Download - 26238Impacts of Shellfish Aquaculture on the Environment

8/12/2019 26238Impacts of Shellfish Aquaculture on the Environment

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April 2008

Shellfish Industry Development StrategyA Case for Considering MSC Certification for ShellfishCultivation Operations


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CONTENTS

Page

Executive Summary 3

Introduction 5

Mollusc Cultivation

MusselCultivationBottom Culture 6

Spat Collection 6Harvesting 7

Suspended Culture 7Longline Culture 8Pole Culture 8Raft Culture 9Spat Collection 10Environmental Impacts 11

ScallopCultivationJapanese Method 13New Zealand Methods 15Scottish Methods 15Environmental Impacts 16

Abalone Cultivation 16Hatchery Production 17Sea Culture 17Diet 18Environmental Impacts 19

Clam Cultivation 19Seed Procurement 20ManilaClams 20Blood Cockles 20Razor Clams 21

Siting of Grow Out Plots 21Environmental Impacts 21

Oyster Cultivation 23Flat Oysters 24Cupped Oysters 24Hanging Culture 24Raft Culture 24Longline Culture 25Rock Culture 25Stake Culture 25

Trestle Culture 25Stick Culture 26


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Ground Culture 26Environmental Impacts 27

Crustacean Culture

Clawed Lobsters

Broodstock 29Spawning 29Hatching 29Larval Culture 30Nursery Culture 30On-Growing 31Ranching 31Environmental Impacts 32

Spiny Lobsters 32Broodstockand Spawning 33

Larval Culture 33On-Growing 33Environmental Impacts 34

Crab CultivationBroodstock and Larvae 34Nursery Culture 35On-growing 35Soft Shell Crab Production 36Environmental Impacts 36

Conclusions 37

Acknowledgements 40

References 40


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EXECUTIVE SUMMARY

The current trend within the seafood industry is a focus on traceability and sustainability withconsumers and retailers becoming more concerned about the over-exploitation of our oceans.The Marine Stewardship Council (MSC) has a sustainability certification scheme for wild

capture fisheries. Currently there is no certification scheme for products from enhancedfisheries1 and aquaculture2. It is the view ofmany producers that the production of shellfishin enhanced fisheries and aquaculture is more sustainable than the wild capture fisheries forthese products and that certification for these products should be considered. The purpose ofthis report was to review the current scientific literature and compare the results to the criteria

required for compliance to Principle 2 of the MSC s Principles and Criteria for SustainableFishing in order to determine whether such enhanced shellfisheries could proceed throughMSC assessment.

Shellfish production can be divided into mollusc cultivation and crustacean cultivation.Mollusc cultivation mainly concerns bivalve molluscs such as mussels, oysters and scallops.For example mussel culture takes place on the seabed or in suspension from rafts and

longlines. Bottom culture is characterised by the re-laying of wild harvested spat ontosubtidal and intertidal beds. The mussels are grown for 1-2 years before harvesting byeither hand-collection, hand-raking or hydraulic dredge. Hand-collection and hand-rakingsupport artisanal fisheries and have little impact on the environment, and as such may complywith the criteria for Principle 2. The use of hydraulic dredges has a greater impact on theenvironment which may prove too detrimental to allow compliance to Principle 2. For adefinitive conclusion to be made research into this specific area should be conducted. Themain issue with suspended culture concerns the increase in sedimentation below farm systems

and the effect sedimentation has on the ecosystem. In the context of this report,sedimentation refers to the settlement of organic and inorganic particulate matter settling from

the water onto the seabed. There are conflicting arguments within the literature;however themajority of research indicates that impacts are minimal and localised. It is possible to showthat suspended culture could comply with Principle 2.

Scallop cultivation is similar to mussel culture in that it can be divided in to suspended cultureor bottom culture, although bottom culture is classified as stock enhancement due to themobile nature of scallops. Suspended culture has the same potential impacts of musselculture with sedimentation being the primary concern. There has been much less researchinto the cultivation of scallops but the current research suggests that there are no adverseimpacts on the environment. The restocking of scallops has a more detrimental impact on theenvironment due to the harvesting method by dry dredge (scallop dredge), which is a high-impact gear which could be destructive if used in sensitive areas.

The environmental impacts of abalone culture have received little if any attention from thescientific community and as such no conclusion regarding compliance to Principle2 could bemade. It is noted that there may be issues with the use of wild harvested algae as a foodsource for the abalone.

Clam culture generally takes place in or on the seabed. As with other bottom cultures, the useof dredging to harvest the product could raise concernsregarding the environmental impactsof this activity if used in sensitive areas. More research is required in this area as theconclusions often have to be inferred from wild capture fisheries which impact much larger

1An enhanced fishery is described as a wild capture fishery where the natural population is enhanced

through the input of hatchery reared juveniles or the introduction of structures to enhance production. 2Aquaculture can be defined in many ways; for the purpose of this report aquaculture is defined as thecontrolled farming of aquatic organisms from the larval stage to commercial size.


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areas. There are also examples of clams being grown in bags placed on trestles. This method

has a reduced impact and may comply with the criteria for Principle 2.

Oyster cultivation is possibly one of the most sustainable types of shellfish culture. There area wide range of methods for culturing oysters, mainly concerned with the use of a structure tosupport the growth of the oysters either in suspended culture or on the seabed. Concerns have

been raised regarding the removal of large quantities of phytoplankton (Chapelle et al, 2000;Newell, 2004) and the increase in sedimentation, however, as with mussel culture there areconflicting arguments within the literature and it appears that local conditions have animportant influence on the extent of impacts from culture systems. The major concern withoyster culture in the USA is the use of Carbaryl, a pesticide, to control burrowing shrimppopulations. By removing organisms from the ecosystem such a practice could result in afailure to meet the criteria of Principle 2.

In contrast to mollusc culture, the culture of crabs and lobsters is in its infancy. Lobsterculture is mainly concerned with taking wild broodstock and rearing larvae to a stage wheresurvival rates are higher than in the wild, at which point the juvenile lobsters are re-

introduced into the natural environment. The use of local broodstock and selective harvestingmethods of the fishery could meet the criteria for Principle 2. Providing the naturalpopulation is not over exploited and healthy, the restocking exercises can improve the wildstocks. Spiny lobsters have received more attention in the tropics and the concerns with theirculture are the low numbers of available larvae from wild stocks.

Crab culture is confined to the tropics where the current trend is to integrate the culture withmangrove regeneration. Integrated cultivation methods of this kind are improving theenvironment and could provide a good example to the rest of the aquaculture industry. Theconcerns over crab culture are the use of wild caught larvae for on -growing and the use of by-catch as a food source. At present the industry is perceived as small and sustainable andwould likely meet the criteria for Principle2, but to support a growth of the industry hatchery

production of larvae and artificial feeds will need to be developed.

In conclusion, it appears that local conditions are vitally important as to whether the impactsfrom shellfish aquaculture are having a detrimental effect. A principle that should beconsidered when assessing the sustainability of a product is the carrying capacity of theculture site. In particular interest of environmental sustainability is the ecological carryingcapacity, which is the level of production that an area can support without having a negativeimpact on the environment. The carrying capacity can be assessed using models and it issuggested that these models are used for each site when considering whether a product issustainable. It should also be noted that shellfish culture can have positive effects on theenvironment by filtering the water column and removing excess nutrients and increasingbenthic-pelagic coupling (Newell, 1988; Mann, 2000) and that this should be considered, andwritten into the criteria for principles of sustainability, as it is an important factor that wildcapture fisheries cannot offer.

Enhanced shellfisheries should be considered as suitable for proceeding through MSCassessment as though a wild-caught fishery as they operate quite differently from traditionalfinfish aquaculture systems.


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INTRODUCTION

The shellfish industry relies on a complex relationship between producer, retailer andconsumer. At present there is a lot of focus from the consumer on traceability, withsustainability of the product being a focal point of this traceability. The demand from

the consumer for sustainable produce has led to retailers to search for such products.Sustainable accreditation is currently administered by the Marine StewardshipCouncil (MSC), who determineswhether a fishery is managed effectively and fishedin a sustainable manner. Currently, the MSC only certifiesun-enhanced wild capturefisheries, but with more focus being placed on certified products, there is a call forcertification of enhanced fisheries and aquaculture products from some of the largerretailers (http://www.msc.org/html/ni_346.htm). The purpose of this report is toexamine the methods used to culture shellfish, or enhance their fisheries, and byreviewing the current scientific research illustrate; any environmental impacts whichmay prevent the sustainability of the activity, the gaps in the research, and whetherthere is scope for these products to be certified by the MSC.

The MSC have proposed that enhanced fisheries could be considered for certificationif they can be categorised under one of the following production systems:

Type 1. Production systems that involve wild harvest followed by a grow outphase.

Type 2. Production systems that involve the introduction of fish either as eggs,larvae or juveniles. The introductions may be of native species(restocking) or exotic species.

Type 3. Production systems that involve the modification of habitats to makeproduction easier or to favour desirable species.

The methods of cultivation will be categorised asone of the above production systems

where possible and if not, additional categories will be proposed. In addition tocategorising the production systems, the environmental impacts will also be assessedaccording to the MSC Principles and Criteria for Sustainable Fishing . In order tobe considered sustainable, the impacts from a culture system must be covered byPrinciple2 of the document which states;

Fishing operations should allow for the maintenance of the structure, productivity,function and diversity of the ecosystem (including habitat and associated dependant

and ecologically related species) on which the fishery depends.The environmental impacts will be assessed according to the criteria of Principle 2and ajudgementwill be made as to whether they would likely pass, fail, or whetherthere is insufficient research in the area to make a definitive decision.
http://www.msc.org/html/ni_346.htm


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MOLLUSC CULTIVATION

MUSSEL CULTIVATION

Mussel production extends to all regions of the globe. The majority of culture takesplace in temperate climatesand in particular Europe (467 658t in 2005) with Spain,the Netherlands, Denmark and Italy the leading producers, along with China (772173t in 2005) (FAO, 2007). The primary species for production are the blue mussel,

Mytilus edulis, and the Mediterranean mussel, Mytilus galloprovincialis, with greenmussels, Perna spp., becoming increasingly important in tropical and sub-tropicalclimes such as New Zealand and the Far East. The cultivation of mussels can becategorised into two main forms; the two-dimensional bottom culture, or three-dimensional suspended culture, and for the purpose of this report each method will beassessed separately.

Bottom Cultivation

The cultivation of mussels on the seabed occurs in intertidal and subtidal areas insheltered bays where there is typically an abundant natural spatfall. The mainEuropean centre for bottom cultured mussels is the Wadden Sea off the Netherlands,Germany and Denmark. The Dutch have leased large plots of the western area of theWadden Sea to mussel producers. In the UK areas such as The Wash and MorcambeBay have traditionally supported large mussel fisheries and production; however inrecent years there has been a regular failure in mussel recruitment, leading tonegligible production. Other areas however have seen increases in production. Onesuch area is the Menai Strait in North Wales. The mussel production in the MenaiStrait relies on collected spat from the Irish Sea, which is re-laid in plots around lowwater springs, where production of the mussels can be maximised. With the use ofanti-crab fencing around these plots the farmers in the Menai Strait have obtainedyields as high as 8:1 (Spencer, 2002). The bottom culture of mussels can be classifiedunder the type 1 production system described previously (a wild harvest followed bygrow out). Below the different stages are discussed with regard to methodology,environmental impacts, and how these effects can be classified.

Spat Collection

Spat are collected at an age of approximately 1 year old, when they measure between10-30mm shell length (SL)(Spencer, 2002). Spat are generally collected locally fromless suitable grounds and re-laid in higher densities in more productive areas. Such amethod of local collection results in a minimal impact, if any, on the local genome.There has been little research regarding the collection of mussel seed by dredging, and

this is likely to be the process which has the greatest effect on the ecosystem. Therehas been significant research into the effects of the heavy-duty dredges used to collectscallops in commercial quantities, with evidence showing dramatic decreases innumbers of species and abundance, with recovery periods in excess of 3 months(Thrush et al, 1995; Curry & Parry, 1996; Jennings et al, 2001). The differencesbetween such dredging activities and the collection of mussel spat are numerous, and

it is likely that such extreme effects will not be encountered. Firstly the mussel spattend to be concentrated in high numbers in distinct areas, unlike the sparsely


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aggregated scallops and clams, leading to a reduction in the area affectedby dredging.

In the UK, licenses to collect mussel spat are issued only following the formation ofmussel mud , which consists of dead shells, silt and pseudofaeces. The mussel mud

creates an unstable layer beneath the spat, which causes the spat to detach their byssalthreads. At this stage the dredges are able to skim off the mussel mud, collecting the

spat whilst leaving the substratum relatively undisturbed. In terms of reducing naturalstocks of mussels by removing the spat, it should be noted that the unstable beds arelikely to be destroyed by winter storms, resulting in the loss of the mussels from thefishery (Kaiser et al,1998). The impacts of mechanical dredging on the environmentare likely to be minimal and limited to small areas. The seasonal nature of spatcollection also allows recovery periods of up to a year, which should be ample timefor the recovery of the ecosystem. One potential impact on the ecosystem from theheavy exploitation of natural mussel beds is the removal of a valuable food source forpredators. In 1991 and 1992 the entire intertidal mussel stock was removed from theWadden Sea, leadingto high levels of mortality in the eider duck population whichutilised the mussel beds as a major food source. It has been suggested that these

effects could persist for many years if beds are not allowed to redevelop and mature(Dankers & Zuidema, 1995). It is the opinion of the author, and others (Kaiser et al.,1998), that providing that entire stocks are not removed, the limited negative effectswill be outweighed by the positive benefits. Although there has been little researchinto the effects of dredging for spat, and as such whether Principle2 is met or not isinconclusive, attention should be paid to the removal of high proportions of spat. The

problems encountered in the Wadden Sea illustrate a possible failure to meet the firstcriteria of Principle2 as the functional relationship between mussel and eider duck isaltered, resulting in high mortalities of the predator.

Harvesting

Harvesting bottom cultured mussels can vary from mechanised dredging and powerdredging as used in the Dutch Wadden Sea and in Poole harbour, England, to simplehand-collection or hand-raking on lower yield operations (Spencer, 2002). As withthe collection of spat there is no research into the environmental impacts of musselcollection using dredges. The dredges differ to those used in commercial capturefisheries and as such analogies should not be made. The gap in the research preventsa conclusion being made as to whether harvesting by dredging would comply withPrinciple2. That said, it should be noted that the high densities of mussel result in arelatively small area being affected, and the plots are usually re-used soon after

harvest and as such the ecosystem could be classified as a permanent commercialmussel bed. Hand-collection and hand-raking, although there is no scientific researchsupporting this view, are likely to have minimal environmental impact. Hand-collection may involve some trampling of benthos, but recovery rates are likely to berapid. It is likely that the latter two methods of harvesting could comply withPrinciple2 of the MSC criteria, providing the scientific research is performed to provethis.

Suspended Cultivation

The methods of suspended culture utilised in different regions originate from a

combination of traditional methods, local availability of materials and site. Theadvantages of suspended culture methods are the increased exposure to water currents


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and the reduction of predation on the cultured population from benthic predators.There are three principle methods of off-bottom culture; longline, pole, and raftculture, and a variety of different designs and methods have arisen to take advantageof local conditions. All three types of suspended culture are classified as type 1production systems.

Figure 1: A mussel longline in the River Fal, Cornwall, UK. Two longlinesapproximately 200m long are supported by a series of buoys.

Longline Cultivation

Longline culture of mussels is the method of choice in more exposed waters due tothe flexibility of the structures. A typical arrangement will consist of a series ofhorizontal ropes that are held in the top 3m of the water column by buoys (Figure 1).From the headlines are a series of dropper ropes, which carry the mussels. Thedropper ropes are typically 4-6m in length and spaced at 50cm intervals (Spencer,2002). The carrying capacity of the longlines is dependant on the volumetric capacity

of the floats, and as the mussels grow, additional floats are required to support thecrop.

Pole Cultivation

The origins of pole culture lie in France, where it began as early as the thirteenthcentury, making this method the oldest of the mussel cultivation techniques. Knownas the bouchot method, it consists of vertical poles driven into the seabed and isessentially a shallow water technique, with access to the poles at low tide or bydiving. The poles are typically 4-7m long (with 2-3m rising above the seabed) and25-30cm in diameter, and are made from hardwoods such as oak and pine, or morerecently aluminium. The poles are places in parallel rows at right angles to the shoreline, and are usually positioned around the ELWM3. For this method the spat arecollected on poles or coir rope attached to metal frames placed in deeper water. The

seed are then transferred to tubular nets, which are then wound around the bouchot3Extreme Low Water Mark, the lowest point the tide reaches on spring tides.


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poles and nailed into place. From these nets the seed will migrate out onto the surface

of the poles and settle. Tropical culture methods based on the bouchot methodinclude the use of bamboo poles placed in the seabed, and although the materialsdiffer the methods are very similar.

Figure 2: A mussel farm on the River Fal, Cornwall, UK. Visible are the rafts (20mx 12m) and the dropper ropes which extend 8m below the rafts.

Raft Cultivation

The use of rafts to cultivate mussels is becoming increasingly popular and is utilisedby many countries including Australia, Chile, USA, Ireland, Scotland, Singapore andVenezuela to name a few. Raft culture is based around a framework of timbersupported by floats, with a series of ropes attached within the framework of the raft.The rafts require a water depth several metres deeper than the length of the ropes toprevent the grounding of the mussels during low tides, which would lead to damagingof the mussels and increased predation. Water quality, current speed and shelter arealso important in the siting of mussel rafts. On the west coast of Scotland theMuckairn raft is extensively used. Wooden beams 11m in length (10 x 8cm), spaced50cm apart, are supported by plastic foam-filled floats (120 x 80 x 65cm). Two

hundred synthetic ropes, each 8m in length are spaced 25-40cm along the beams(Spencer, 2002). These rafts have a carrying capacity of over 10t each, equating to6kg of mussel per metre of rope. In the River Fal, Cornwall, a Spanish raft design isused. Timber rafts are 20m long and 12m wide and support 600 dropper ropes, each8m in length (Figure 2). There are ten rafts in total which results in a culture system48km in length, producing 200-300 tonnes of mussels a year (S. Kestin,pers. comm.).The farm is situated in 12-14m of water, with depths in the estuary reaching 30m inthe main channel. There is considerable current flow through the farm, yet it is stillregarded as sheltered as there is little wind or wave action. Salinities are stable at34ppt as freshwater influence does not reach the farm site. Spat collection occurs atthe farm on ropes attached to the raft. The seed are then stripped from the ropes and

placed into cotton socking which is then reattached to the ropes (Figure 3), which are


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then replaced onto the rafts. Thinning of the mussels occurs throughout the grow outprocess.

Figure 3: A hopper being used to fill cotton socking with mussel seed and attachingthem to grow out ropes on a raft culture site, River Fal, Cornwall, UK.

Spat Collection

The continuity of suspended mussel cultivation is greatly dependant on the acquisition

of spat. Currently the majority of spat is obtained from natural populations. Hatchery

produced spat are available; however, for the majority of markets the use of these spat

is uneconomical.

The collection of wild spat is highly dependant on local conditions and a successfulfarm manager will be able to judge when conditions for spatfall are optimal, and timethe collection appropriately. A wide variety of materials are used for spat collectiondependant on local availability, durability and cost. In the Far East bamboo poles areused in a similarway to the bouchot method, whereas the most common method is the

use of ropes. This is mainly due to theirrelative cheapness and availability. Natural

fibres are excellent for the collection of spat as the hairy nature provides an admirablesubstrate for settling. In the more developed countries, synthetic ropes are used due totheir increased durability. Although they may not be as attractive to the spat as thenatural fibres, settling on synthetic ropes can be increased by inserting lengths ofunravelled rope to create crevices in which the spat can find shelter. The collectingropes are attached to longlines or rafts, similar to those used for on-growing. The spatare removed from the collecting ropes when they reach 10mm SL and are placed inmesh sacks which are then attached to the culture ropes. The spat migrate out of thesacks and settle on the ropes using their byssus and are then cultivated as previouslydescribed.


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Environmental Impacts

The impact on the marine environment and ecosystems associated with suspendedmussel culture is covered more extensively than most shellfish groups, the onlyexception perhaps being shrimp. There are many conflicting results in the literature

and this stems from the characteristics of each individual test area. The main concernwith suspended culture is a predicted increase in sedimentation beneath the rafts andlonglines, caused by reduced flow because of the suspension structure, and increasedlevels of faeces and pseudofaeces from the mussels and associated epibiota.Generally investigations into the effects of suspended culture on sedimentation rateshave shown that the effect is minimal and that the risk of adverse effects is low (Grant

et al, 1995; Jeffset al, 1999; Crawford, 2001; Crawford et al, 2001; Crawford, 2003;Crawford et al, 2003; Danovaro et al, 2004) although there are examples ofsuspended mussel culture having adverse effects on the local ecosystem (Freire etal,1990; Stenton-Dozey et al, 1999).

Hartstein and Stevens (2005) showed that there was no significant difference insedimentation rates between farm and control sites in Marlborough Sound, NewZealand. They did however;find a significant difference in the total organic matter(TOM) between farm sites and control sites in sheltered bays. There were nosignificant differences between farm and control sites in more exposed areas,illustrating the importance of local hydrodynamic forces on the effects thataquaculture can have on the local environment. Increases in sedimentation have beenattributed to the presence of mussel culture by several authors (Dahlbck &Gunnarsson, 1981; Grant et al, 1995; Chamberlain et al, 2001; Giles et al, 2006). Theincrease in sedimentation below mussel culture is due to the excretion of faeces andpseudofaeces by mussels and the associated epibiota which grow amongst themussels. The faeces are characterised by high levels of organic matter andphaeaopigments. Biodeposition can alter the characteristics of the sediment belowculture systems. Sediments below culture sites tend to be characterised by finer grainsizes and increased silt/clay content, with decreased porosity and water content (Gileset al, 2006). Faecal based sediments are also characterised by increased C:N ratiosand increased organic content (Christensen et al, 2003). An increase in sedimentation

and change in sediment character does not constitute a failure to meet the criteria ofPrinciple 2, and as such the impact on the environment could comply with the criteria.

However, the sediment can itself affect the ecosystem and these effects are nowconsidered separately.

A change in sedimentation rate and sediment characteristics can potentially lead to achange in the macrobenthic community below culture systems. A study by Giles et al(2006) showed that although there was a significant difference in sedimentation rate,the benthic macrofaunal community was not significantly effected, along with molarC:N ratio, and organic matter content; a fact mirrored by other investigations (Grant et

al, 1995). Stenton-Dozey et al(1999) found that the presence of mussel farms had asignificant effect on macrobenthic community structures in Saldanha Bay, SouthAfrica. The general pattern observed under mussel rafts and long-lines is a shifttowards opportunistic deposit feeders and carnivores (which scavenge on musselsfalling from the rafts). Outside the farm areas communities tend to be dominated by

deposit and suspension feeders, although this obviously varies between areas(Stenton-Dozey et al, 1999; Christensen et al, 2003). Trophic webs can also be


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altered by the presence of mussel culture structures. Freire et al(1990) demonstratedhow the diet of the Arch-fronted swimming crab, Liocarcinus arcuatus, wassignificantly affected by mussel rafts in the Ra de Arousa, Spain. They concludedthat the change in diet was due to a change in the macrobenthic community, and thatthe crabs were responding opportunistically to these changes, and taking advantage of

the more abundant species below the rafts. Such changes in macrobenthic communityand trophic relationships could constitute a failure to meet Principle2 according toboth criteria 1 and 2. There are however conflicting results in the literature, whichposes a problem when regarding the culture system as a whole. A study byChamberlain et al (2001) in southwest Ireland helps to clarify the problem. Theycompared transects at an exposed site and a sheltered site, and found that there waslittle difference in species diversity along the transect at the exposed site, illustratingthat the farm had little effect. However, at the sheltered site there was a decrease indiversity closer to the farm, and the community was impoverished, becomingdominated by opportunistic, deposit feeding polycheates. Insheltered areas there islittle movement of water leading to impacts confined to a localisedarea. The study byChamberlain et al(2001) demonstrated little or no effect beyond 40m of the farm site.In exposed areas the sediment arising from mussel culture is likely to be spread over a

wider area, although this will have a dilution effect so whilst a larger area may beaffected the effect will be reduced. The local hydrographic conditions shouldtherefore be considered before reaching a conclusion as to whether a culture operation

is having an adverse impact on the macrobenthic community.

Another concern regarding the effect of mussel culture on the environment is therecycling of nutrients, and the changes to the cycle created by aquaculture operations.It has been noted that aquaculture operations can create a concentration of nutrients in

the culture area (Christensen et al, 2003). In the mid-1980 s there was a fluctuation inthe meat quality of P. canaliculus, which was attributed to differences in foodavailability (Kaspar et al, 1985). The shortage in food was linked back to a reductionin nutrients required by phytoplankton, possibly linked to the removal of nutrients bymussel culture. A study by Kasper et al(1985) found that at a reference site 97% ofecosystem nitrogen was found in the sediment, whereas at a farm site between 59%and 71% of ecosystem nitrogen was found in the sediment with as much as 40% being

present in the mussels. Although the mussels excrete ammonium which is directlyavailable to phytoplankton and increases primary production, the harvest of musselsleads to an export of nitrogen from the system, resulting in an imbalance in nutrientcycling within the ecosystem. A calculation by Christensen et al (2003) estimated

that a harvest of 4200t mussels from Beatrix Bay, New Zealand, would result in anexport of ~ 59t of nitrogen. The alteration to nutrient cycles could potentially result inchanges to community structure or trophic webs, which could result in a failure tomeet criteria 1 of Principle 2. There is no evidence for such an effect althoughspecific research into this area has not been performed. It should also be noted that in

areas of enrichment, the removal of nitrogen could be beneficial to the ecosystem.Reviewing the current literature there is no adverse effect on the environment and assuch the culture system could comply with Principle 2 with regards to nutrientcycling, however, it should be noted that there is a potential risk.

One of the major problems associated with sediment accumulation below finfish

aquaculture systems is the formation of anoxic sediments. The majority of studiesconcerning mussel culture have demonstrated that the formation of such sediments


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doesnot occur (Grant et al, 1995; Christensen et al, 2003; Giles et al, 2006). Thereis however one exception where the presence of Beggiatoa spp on the sedimentsurface was noted, indicated an anoxic environment beneath a mussel farm in Sweden(Dahlbck & Gunnarsson, 1981). The formation of anoxic sediments creates asignificant impact to the ecosystem and could constitute a failure to meet the criteria

of Principle 2; however, with the majority of studies failing to demonstrate theformation of anoxic studies it should be considered that Principle2 could be met inthis regard.

SCALLOP CULTIVATION

Of the numerous pectinid species occurring across the globe, only 33 species areviewed as commercially important, 5 of these species coming from European waters.Global scallop culture production totalled 1.3 million tonnes in 2005 (FAO, 2007), a

decrease from previous years (reaching 1.7 million tonnes in 1997 [FAO, 1999]).Scallop production is a relatively young venture, starting in the 1930 s in Japan,where many of the major advances were made, particularly during the 1960 s when arapid decline in wild capture fisheries led to increased concentration on themariculture of pectinid species, particularly the Japanese Scallop, Patinopectin

yessoensis (Spencer, 2002). The Japanese and Chinese continue to dominate globalscallop production, contributing 97.9% of global production in 2005 (FAO, 2007).Although many countries have tried to use the methods developed by the Japanese,different species of scallops require specific conditions and have required furtherresearch, particularly in the area of hatchery production of spat. Here, methods usedin different countries are discussed in order to illustrate the differences, whilstattempting to represent the global picture.

Japanese Culture Methods

Although the majority of scallop production in Japan is concentrated aroundPatinopectin yessoensis (a cold water species), two temperate species are alsocultured towards the southern islands, namely Chlamys senatoria nobilisand Pectenalbicans. The Japanese have developed successful methodology for the hatcheryproduction of spat, however it remains relatively expensive and the high abundance of

natural spat provides a more economically viable option. Scallops spawn from March

to April and spatfall occurs after 5-6 weeks, between April and June, and is largelydependant on the area and local conditions. Following larval development, the spatsettle onto suitable surfaces, attaching themselves with byssal threads. Thisattachment is temporary and is followed by settlement on the seabed. The spat arecollected in mesh sacks, filled with monofilament, which are suspended fromlonglines and deployed in anticipation of spatfall (a method used globally for thecollection of wild seed). Spatfall for P. yessoensis can be predicted whenapproximately 50% of developing larvae reach a size of 200 m.

Spat are removed from the collecting sacks between July and September, whenapproximately 5mm SL in size, and graded into size groups. At this young stage the

spat are placed into pearl nets at around 100 individuals per net, and suspended fromlonglines. Stocking densities are continuously reduced as the scallops grow and can


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be expected to reach 50mm SL within a year, with survival rates as high as 90%.When the scallops reach 30mm SL in size they can either be placed in lantern nets,again suspended from longlines, for on-growing to market size (100mm SL), whichusually occurs within 2 years. Although lantern nets can produce higher growth ratesthan bottom culture methods, they are also liable to produce misshapen scallops due

to the scallops biting each other within the nets. In some areas pocket nets areutilised in a similar way to lantern nets. The pocket nets consist ofa vertical nettingcontaining pockets, which is suspended in the water column. Scallops are stocked atdensities of 2-3 per pocket and can achieve similar growth rates to lantern nets, whilstutilising two-thirds of the area. The grow out of scallops in lantern nets can becategorised as a type 1production system as described in the introduction.

Scallops can also be re-laid, or sown , on the bottom for on-growing after dredginghas occurredto remove starfish predators from the culture plots. Scallops tend to bestocked at around 5 individuals per m2 and can reach market size in 2.5-3.5 years.Although growth is slower in bottom culture and survival reduced (around 30% for

bottom culture) the scallops are not misshapen. Harvesting is usually performed bydredges and can result in up to 80% of the stock being recaptured. The relaying ofscallops for bottom culture can be classified as a type 2 production system. As thereis no containment of the scallops,it is basically a restocking exercise.

Figure 4: Chilean Scallops (Agropecten purpuratus) being prepared for ear-hanging culture. The scallops are connected to the rope through a hole drilled intothe ear of the shell.

Another method of culture used in Japan is the ear-hanging method (figure 4). Thismethod can only take place in more sheltered conditions away from wave and winddisturbance, and generally takes place in areas with depths of less than 10m. Ear-hanging is labour intensive at the initial phase, as each year-old scallop (40-60mmSL) is attached to a synthetic string or hook passed through a hole drilled into itsear . The scallops are attached to dropper lines, suspended from longlines, and

spaced 8-15cm apart. A longline of 100m can support up to 45 000 shells, a capacityof 3 times greater than a longline supporting lantern nets. Due to the freedom ofwater flow around the scallops, growth rate, survival and shell shape are all improved


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when compared to lantern net culture, and although initially labour intensive, there islessexpenditure on structures and servicing of the culture area. Ear-hanging is similarto lantern net culture in that both systems involve suspension from longlines. Thesimilarity is continued with the classification of ear-hanging under the type 1production system described in the introduction.

New Zealand Culture Methods

New Zealand is an example of a country where the Japanese methods have provenunsuccessful. Although large amounts of seed from the New Zealand scallop, Pectennovaezelandiae, can be collected using mesh bags, the on-growing of the spat hasbeen plagued with fouling problems. Removal of fouling organisms is labourintensive and has proven too costly to allow intensive culture economically viable.The New Zealand methods have since become extensive and since the early 1980 sthe wild caught seed have been sown in the traditional wild capture fisheries aroundMarlborough Sound. The wild fishery has since been abandoned for a managed

fishery, which is funded by a levy placed on the harvested scallops. This is anotherexample of restocking and can be classified as a type 2 production system.

Scottish Culture Methods

Although there are many natural beds of scallops around the UK coastline, the switchto cultivated stocks has centred on Scotland. The main reason for this are thesheltered, clean and cool sea lochs along the west coast that lend themselvesfavourably to good spat settlement and growth. Initial research focussedon both theKing Scallop (Pecten maximus) and the Queen Scallop (Chlamys opercularis),however it was soon realised that the low value of the latter would make cultivationuneconomical. This created a problem for the culturists, as the collection of spatinvariably resulted in a mixture of spat from both species, with C. opercularisdominating. A method of separating the two species has now been developed, wherea plastic grid is placed in the tank of mixed spat. Due to the more active behaviour ofC. opercularis spat the two populations become separated. Following occasionalfailures in natural spatfall, the industry has looked towards hatchery production ofseed as a more reliable source and using the methods outlined by the FisheriesLaboratory in Conwy, North Wales, a successful hatchery has been established inOrkney. Transportation of spat is a risky business, as too much stress can cause highmortalities during transit and lead to low survival once placed into the culture system.

In order to minimise the risk to the spat they must be kept cool and moist.Grow out methods used in Scotland are similar to those in Japan, adjusted for thelocal conditions, and market sizes are reached in 4-5 years for P. maximus (100mmSL), and 3 years for C. opercularis (60mm SL). As in other scallop producingcountries, the Scottish industry has encountered problems with fouling and the highcost of labour and equipment associated with the removal of fouling organisms.Bottom culture is the best way to reduce these costs, however the problem ofpredation then arises from organisms such as starfish (Asterias rubens), brown crab(Cancer pagurus), and the velvet crab (Necora puber). Utilisation of lantern netculture and ear-hanging results in Scottish culture being classified as a type 1

production systems.


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The overstocking of Zhikong scallops, Chlamys farreri, has been linked toenvironmental issues in Sishilli Bay, China (Zhou et al, 2006). The main result ofthese impacts has been increased mortality rates and stunted growth of the scallops

themselves. This is an important factor to be considered for all bivalve culture.Bivalves require a high standard of water quality in which to grow, and although theyhave been shown to improve water quality in heavily polluted waters, these organisms

are likely to be unfit for human consumption. It is in the interests of the shellfishfarmers to ensure that the water quality around the culture site is of a high quality, and

there are often conflicts between shellfish farmers and other water users who locallycause pollution. The presence of scallops has been shown to decrease seston andchlorophyll a concentrations, and increase sedimentation rates. The two trends areconnected through the filtration of the water column and the subsequent production of

faeces and pseudofaeces (Zhou et al, 2006). It has been noted that overstocking ofscallops, and bivalves in general, can have a detrimental impact on the environment,

but the general belief is that if the carrying capacity (see conclusion) is not exceededthe benefits of scallop culture systems would far outweigh the minimal costs. Therehas been no research suggesting that the suspended culture of scallops has a negativeimpact on the environment and as such, suspended culture could fulfil all the criteriadescribed for Principle2. The restocking of scallops to enhance wild fisheries maynot comply with Principle 2. Due to the harvesting methods of the wild capturefishery, i.e.the use of mechanical dredges, there are many possible adverse effects onthe environment. Scallops are dredged using dry dredges, which consist of a steelframe that has steel rings and netting attached. As the dredge is pulled along theseabed the weight of the dredge, and sometimes the addition of steel teeth, dig into the

sediment facilitating the capture of the scallops (Messieh et al, 1991). The potentialimpacts of dredging activity include changes to sediment characteristics, benthiccommunity structure and biodiversity (Jones, 1992).

In parts of the UK, and possibly globally, there are small artisanal fisheries wherescallops are collected by divers. Although the scallops are collected from wildpopulations there is scope for such fisheries to be assisted by stock enhancementthrough bottom culture. There are no scientific reports regarding the effects of handcollection by divers, but it is likely that any effect will be minimal, as there is nodisturbance of the seabed and landings are limited by the weight that can be carried by

a diver and the time constraints related to diving activities (C. Pringle,pers. comm.).The method is also very selective as only scallops above the minimum landing sizeare harvested. If such fisheries were enhanced through bottom culture then theywould be classified as type 2 production systems.

ABALONE CULTIVATION

Abalones are gastropod molluscs with the generic nameHaliotis, and they possess ashallow, ear-shaped shell with a series of holes along the dorso-lateral margin,through with respiratory tubes protrude (Spencer, 2002). They feed on algae andbenthic diatoms by scraping away the layers with their radula, and as such aredifferent to the filter-feeding bivalve molluscs previously discussed. The landings ofabalone are a very small proportion of global landing statisticsaccounting for only


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0.16% of shellfish landings (quantity) in 2005 (FAO, 2005); however they are ahighly prized gastronomic delicacy in many countries, particularly in the Far Eastwhere they can command a high price (a global average of US$10.57 kg-1 [FAO,2005]). This high price has made the abalone a strong candidate for commercialculture operations.

The culture of abalone began in Japan at the end of World War II following thecollapse of wild fisheries. Originally hatcheries were used to produce juvenileabalone, 15-20mm SL, for restocking into the natural environment (Spencer, 2002).This practice has continued to the present day, however some abalone are nowretained and grown-on to a relatively small market size of 50-80mm SL, which can beachieved in 3-4 years. There are three main areas of abalone cultivation; (1) spatprocurement (hatchery produced to 8-10mm shell length), (2) growth of juveniles (8-26mm SL), (3) and final ongrowing to market size (26-65mmSL) (Mgaya & Mercer,1995). Different species of abalone require different conditions and rearingtechniques, however the general set up of culture facilities are similar.

Hatchery Procedures

The first step in cultivating a species is the successful spawning, fertilizing andhatching of larvae (Fleming & Hone, 1996). InHaliotis tuberculataspawning can beachieved by placing organisms in individual tanks and exposing them to ultra violetirradiated water (irradiation causes an increase in hydrogen peroxide which stimulatesspawning) (Hayashi, 1982). Generally spawning can be induced over a longer timeperiod by conditioning, involving a light regime of 12 hours of light and 12 hours ofdarkness, in conjunction with filtered seawater at summer temperatures (10-20 C).Conditioning in this way usually takes 1000-1500 degree-days (number of days x

temperature above biological zero [7 C in H. tuberculata]) (Spencer, 2002). Eggsusually hatch within 18 hours of fertilization (Hayashi, 1982). Trochophore larvae are

transferred into stock tanks containing settlement plates. These plates are conditioned

(with a film of benthic diatoms and microalgae) prior to the introduction of larvae inorder to provide the larvae with a food source. Following settlement the abalone aretransferred to larger vessels for ongrowing to juvenile size or market size (figure 5).The nature of such vessels can vary according to the species being cultivated and theregion in which the culture facility exists, typically cages or barrels are used (Fallu,1991).

Sea Culture

The use of barrels and other such containers suspended from longlines for thegrowout of abalone is common place and occurs in sheltered bays. The requirementsfor successful abalone growth are clean waters with low suspended particulate matter.Ease of access is also important in order to service the culture structure and feed theabalone. The barrels used are either purpose built or adapted from other uses, withthe ends removed and replaced with a mesh to allow increased water flow through thesystem. The use of cages is also undertaken in areas of California. The cages contain

vertical sheets of plastic which are spaced 20cm apart, which act as a refuge for theabalone. Vigorous aeration below the cages ensure that the inner most levels of thecage receives well-oxygenated water and that waste products are removed effectively.Up to 20 000 juvenile abalone can be stocked in a cage, which is 10 times greater than


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a conventional 250l barrel, and growth rates are said to be 25% better than thoseachieved from comparable barrel culture (Spencer, 2002).

Figure 5: An abalone hatchery system where the abalone have settled onto plates and

are being grown on before being placed into sea culture systems. The tanks are

connected to a flow-through system and contain corrugated plastic as a shelter for theabalone.

Picture courtesy of wikipedia.com

Growth rates obtained during culture can vary widely dependant on the environmental

conditions, for example temperature and stocking densities. Lopez et al (1998)obtained relatively high growth rates of 3.21, 3.78, and 3.81mm shell length permonth at 15, 18, and 22 C respectively, when compared to other authors who haveobtained growth rates around 1mm shell length per month (Hayashi, 1982; Kelly &Owen, 2002) although it should be noted that these were laboratory based growthtrials. In sea cages in Korea, growth rates of around 0.83mm shell length per monthare achieved, when averaged over a yearly cycle. At these growth rate, the juvenilestake around 4 years to reach market size (>60mm SL) (Spencer, 2002). Stockingdensity is also known to affect the growth of abalone and it is generally accepted thatlower stocking densities tend towards increased growth, and it is the aim of a farmmanager to find the correct balance between optimal growth and optimal economicgain.

Diet

Anotherimportant aspect of commercial abalone culture is the use of artificial dietsand the sourcing of suitable feeds to optimise growth and reduce economic input.Artificial diets have been studied for over 30 years in Asia (Fleming et al, 1996).Nutritional value is obviously an important aspect of artificial diets, however otherfactors are equally important, including cost-effectiveness, availability off nutrients,


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stability and palatability. Despite the differences in nutritional requirements betweenabalone species, proximate analysis of artificial diets reveals that there is a high levelof similarity between them (Fleming et al, 1996). In general, an artificial diet forabalone will be high in protein (20-50%) and carbohydrates (30-60%), with low levelsof lipids (1.5-5%) and fibre (Wilding, 2006).

Although much of the current research is concerned with artificial diets, there is stillinterest in the use of cheaper algal feeds. These algal feeds can be found in thenatural environments, or more recently, can be cultured, particularly in polyculturesystems where algae such as Ulva lactuca are used as effective biofilters. This cheapfood source has been found to be an acceptable diet for abalone culture (Shpigel &Neori, 1996), and is sustainably sourced. Other algal species suitable for the cultureof abalone include Gracilaria cornea, Hypnea spinella, Hypnea musciformis, andPalmaria palmate(Mai et al, 1995; Viera et al, 2005).


As the production of abalone in a culture environment generally utilises hatcheryreared larvae, it cannot be classified under any of the production systems described inthe introduction. A new category must therefore be proposed. As the cultivationprocedure is closed from the wild population the new category shall be namedclosed production . Production systems categorised as closed systems should not

involve the use of wild caught broodstock and should utilise hatchery producedlarvae. The on-growing stage of production should occur in cages, or similarstructures in the natural environment,to prevent escape,or in land based facilities.

There are no known studies into the environmental impacts of abalone culture, and as

such it would be inappropriate to comment on whether abalone culture could complywith criteria for Principle2. Points that can be inferred from the culture methods arediscussed here but it is imperative that before decisions are made regarding theenvironmental impacts of abalone culture that specific studies are made, whetherthese are field studies or through the use of reliable models. The hatchery productionof abalone larvae is now widespread and as such there is no impact on theenvironment with regard to obtaining larvae. One possible issue that can be raised isthe use of algae which is taken from the wild. Whilst algae can be seen as lesspolluting than formulated diets, the use of algae taken from the wild could affect thelocal ecosystems by removing an effective filter and basic food source. The switch toartificial diets could also lead to problems with high nutrient loads in effluent waters,

although recirculation systems could reduce these effects. The best practice could beto utilise algae which is cultured, as this removes the strain on wild populations andcan also be used to reduce organic loads of effluent water, and as such is an excellentcandidate for sustainable poly-culture.

CLAM CULTIVATION

Clams include cockles, razor-clams, manila clams, and quahogs among others. Themost important of these species in a commercial sense is the Manila clam (Tapes

philippinarum) which accounts for 70.57% of all global clam landings (FAO, 2007).The majority of world clam landings come from wild capture fisheries, with many


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stocks being regulated in order to prevent over-exploitation. Regulatory measures areoften placed on the fisheries restricting the number of licences issued, gear used,season, minimum landing size (MLS) and quotas. Examples of regulated fisheries are

the cockle (Cerastoderma edule) fisheries in the Netherlands and the UK. In someregions of the world there are fisheries for spat which are then relayed in shellfish

parks for cultivation, where there is more control over environmental conditions.Quantities of commercially harvested clams are difficult to derive and aren t preciselyknown as they are often not distinguished from wild caught clams when landed orreported. This problem is compounded by the grey area in defining what is classed ascultivated product and what is classified as regulated fisheries (Spencer, 2002).

Seed Procurement

The source of cultivated clams can come from either wild seed or hatchery producedseed. In general, clam seed is easy to produce in a hatchery, however there are only afew species for which hatchery produced seed is available in commercial quantities.

Hatchery production of seed tends to be for species which locally suffer fromunpredictable spatfall in the wild. In the UK there is hatchery production of Tapesphilippinarum and Tapes decussates(Grooved Carpet Shell) on a regular basis withMercenaria mercenaria (Hard Shell clam) occasionally being produced (Spencer,2002). The Manila clam was introduced into the UK and as such is unable toreproduce in the natural environment. The cultivator is therefore reliant on hatcheryproduced seed, which can be obtained at a range of sizes between 5mm and 30mmshell length.

The collection of wild seed can range from simple hand collection using rakes andsieves to the mechanised use of hydraulic dredges which allow access to deeper water

where dense patches of seed may occur. Once obtained, the seed are sown in cultureplots at more productive densities; however, in order to obtain improved productionlevels the stock must be protected from predators. One of the main users of wildcaptured seed for clam cultivation is China, with the focus being on three mainspecies; the Manila clam, the blood cockle, and the razor clam.

Manila Clams

Shallow ponds are created low on the intertidal zone, each several hectares in area, tocollect the settling spat. The ponds are inoculated with algae such as Chaetocerosinorder to provide the larvae and spat with a food source. The tidal flushing of the

ponds is controlled in order to prevent the loss of larvae, spat and food. The yield ofsuch ponds is typically between 750-1500 clams (5mm SL) per m2. The small clamsare re-laid into prepared plots at densities of 180 per m2, and can produce yields of 2-4kg m-2within a year (Spencer,2002).

Blood Cockles

As with the Manila clam, shallow ponds are used for the collection of spat and initialgrowout stage. If spatfall is particularly dense in an area then thinning may berequired where the spat are re-laid in intertidal beds. Growout is usually around 2years, with cockles reaching 20mm SL(Spencer, 2002).


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Razor Clams

Plots are conditioned by loosening and smoothing of sediment in intertidal areas.Larval content of the water column is monitored to predict the time of spatfall, and 3-5 months following the predicted spatfall the seed are collected. At this stage they are

approximately 10mm in length, and are re-laid into growout beds at a density of 900-1800 per m2. After 6 months the clams have reached 50mm SLand are harvested(Spencer, 2002)

Siting of Growout Plots

Clam cultivation plots are generally in more sheltered areas away from extreme windand wave action. Although cultivation is possible in more extreme environments, thestructures need to be more robust, with service and labour costs increased. There isalso a danger that net-covered clams can be smothered before the problem can berectified. Access to the culture plots is also an important aspect of site choice. If the

sediment is too soft then the use of boats is required to access the plots, whereasharder substrates can support wheeled and tracked vehicles, as in France. Manysuitable substrates can be found in estuarine areas, however some species such as theManila clam and the Palourde prefer salinities above 24 psu, and as such are notsuited to estuarine areas where the salinity is likely to drop below this level on a dailybasis. Although clams can grow effectively high on the intertidal zone, improvedgrowth is achieved further down the shore where they are covered by water for longer

periods and hence feeding for longer periods.

There are different methods of grow out for clam cultivation, each being categorisedunder a different production system described in the introduction. The simplest form

of culture is the re-laying of spat into culture plots. With little or no modification tothe plots such methods can be classified as type 2 production systems; i.e. restocking.The use of growbags to culture clams, generally on trestles, can be considered as type1 production, with wild harvested spat being cultured through a grow out phase. Thefinal method is similar to re-laying spat in the sediment;however the use of netting toreduce predation creates a favourable environment and can be classified as a type 3production system.


Little attention has been paid to the environmental impacts of clam cultivation, and

although analogies can be made to other cultured species that have been moreextensively researched, it should be noted that there are likely to be differences andthat such analogies can only suggest possible impacts, and the actual effects shouldbecome the focus of future research. The anticipated environmental impacts causedby the cultivation of clams can be separated into three main areas; 1. The effects ofcollecting spat from wild populations. 2. The effect of stocking clams at highdensities on the local ecosystem. 3. The effect caused by harvesting the clams.

There is little research into the effect of removing spat from the natural populations to

support clam cultivation, although evidence from mussel studies suggest that if toomuch of the natural spatfall is removed natural populations fail, causing increased

mortality in organisms which predate in them. An example of this is the increasedmortalitiesof eider ducks witnessed in the Wadden Sea, the Netherlands, in 1990 and


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1991, following the removal of the natural spatfall (Kaiser et al, 1998). The removalof clam seed by hydraulic dredges could potentially damage the benthic ecosystemand the communities residing there, although it should be noted that there has as yetbeen no research into such activities. In the UK the majority of clam seed is produced

by hatcheries and thus removes the problems associated with wild spat collection.

The use of hatchery seed will allow the spat procurement phase to comply withPrinciple2. Further research is needed in order to assess whether wild collection ofspat could complywith the criteria for Principle2.

The on-growing of clams occurs either in or on the bottom, and as such results in alower stocking density per m2 of seabed than that of mussel culture. It is possiblehowever, that similar effects associated with sedimentation and removal ofphytoplankton (and nutrients) could occur. A study by Mojica and Nelson (1993)investigated the effect ofMercinaria mercinariaculture (in growbags) in a lagoon inFlorida. Results indicated that, as with mussel culture, the sediment below the culture

system had a lower mean grain size than the control sites, with an increased

percentage silt/clay component. The redox layer was also much shallower within thefarm site at 0.5mm, compared to 24.33mm and 16.5mm at the control sites. The study

showed significant variation in nutrient and plankton concentrations between the sites,

although no trend could be detected. No significant effect on the benthic communitywas detected although there was a reduction in seagrasseswithin the culture site. Theauthors concluded that, although the significant change in sediment characteristicscould potentially alter the ecosystem dynamics, such alterations were not evident andthat there were no negative impacts on the environment from the culture site. Itshould be noted that no stocking densities were given and that an increase in intensitycould result in negative impacts. A negative impact on seagrass beds couldpotentially lead to a failure to meet the second criteria of Principle 2, as manyseagrass beds are endangered or protected habitats. The seed for this operation wereproduced in a commercial hatchery and harvest was by hand, by removal of thegrowbags. Such activities are likely to cause minimal environmental impacts, if at all.

With regards to growbag culture, impacts from harvesting are likely to be minimal asbags are removed by hand, with the possible assistance of motorised vehicles to carrythe bags to shore. Such activities limit the effects of trampling to small tracksbetween the rows of growbags. With harvesting likely to be limited to a yearlyactivity at its most frequent, recovery of these tracks is likely to be rapid. Suchproduction systems will comply with the Principle 2 criteria and could be seen assustainable.

Harvesting of cultured clams has received little attention from researchers, althoughthe effects of the different harvesting methods can be inferred from other harvestingactivities. This is especially true for the hydraulic dredging of clams from sediment,which has received considerable attention regarding the wild capture fisheries. Thehydraulic dredge works by blasting water infront of the dredge, which liquefies thesediment allowing a blade to cut through it and harvest the clams (Messieh et al,1991). The action of the hydraulic dredge has a significant impact on the physicalappearance of the sediment and on the benthos. The sediment is left scarred and thescars, which can be detected with side-scan sonar, can remain there for long periods

of time (over 3 months [Thrush et al, 1995; Curry & Parry, 1996]) and if harvesting isrepetitive on a short timescale, full recovery may never be achieved (Jones, 1992).


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Other impacts that can occur include; increased mortality and damage to non-targetspecies, increased predation of infaunal species due to increased exposure, changes tothe chemical and physical characteristics of the sediment (Messiehet al, 1991). Suchdramatic changes to the sediment or the benthos are unlikely to be covered by thecriteria of Principle 2, therefore harvesting by hydraulic dredge could result in a

failure to comply with Principle 2.

A further cause of impacts worth mentioning is the structures associated with theculture of clams. When clams are re-laid into the sediment they are vulnerable topredation. Netting can be placed above the clams to reduce mortality from predation,but such activities can potentially affect sedimentation and local hydrography. Astudy by Munroe and McKinley (2007) demonstrated that although the nettingappeared to increase the population of clams there were no significant effects onsediment composition. An increase in organic carbon was detected beneath the netted

areas, but this was attributed to the increased density of clams beneath the netting,rather than increased sedimentation caused by the structure itself. They concludedthat the netting had little effect on the environment and that increasesin sedimentationnoted in other studies are likely to be due to the individual oceanographiccharacteristics of each site. As the netting is not having an effect on the ecosystem orthe benthos the use of netting could complywith the criteria for Principle2.

OYSTER CULTIVATION

Oyster cultivation is centred on one main species, the Pacific Oyster, Crassostreagigas, a cupped oyster. Originally from the western Pacific, C. gigas has been

introduced to almost all areas of the globe whether it was intentionally orunintentionally. Between the 1920 s and 1960 s several consignments of oysters wereintroduced to the USA and Canada and they have since established naturalpopulations and now support fisheries. France imported Pacific oysters in the 1960 sand 1970 s to support its Portuguese oyster fishery which was affected heavily bydisease during this period. Natural populations have now established along theFrench coast op to the Brittany peninsula and now supports a successful spatcollection each year, which when grown on in trestles result in annual landings of140000t (Spencer, 2002). It has not always been a welcome introduction however,due to its success in the natural environment it is causing problems in Australia where

it is competing for resources with the Sydney Rock Oyster, with similar problems

being encountered in New Zealand. In the UK Pacific oysters were first introducedfrom Portugal in 1926 to the River Blackwater in Essex as a cultivation crop. Havingestablished that conditions in the UK would prevent the establishment of wildpopulations, broodstock was sent to hatcheries across the country to widen the areawhere oyster cultivation could take place.

Although landings of flat oysters are low throughout the world, they are highly sought

after gastronomically, and can command a higher price than the cupped oysters. TheUK once had large populations of the European flat oyster, Ostrea edulis, whichsupported successful fisheries. They have since been decimated by the diseasebonamiasis. This disease is caused by the microscopic organismBonamia ostreae,which invades the blood cells of O. edulis, causing mass mortalities in the UK,Ireland, France and the Netherlands.


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Flat Oyster Cultivation

Although greatly reduced by disease, the high market price of flat oysters supportssmall operations in Europe and New Zealand. In England the main fisheries are in the

Solent and in the River Fal, Cornwall, with a derived fishery in Essex using seed re-

laid from the two former sites. Techniques used to culture flat oysters have remainedrelatively unchanged since the 19th century. In order to encourage spat settlementlime is placed in areas of the lower shore where settlement is known to take place.Originally the lime was in the form of roofing tiles (and this practice persists in someareas of France), before cheaper alternatives were discovered including old musseland oyster shells. In Norway, Italy and Croatia the use of bundles of twigs is stillused to collect spat, with new materials being investigated. Once the spat have settled

they are removed and placed into mesh bags for continued growth. These culturesystems are categorised as type 1 production systems.

Cupped Oyster Cultivation

The cultivation of cupped oysters is the most commercially important and widespreadof all oyster culture. Naturally warm water species they have been transferred tomany other countries as previously described. There are a variety of growingmethods used across the globe based on either traditional methods or more modernmethods developed and copied from the methods of other countries.

Hanging Culture

Originating in the 1920 s this is one of the oldest of the modern methods, although itsuse today is decreasing with new methods using its basic principles proving more

successful. The hanging method uses scallop or oyster shells threaded onto a wire orren , which are suspended from bamboo frameworks placed near the low water mark.Spat settle on the shells and grow in a three-dimensional environment which tendstowards increased growth and fattening. This method also removes limitations withtype of seabed and can be placed below the low water mark, allowing increasedfeeding and avoidance of benthic predators. Following settlement the oysters areplaced on racks positioned progressively further up shore for a period of 6 months,during which time hardening occurs, before returning to the low shore for ongrowingto market size. Hanging culture is another example of a type 1 production system.

Raft Culture

Typically, rafts of 20m x 10m, made of bamboo, are linked together in chains andanchored to the seabed. To the structure 1200 rens are suspended, eachapproximately 2m long and holding 60-70 shells. Deployment of the rens is assistedby sampling the water for the presence of eyed larvae, indicating that settlement isimminent. As with hanging culture the young oysters are placed along the highershore for a period of hardening before they are returned to the rafts. At the on-growing stage the oysters are restrung 20cm apart on a 9m wire and placed in deeperwater. Similar to hanging culture this is another example of type 1 production.


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Longline Culture

Longline culture consists of two parallel ropes anchored at each end by concreteblocks and suspended by a series of buoys, whose number depends on the load ofoysters being cultivated. As with raft culture, shells with settled spat are suspended

from the ropes. The advantage of longline culture over raft and rack culture is theability for the equipment to be placed in more exposed conditions offshore, and themajority of longline farms exist in coastal rias and offshore. Longline culture is atype 1 production system.

Rock Culture

Typically a Chinese method of cultivating oysters, rock culture involves thedeployment of large stones and boulders, often in bridges to raise the oysters off theseabed. The stones are frequently marble, which is cleaned and covered with lime toencourage settlement. As many as 60 000 stones can be placed in a hectare and can

produce up to 3t of flesh. For the culture of Crassostrea ariakensis, lime tiles areused for the settlement of spat. These tiles are easily moved and can be thinned outduring the grow out process to prevent overcrowding and so maximise the yieldobtained from each spatfall. The grow out of C. ariakensistakes 4 years and with upto145 000 tiles per hectare, yields of 15t of oyster flesh per hectare can be obtained.Rock culture is considered to be type 3 production, since the rocks modify the naturalhabitat to encourage settlement and improve growth of the oysters.

Stake Culture

Used in areas with a soft substrata, stake culture involves the use of 1.2m stakes, 20-

30mm in width, placed in a variety of patterns in the seabed. Initially the stakes areplaced in bundles until spat have settled on the surface, at which point they arethinned out and placed into grow out patterns. The grow out period is about 18months and yields around 0.5t per 10 000 stakes, up to 6t per hectare. Stake culture is

similar to rock culture in that there is modification to the natural environment toimprove settlement and growth, and as such is classified as type 3 production.

Trestle Culture

Both spat procurement and growout can be performed on intertidal trestles, which are3m long and 0.5m high. For spat collection the trestles can hold a variety of

settlement surfaces from traditional stones and slate to more modern materials such asPVC tubes, which have a corrugated surface to improve settlement. Spat are removedfrom the collectors at 20-30mm shell length and placed into bags, called pches inFrance, which are 1m long and 0.5m wide. The oysters are initially stocked at 5kgper bag, which results in a yield of 15-20kg per bag. The trestle method is the mostcommonly used method for culturing Pacific oysters in France (figure 6). Trestleculture is another example of a type 1 production system.


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Figure 6: The cultivation of oysters using trestles in Belon, France.

Picture courtesy of wikipedia.com

Stick Culture

The Sydney Rock oyster is a species typically grown using stick culture. An intertidal

method of culture, it comprises of hardwood sticks coated in tar to resist boringorganisms. The sticks are 180cm long x 2.5cm square cross-section, and are secured

in layers on to a frame to create a crate , which is in turn secured to a rack positionedclose to low water in spat collecting areas. Spat settle and attach directly to the sticks,

and once the spat have grown to 2-3cm SLand are more resistant to predators, thesticks are removed from the crates and laid individually on the racks, about 20cmapart. The oysters reach market size in approximately 1-2 years, and when the oystersare harvested they are graded for plate or bottling, with smaller oysters beingreattached to the sticks to continue grow out. Some farms remove the oysters at asmaller size and complete the grow out process with 3-15 months in trays, producinga better shell shape and improved flesh content. Stick culture could fall into type 3production as the grow out stage takes place on the same structure as the settlementstage, however the structure is rather complex with modifications occurring

throughout the grow out process, and as such is more likely to be considered type 1production.

Ground Culture

Along the Atlantic and Gulf coasts of the USA American oysters, Crassostreavirginica, are fished from public grounds rather than cultured on private plots;however some additional materials are added to the environment to improve theproductivity of the fishery. The use of cultch (shell fragments obtained fromprocessing facilities or dredged from the seabed) enhances the seabed to improvesettlement of spat. Once the spat have settled a variety of methods can be used for the

grow out of the oysters as described above, however some are left on the seabed tosupport the fisheries and continue to grow on the cultch. The growth of oysters on


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shells is common in many species and this can lead to the formation of reefs. Thesereefs were once extensive across Chesapeake Bay, USA, before over fishing andpollution decimated them. Ground culture consists of two production types. Groundculture that continues throughout the grow out stage is type 3 production, since theaddition of cultch is an environmental modification. If other grow out methods are

used, then the culture becomes type 1 production.


The anticipated environmental impacts caused by the presence of oyster cultureoperations are similar to those of mussel culture, as both bivalves can be grown insuspended or bottom culture. The presence of oyster rafts has been shown to alter thecharacteristics of the sediments below the farms (Hayakawa et al, 2001) although theeffect of such a change in sediment character was not considered. The use of a modelby Chapelle et al (2000) illustrated that in an enclosed Mediterranean lagoon, anoyster farm led to increased ammonium concentrations, and decreased concentrations

of phytoplankton, zooplankton and oxygen. The increase in ammonium wasattributed to direct excretion by the oysters, with decreases in plankton and oxygendue to filtration and respiration of the oysters. As has been found in other studies,sedimentation was greater in the area of the farm, leading to higher levels of organicmatter. Using the model, Chapelle et al (2000) found that by halving the oysterbiomass, there was an increase in phytoplankton and zooplankton. They alsoillustrated a change in the dominance and succession of both phytoplankton andzooplankton and this is likely to be due to a shift from oyster predation to zooplankton

predation on the phytoplankton. An alteration in phytoplankton and zooplanktonconcentrations represents a shift in the trophic balance within the ecosystem and assuch may resultin a failure to comply with criteria 1 of the MSC s Principle 2.

Cultivation of oysters, namely Crassostrea gigas, has been shown to affect themacrobenthic community. De Grave et al(1998) took samples from a trestle culturesite, from both beneath the trestles and the access lanes, and compared the results tosamples from a control site. The diversity of organisms beneath the trestles was lower

than the control site, with lower numbers of individuals and a lower number ofspecies. The samples from the access lanes, however, showed an increased diversitywhen compared to the control site. De Grave et al (1998) concluded that the presence

ofoyster trestles, whilst not increasing the organic content of the sediment, induced aslight shift in total species and displaced some species. It was also noted that thetrestles acted as a refuge for mobile scavengers such as Carcinus maenas andPaleamon serratus as few were found on open sand compared to larger numbersbeneath the trestles. Results from this study suggest that although there is a minoreffect on the benthic community structure, there is still a stable community presentand when compared to other aquaculture systems the effects are negligible. Adecrease in biological diversity could constitute a failure to comply with the secondcriteria of Principle 2. Although it should be noted that the area under the trestleswhich exhibits a decreased biodiversity is only a small area of a wider ecosystem, and

if the ecosystem were considered to include the areas outside of the farm then theeffect is likely to be negligible. Providing that culture sites do not dominate anecosystem, then the second criteria could be met and Principle 2 complied with.

A major impact of oyster culture on the environment is the use of Carbaryl, a broadspectrum pesticide, which is used in the USA to control burrowing shrimp


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populations. There are two species of burrowing shrimp,Neotrypaea californiensisand Upogebia pugettensis, which burrow into the mud beneath oyster reefs in thePacific Northwest region of the USA (Dumbauld et al, 2006). The Carbaryl is addedto the sediment every 6 years as part of ground cultivation plot preparation(Simenstad & Fresh, 1995). The use of Carbaryl can potentially alter the community

structure and trophic web and could result in a failure to comply with Principle 2criteria.

There are also positive impacts of cultured oysters, as with other bivalve molluscculture, which are often overlooked. Oysters are efficient filters of the sea and canremove heavy metals, organic matter, suspended solids and phytoplankton, which canbe especially useful in heavily polluted waters and eutrophic conditions. Oyster reefshave been shown to effectively remove faecal coliform bacteria and chlorophyll a(inthe form of phytoplankton) from the water column, and form an important part of thenutrient cycle releasing ammonium into the environment (Cressman et al, 2003). Itshould be noted that oyster beds were once much more prolific (Keiser et al, 1998)andas such theirre-introduction into estuaries could be an effective way of resettingthe balance of these ecosystems by removing urban and agricultural waste enteringthe marine environment through the rivers and runoff. An example of how prolificoyster beds were in previous times is illustrated in Chesapeake Bay, USA. Theestimated biomass of oysters occurring in the bay pre-1870 was 188 x 106 kg (dryweight) and the oysters could filter the entire volume of water within the bay in 3-6days. The biomass in 1988 was calculated at 1.9 x 106 kg (dry weight) and theturnover time had increased dramatically to 325 days (Newell, 1988). The dramaticdecrease in oyster biomass has been attributed to continued overexploitation bycapture fisheries and by the pressures of disease. The result of the dramatic decreasehas been the formation of an anoxic layer below the pycnocline during the summermonths. The re-introduction of oysters would help to reverse these effects throughtheir role as a major benthic-pelagic coupler (Mann, 2000), and is an example of howthe enhancement of shellfish fisheries and shellfish culture can improve the naturalenvironment and stabilise endangered ecosystems.

Oysters have also been shown to successfully reduce the organic load in aquacultureeffluents. The Sydney Rock oyster, Saccostrea commercialis, reduces the totalsuspended solid (TSS) content of shrimp pond effluent to 49% of initial levels whenstocked at high density (Jones & Preston, 1999). Such studies show promise for thepoly-culture of oysters with other commercially cultured species, reducing theenvironmental impact of the operation and increasing the economic gains. The use ofpearl oysters to remove heavy metals has been investigated and illustrates a method of

aquaculture to improve the environment whilst producing an economically viableproduct without the concerns over human consumption (Gifford et al, 2004). Theproduction of pearl oysters could comply with the criteria for Principle2, althoughwhether the flesh would be edible is not the focus of this report and would bedependant on thelevel ofpollution in the locality of the culturesite.


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CRUSTACEAN CULTURE

CULTIVATION OF CLAWED LOBSTERS

The cultivation of clawed lobsters involves two species; the American lobster,

Homarus americanus, and the European lobster,Homarus gammarus. Technically theculture of homaridlobsters is simple, however problems arise throughout the cultureprocess with cannibalism and fighting, resulting in the need to individually confinethe lobsters.

Broodstock

The use of wild caught broodstock is common place and relies on either the capture of

berried (egg-bearing) females, or the conditioning of females in captivity. The formeris the easiest method of obtaining eggs, however the landing of berried females isprohibited in many areas as a conservation measure and licences are required. The

mating of captive broodstock can be achieved using natural copulation of intermoultfemales, or through artificial insemination. Once lobsters have been acquired theycan be stored either at sea in boxes moored to buoys or in land-based pond facilities,which provide easier access. In the land-based systems the lobsters are held incommunal tanks, with hides added to provide refuge for the lobsters and reduce stress.

Various hides have been tried and tested and a popular choice is the use of concreteridge tiles (Burton, 1992). To prevent fighting between the lobsters the claws arebanded (Europe) or pegged (USA). Pegging involves a small wooden peg beinginserted into the articulation joint, preventing the use of the claw, although it shouldbe noted that this can lead to increased susceptibility to infection (Lee & Wickens,1992).

Spawning and Incubation

The conditioning of lobsters for spawning and incubation differs between the twoHomarus species. The National Lobster Hatchery, Padstow, Cornwall, follows theprocedure described by Burton (1992) (D. Boothroyd,pers. comm) where lobsters areacclimated to 16 C and held at this temperature during the incubation process. Thesalinity is maintained above 30ppt. If the temperature is raised above 16 C the larvaldevelopment time is reduced, however the survival rate of the larvae is also reduced.

Environmental conditionsare important for the success of attachment of eggs to the

pleopods following spawning. Homarus americanus is held at winter temperatures(0-5 C) for 5 months prior to spawning to improve this attachment, although forreliable spawning the lobsters have to be exposed to this conditioning for a number ofyears. An incubation period of 4-18 months, depending on temperature, followsspawning. The development of the eggs during this time can be monitored by thecolour change that occurs, and later by the size of the eye of the developing larva. By

monitoring in this way the hatching time can be predicted to within a few weeks.

Hatching

The period of hatching usually lasts 3-5 days, with the first eggs from a brood likely

to be more robust than those hatching later. Often only the larvae which hatch in thefirst couple of days will be used in culture as the rate of mortality for the later


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hatching

Download - 26238Impacts of Shellfish Aquaculture on the Environment

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