation program could average as high as 90% survival, while a less well-managed program could average 70% survival or less through the acclimation period. Unanticipated mortality during the acclimation period is more costly than anticipated mortality if stocking rates of production tanks are affected. If the producer anticipates a survival of 70% through the acclimation period, then additional postlarvae can be purchased and acclimated to ensure that the production tanks are stocked at the desired density. The net effect is only a higher seed cost. However, if the producer bases postlarval purchases on an anticipated 85% acclimation period survival, and only achieves a survival of 70%, the production tanks will be understocked and overall productivity will suffer. A sensitivity analysis was carried out to examine the effect of both postlarval price and acclimation survival rates on enterprise profitability. The analysis assumes the acclimation mortality is compensated for in the number of postlarvae purchased, and production tank stocking density is maintained constant. If we assume that production tank survival averages 61% and the shrimp reach 18 grams after 180 days, no realistic combination of lower seed prices and higher acclimation survival rates resulted in acceptable rates of return for the project. However, if we assume an average survival of 70% in the production tanks, seed costs and acclimation survivals would likely determine the acceptability of the rate of return for the project (Table 10-18). If the price of postlarvae increases to $12 per thousand, the project is unacceptable even if acclimation survival rates are 90%. However, if postlarvae can be purchased for $6.00 per thousand, positive net present values are obtained even if

Table 10-18: Sensitivity analysis of the effect of postlarval price and acclimation survival rates on NPV and IRR, assuming production tank survival rates of 70% and a growout period of 180 days.

Acclimation Survival Rates PL Price

($/1000 PLs) 75 % 80 % 85% 90% NPV $ 16, 968 $ 34,587 $ 50,134 $ 63,953 $6.00 IRR 28 % 29 % 31 % 32 % NPV ($ 9,836) $ 9,459 $ 26,484 $ 41,617 $8.00 IRR 25 % 27 % 29 % 30 % NPV ($ 36,640) ($ 15,670) $ 2,833 $ 19,280 $10.00 IRR 22 % 24 % 26 % 28 % NPV ($ 63,444) ($ 40, 799) ($ 20,818) ($ 3,056) $12.00 IRR 20 % 22 % 24 % 26 %


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acclimation survival drops as low as 75%. At intermediate seed prices, the acclimation survival rates will determine whether or not the net present value of the project is positive or negative.

Market Prices The economic model assumes the shrimp can be sold head-on for a price of $5.24/lb, or 50% above the current wholesale price for a 36-40 count shrimp. A sensitivity analysis was conducted to determine the minimum heads-on price the producer must receive for the shrimp in order for the investment to be an acceptable risk. This was accomplished by determining the product price at which the net present value is equal to zero, using a discount rate of 26% (6% opportunity cost + 20% risk premium). The analysis was carried out for each of four different production tank survival rates and two different growout times (180 days and 150 days). The results are presented in Table 10-19. The baseline scenario for the model assumed that the shrimp can be sold as a whole, heads-on product for $5.24 per pound. Presently there is insufficient market information to know whether or not the product can be sold at that price on a consistent basis. The price used in the baseline scenario is not really an important price to consider. The prices shown in Table 10-19 are much more significant. The breakeven price to cover cash costs is the cost of producing a pound of shrimp under a given set of conditions. Production should not even be considered unless this price is virtually guaranteed. The breakeven price for all costs is a measure of the total cost of production per pound of shrimp when depreciation of capital items and income taxes are added to the total operating costs. This is the minimum price that must be received to guarantee that the enterprise will not lose money in the long run. Unless this price can be earned for the shrimp, production will never pay back the capital costs associated with the project. The minimum economic price is the price at which the net present value is equal to zero under a given set of assumptions regarding production, production costs, and capital costs. This is the price that must be received to cover the

Table 10-19: Sensitivity of breakeven prices and minimimum economic prices to

survival and growout time.

Survival Growout Time Item

60% 65% 70% 75% Breakeven Price (Cash Costs) $3.56 $3.35 $3.15 $2.99 Breakeven Price (All Costs) $4.88 $4.57 $4.28 $4.04 Minimum Economic Price (Heads-on) $5.98 $5.59 $5.23 $4.93 Minimum Economic Price (Heads-off) $9.49 $8.87 $8.30 $7.83

180 days

Percentage of Current Frozen Price 171 % 160 % 150 % 141 % Breakeven Price (Cash Costs) $3.15 $2.97 $2.79 $2.65 Breakeven Price (All Costs) $4.22 $3.96 $3.72 $3.51 Minimum Economic Price (Heads-on) $5.08 $4.76 $4.46 $4.20 Minimum Economic Price (Heads-off) $8.06 $7.56 $7.08 $6.67

150 days

Percentage of Current Frozen Price 146 % 136 % 128 % 120 %


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opportunity cost of capital and the risk premium associated with the use of that capital in a non-secure venture. This is the minimum price that must be received for the investor to be indifferent to the investment. Below this price it would be better for investors to leave their money in a secure investment such as a CD or treasury bond. Only if the price is greater than the minimum economic price would it make sense for an investor to consider putting money into the project. For reference, the minimum economic price is computed for each scenario as a heads-off price (Heads-off price = Heads-on price/lb ÷ 0.63 lb tails/lb whole shrimp). The heads-off price is then expressed as a percentage of the current price for frozen 36-40 count shrimp (Fulton Fish Market, June 22, 1999). Note that the minimum economic price (heads-off) is 120% of the current wholesale price for the most favorable scenario (75% survival, 150 day growout time). The percentage of the current frozen price ranges from 120% to 146% for the 150 day growout scenarios, and from 141% to 170% for the 180 day growout scenarios. These figures clearly illustrate the need to market shrimp grown in this kind of production system through direct marketing channels as a premium product. In order for the producer to receive the needed prices, the product must be clearly recognized by the consumer as a unique product, clearly superior to the frozen tails available at much lower prices. Good information is needed on such issues as shelf-life, consumer preferences, and the volume of shrimp that can be sold through direct marketing channels at premium prices.

Conclusion The economic analysis shows that an intensive shrimp production facility will have to achieve better growth rates and survivals than were observed in Harbor Branch’s FDACS shrimp demonstration project in order to be economically feasible. A baseline scenario based on the actual stocking densities, survivals, growth rates and costs obtained during the one year demonstration project only produced a 13% internal rate of return. However, sensitivity analyses demonstrated that if growth rates can be improved so that the shrimp grow at rates typically observed in ponds, this kind of an enterprise could earn an economic profit. Improving survival rates from 60% to 70%, holding other variables constant, would also result in economic profitability. If both growth rates and survival can be improved, the economic potential improves dramatically. Economic returns are less sensitive to seed costs, but seed costs could tip the scales one way or the other for scenarios that are marginally profitable. With modifications in the management and/or design of intensive production systems for shrimp, it seems likely that growth rates and survival rates can be improved sufficiently for shrimp production in intensive freshwater recirculating to become a profitable business. It is strongly recommended, however, that anyone considering this type of enterprise begin on a small scale to demonstrate whether or not the contemplated production system and management regime can, in fact, achieve the required survival, growth rates, and cost structure for the business to succeed.


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The profitability of an intensive shrimp production system such as this requires that the shrimp be sold at prices from 20% to 70% above the current wholesale price for frozen shrimp. The volume of shrimp that can be sold and the price that can be received through direct marketing channels will depend, in large measure, upon the marketing efforts of the producer. Potential investors should thoroughly research their markets before attempting this type of enterprise.

Literature Cited Moss, S.M. (1999). Biosecure Shrimp Production: Emerging Technologies for a Maturing

Industry. Global Aquaculture Advocate 2(4/5): 50-52. Moss, S.M., G.D. Pruder, K.M. Leber, and J.A. Wyban. 1992. The relative enhancement of

Penaeus vannamei growth by selected fractions of shrimp pond water. Aquaculture 101: 229-239.

Scura, E.D.. 1995. Dry season production problems on shrimp farms in Central America

and the Caribbean Basin. In, C.L. Browdy and J.S. Hopkins, editors. Swimming Through Troubled Waters, Proceedings of the Special Session on Shrimp Farming, Aquaculture ’95. World Aquaculture Society, Baton Rouge, Louisiana. pp. 200-213.

Appendix A – Ammonia Mass Balance Analysis

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Appendix A

AMMONIA MASS BALANCE ANALYSIS


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Ammonia Mass Balance Analysis by

Peter Van Wyk Harbor Branch Oceanographic Institution

. The required flow rates in a recirculating aquaculture system can be estimating using a process called mass balance analysis. This approach is based on the physical law of conservation of mass, which says that mass cannot be created or destroyed, but only transformed. Losordo (1991) demonstrated how to use this concept to estimate the required flow rates in recirculating aquaculture systems. What follows here is based on Losordo’s mass balance approach for estimating required flow rates. In order to apply the mass balance approach, the following steps must be taken:

1) The system boundaries must be defined. 2) All flow streams crossing boundaries must be identified as eith input or output. 3) The material to be balanced must be identified. 4) The processes which occur inside the system to transform the material must be

identified. Once these steps have been taken, the mass balance equation can be written:

The Rate of Accumulation of Mass Inside the System =

The Rate of The Rate of The Net Rate of Flow of Mass - Flow of Mass + Transformation of Mass Into the System Out of the System Within the System

The transformation of mass within the system can result from processes which generate the particular form of the material of interest (ammonia, for example), and from processes that consume that material. Rewriting the mass balance equation to express this idea: (1) Accumulation = Input - Output + Generation - Consumption When applying this analysis to a recirculating system, the system boundaries include the culture tank and the water treatment system, as well as all the plumbing required to make a complete circuit ( Figure 1). Makeup water is a flow crossing this boundary, so any new material entering the system in the makeup water would be considered input. Any material lost from the system in the effluent is considered output. Generation and consumption would result from biological or chemical transformation processes within the system. In a steady state system there is no net change in the amount of the material in question. The accumulation term in Equation (1) is zero. Under these conditions, input plus generation are exactly balanced by output plus consumption: (2) Input + Generation = Output + Consumption.


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If we apply the mass balance approach to analyze a particular water quality variable, such as ammonia, we can determine the conditions that will be required to attain a steady state condition with respect to that variable. For example, we will make the assumption that after the system adjusts to a given feed rate and water exchange rate, the ammonia concentration in the system will settle down to some equilibrium concentration. For design purposes, we want to make sure that when the system is at maximum capacity the equilibrium concentration of ammonia is within acceptable limits for the culture species. System Ammonia Mass Balance Ammonia is one of the most critical water quality parameters in nearly all recirculating aquaculture systems. Ammonia is generated within the system as a by-product of protein metabolism. The rate of ammonia generation is a function of the feeding rate. In a clear system (no significant algal biomass), ammonia is consumed by nitrifying bacteria residing largely in the biofilter. The rate of nitrification by these bacteria depends upon the amount of ammonia passing through the biofilter. A key design question is “What flow rate will the biofilter require to make sure that the steady state equilibrium concentration of ammonia is within the acceptable limits for the culture species?” To answer this question, we need to be able to express mathematically the input, output, generation, and consumption terms in Equation (2).

Figure 1: System Diagram for Ammonia Mass Balance


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The input of ammonia into the system will be quantity of ammonia entering the system in the makeup water. This is calculated by multiplying the concentration of TAN in the makeup water by the flowrate of makeup water into the system: (3) Input = (Q * CTANin) where, Q = flow rate of makeup water CTANin = concentration of TAN in makup water. The output of ammonia from the system will be the quantity of ammonia exiting the system in the effluent. This is calculated by multiplying the concentration of TAN in the effluent by the effluent flow rate. If the system volume is not changing over time (the system is neither filling up nor being drained), the effluent flow rate will exactly equal the flow rate of makeup water into the system. Output can be expressed mathematically as follows: (4) Output = Q* CTANout where, Q = flow rate of effluent (= flow rate of makeup water) CTANout = concentration of TAN in the effluent. The generation of ammonia within the system is the result of metabolism of the proteins in the feed. The rate of ammonia production is dependent upon the feeding rate, the protein content of the feed, the fraction of protein nitrogen that is excreted as TAN, and the rate of TAN excretion:

(5) P TAN =

FA x PC x 0.092t

where, PTAN = Rate of Ammonia Production FA = Amount of Feed per Feeding PC = Protein Content of the Feed, expressed as a decimal fraction t = time in which all of the TAN from a given feeding is excreted. The number in the formula, 0.092, is the fraction of protein nitrogen that is excreted as TAN. This is calculated by multiplying the fraction of nitrogen in protein (0.16) by the fraction of nitrogen in the feed protein that is assimilated (0.80), the fraction of assimilated nitrogen that is excreted (0.80) and the percentage of excreted nitrogen that is excreted of ammonia (0.092 = 0.16 x 0.8 x 0.8 x 0.9). There are a couple of simplifying assumptions that are being made here. We are assuming that all of the non-assimilated nitrogen in fecal material is removed rapidly from the tank before the heterotrophic bacteria have time to break down the protein in the feces and excrete more ammonia into the system. We also assume that 100% of the TAN is excreted


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within t hours after a feeding. This assumption requires that the time interval between feedings must be greater than t, the ammonia excretion time. The consumption of ammonia within the system is the rate at which ammonia is converted to nitrite and nitrate within the biofilter. This will be a function of the rate at which ammonia enters the biofilter, and the efficiency of the biofilter. The biofilter efficiency is the fraction of TAN removed in one pass through the biofilter. The rate of ammonia removal by the biofilter can be expressed mathematically as: (6) RTAN = Qf x CTAN x E where, RTAN = ammonia consumtion rate Qf = flow rate to the biofilter CTAN = concentration of ammonia entering the biofilter E = ammonia removal efficiency of the biofilter. Estimating Required Recycle Flow Rates Based on Ammonia Mass Balance If we know the values of the other variables in the mass balance equation, we can calculate the required recycle flow rates by rewriting Equation 2 using the mathematical representations of the input, output, generation and consumption terms, and the solving for the recycle flow rate, Qf. The following equation is Equation 2 expressed in mathematical form:

6)

(Q x CTANout) + (Qf x CTAN x E) = (Q x CTANin) + PTAN output consumption input generation

If we know the values of the other variable, we can solve for the recycle flow rate, Qf., by rearranging to express Qf in terms of the other variables:

7) E x CTAN

CTANout) x (Q - P CTANin) x (Q Qf TAN+=

In most cases we will have good information about the values of the variables on the right side of Equation 7. Q is the flow rate of makeup water and is typically assumed to produce an exchange rate equivalent of between 5-10% of the system volume per day for most recirculating systems. CTANin is the concentration of ammonia in the makeup water. This value should be zero or close to zero. In some locations the well water comes out of the ground with relatively high levels of ammonia. If this is the case, the water should be pretreated through a biofilter so that CTANin is as close to zero as possible. PTAN is dependent on the feed rate (FA), the protein content of the feed (PC), and the ammonia excretion time (t). For design purposes, FA should be calculated based on the highest anticipated biomass the system will experience:


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(8) FA = Shrimp Biomass x

Feed Rate (% Bodyweight/Day)Number of Feedings/Day

The protein content of the feed should be known in advance based on published studies and the feed manufacturer’s recommendations. Most diets for shrimp grown in high density recirculating systems contain between 35% and 40% protein. The ammonia excretion time is approximately 6 hours. The value generally used for CTANout is the maximum value of TAN that the shrimp can tolerate with no appreciable effect on growth or health. Levels of unionized ammonia of greater than 0.03 mg/l begin to affect the physiology of the shrimp. We can use unionized ammonia tables to determine the TAN that would produce the maximum acceptable unionized ammonia concentration at a specific pH and temperature. To be conservative, select a pH and temperature combination at the upper end of the expected values for these parameters. For example, at a pH of 8.0 and a temperature of 32°C the fraction of unionized ammonia is 0.075. Under these conditions, the maximum acceptable TAN (CTANout) would be 0.4 mg/liter (0.4 = 0.03/0.075). The biofilter efficiency, E. is difficult to estimate because biofilter efficiency is a function of many factors, including ammonia concentration, hydraulic loading rate, filter surface area, filter media type, temperature, pH, salinity, alkalinity, DO, TSS, and filter type (submerged, trickling, fluidized bed, etc.). One of the reasons for building a prototype of the production system is to gather information on biofilter performance under different conditions and to determine a reasonable value of E to use in the mass balance analysis. Under normal operating conditions a properly-sized biofilter should remove about 50% of the TAN in the water in a single pass. In fact, the biofilter is not the only place in the system where nitrification takes place. Up to 30% of the nitrification in a typical recirculation system takes place outside of the biofilter (Losordo, personal communication). Because of this, the method outlined here tends to overestimate the required recycle rate by about 30%. However, if the assumption that the fecal wastes are removed immediately is not met, then the amount of ammonia that would need to be removed by the biofilter would be higher. It may well be that these two error factors in the calculation balance out. This is likely to be the case in a system which accumulates solid wastes for 12-24 hours between backwashes. Example: Assume we have a 50,000 liter recirculating aquaculture system with a maximum biomass of 0.05 lbs/gallon (0.006 kg/liter). These shrimp will be fed a 35% protein feed at a rate of 2% of their bodyweight per day in four equal feedings 6 hours apart. Water will be exchanged at a rate of 5% of the system volume per day. The makeup water contains 0.1 mg/l ammonia nitrogen. The biofilter will have an efficiency of 50% (50% of the TAN will be converted to nitrate in a single pass).


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What should the recycle flow rate be to maintain the total ammonia nitrogen at a value less than or equal to 0.4 mg/l ? Step 1: Calculate the weight of feed per feeding and the rate of production of total ammonia-nitrogen (PTAN ). Maximum biomass = 50,000 liter x 0.006 kg/liter = 300 kg of shrimp FA = (300 kg shrimp x .02 kg feed/kg shrimp) / (4 feedings/day) = 1.5 kg feed per feeding

PTAN =

1.5 kg feed/feeding x 0.35 x 0.092 x 106 mg / kg 6 hrs

= 8,050 mg TAN/hr Step 2: Calculate flowrate (Q) of makeup water and wastewater: Makeup water = 5% of System Volume/day x System Volume (liters) = .05/day x 50,000 liters x 1day/24 hours = 2,500 liters/day x 1 day/24 hours = 104 liters/hour Step 3: Calculate recycle flow rate using Equation (7): Eq. (7): Qf = (Q x CTANin + PTAN - Q x CTANout ) / (CTAN x E ) Q = 104 liters/hour (from Step 2) CTANin = 0.1mg TAN/liter (given) PTAN = 8,050 mg TAN/hour ( from Step 1) CTANout = CTAN = 0.4 mg TAN/liter (by assumption) E = 50% (by assumption) Qf = ( (104 l/hr x 0.1 mg TAN/l) + 8,050 mg TAN/hr - (104 l/hr x 0.4 mg TAN/l) ) / ( 0.4 mg/l * 0.50 ) = (10.4 mg TAN/hr + 8,050 mg TAN/hr - 41.6mg TAN/hr) / 0. 20 mg TAN/liter = 40,094 liters/hour (176.5 gpm) This corresponds to one turnover every 1.25 hours, or every 75 minutes.


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Literature Cited: Losordo, T.M.. 1991. An introduction to recirculating production systems design. In, M.B.

Timmons and T.M. Losordo, (eds.) Engineering Aspects of Intensive Aquaculture. Proceedings from the Aquaculture Symposium, Cornell University, April 4-6, 1991. Northeast Regional Agricultural Engineering Service, Ithaca , New York. NRAES – 49, pp. 32-47.

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APPENDIX B

FRICTION LOSS TABLES

farming marine shrimp in recirculating freshwater systems

Documents