Aquacultural Engineering 5 (1986) I - 15

A Mechanized Seine Harvest System for Freshwater Prawns*

Thomas M. Losordo,~ Jaw-Kai Wang,~- Mark J. Brooks,~ and Chee Yin Lamt

. Agricultural Engineering Department, .f.Animal Sciences Department. University of Hawaii. Honolulu, Hawaii 96 822, USA

A BSTRACT

This paper describes new developments in a selective harvest system for freshwater prawns. The net system can be pulled either mechanically or by hand. It produced a consistently high harvest efficiency and yield. The system inchlded a low cost, light weight protot)pe boom designed to replace the harvest crew in the pond. Components of the net system were attached to each other by nylon fish net zippers. The zippers were also used as a closing mechanism to increase harvest efficiency. The mean harvest efficiency of marketable prawns of the new system in earthen production ponds was estimated to be 63"5%.

INTRODUCTION

The commercial production of the freshwater prawn Macrobrachium rosenbergii in Hawaii was begun in the early 1960s (State of Hawaii, 1978). In 1983, approximately 96 ha of water surface area were in com- mercial production on 21 individual sites. Four major farms make up approximately 84 ha of the productive area (State of Hawaii, 1983).

Regardless of the farm size, one of the most costly activities involved in prawn grow-out operations is selective harvesting of market size prawns with nets (Fujimoto et al., 1977). Gibson and Wang (1977) esti- mated that the traditional net-harvesting operation required 7"5 man- hours per 0.5 ha pond, accounting for approximately 9% of the total farm operating cost. Similarly, Shang (1981) set harvesting labor require-

*Journal Series No. 2958, The Hawaii Institute of Tropical Agriculture and Human Resources.

I Aquacultural Engb~eering 0144-8609/86/S03.50--© Elsevier Applied Science Publishers Ltd, England. 1986. Printed in Great Britain

2 7". M. Losordo, J.-K. Wang. M. J. Brooks, C. Y. Lain

ments at 7-5 man-hours per pond per harvest, about 5-7% of the total operational cost, and 16-37% of all farm labor costs.

While the traditional net-harvesting operation is labor intensive, its harvest efficiency has been low. Harvest efficiency is defined as the number of market size prawns caught in a single harvesting operation divided by the total number of market size prawns presented in the pond prior to harvesting.

Peterson (1982) described two methods of net harvesting prawns from rectangular production ponds. He found that harvest efficiencies ranged from 6-23% with an overall average of 15%. Lam and Wang (1985), in a study on estimating harvesting efficiencies on the same farm (as Peterson's study), found harvesting efficiencies from 2-46% with an average of 23%. This data is significant as farmers and researchers have, in the past, considered harvesting efficiency to be between 50-75% (Gibson and Wang, 1977; Polovina and Brown, 1978; Williamson and Wang, 1982).

Williamson and Wang (1982), in an effort to increase harvest effi- ciency and reduce labor, designed, fabricated and field-tested a 'first generation' mechanized seine harvest net system for the Hawaiian prawn industry. The net system consisted of a monofilament nylon seine-net and a rigid PVC harvest grader box between the primary net wings. The net system was pulled through the pond by prime movers utilizing a multiple line bridle system to pull on the lead line and floater line. Variations of the system were fabricated and used on two major farms from 1979 to 1980. The systems did not come into wide use because of the time required to set up and move the complicated bridle system (Rietow, personal communication) and suspected damage to prawns harvested in heavily silted ponds (Wallace, personal communication).

A 'second generation' mechanized net system was developed by Amfac Aquatech-Hawaii, a now discontinued pilot farm operation on the island of Kauai. The Amfac system consisted of a No. 18 twine multi- filament nylon seine-net with a center-mounted net harvest bag. The system was pulled from each end with a prime mover and depended on a large number of bottom line lead weights and a double lead line with a net ~skirt' in between to secure an adequate seal with the pond bottom. While the net achieved harvest efficiencies from 66 to 70% in new ponds, it did not perform well in heavily silted older ponds with eroded banks.

This paper describes a "third generation' mechanized harvest net system that could be used in harvesting both new and old production ponds. It can be pulled either mechanically or by hand and produces consistently high harvest efficiencies and yields.

A mechanized seine harvest system forfi'eshwater prawns 3

Prawns are benthic or bottom-dwelling animals, and as a result the most critical factor in net system design is its ability to maintain a good seal between the net system and the pond to prevent prawns from elud- ing capture.

There are three major causes for poor seals between nets and ponds:

1. Pond irregtdarities: As the pond ages, it accumulates mud and silt. Large 'bull' male prawns and "trash" fish, such as common tilapia, can make deep depressions in the soft mud and silt. Another cause of depressions in the bottom is that of the foot-prints of the harvest crew. Older ponds are also likely to have severe berm erosion caused by wind driven wave action.

2. Long bank grass: Due to the continued belief by many farmers that long bank grass is necessaD' for good production, grass is often kept at extremely long lengths protruding into production ponds. The presence of such grass tends to lift the leading edge of the net system's bottom line. Unless the grass is cut or lifted out of the way, it is impossible to achieve a good seal.

3. Improper pulling forces: Most hand-pulled seine-nets in Hawaii were designed to be pulled by a harvest crew, using their feet curled inside the bottom line. The crew is usually spaced at regular intervals along the seine. When the crew encounters excessive amounts of bottom silt or mud, they often will lift with their feet or they will pull the net by the floater line. This situation is especially common during the net ~closing' operation. This operation requires that the two ends of the net be 'closed" together and removed from the pond without losing the seal against either the bottom or herin. As the net is gradually moved on land, the prawns are crowded near or into the harvest bag. However, prior to entry into the bag, the densely concentrated prawns can escape in large numbers with the slightest lifting of the net.

For a mechanized net system pulled from the pond berm, the pulling force at the end of the rope has a vertical component that may cause lift- ing of the bottom lead line in the area where the pond bottom intersects the berm. This area is commonly called the "toe' of the pond and is suspected to be an area where prawns elude capture due to a poor seal.

OPERATIONAL AND DESIGN CONSIDERATIONS

Net pulling systems In order to maintain a good seal, ideally the only force applied to the net bottom line should parallel the pond bottom. In practice, it means that

4 7-. M. Losordo, J.-K. Wang. M. J. Brooks. C. K Lain

long lines need to be used to pull the net bottom line to minimize the angle between the pulling line and the pond bottom.

In the improved net system, 18"3 m (60-ft) rope tethers are attached to harnesses, which are then used to apply pulling forces to the bottom lead line. Two persons wearing harnesses then walk backwards in the pond pulling the net, following the toe of the pond. Disregarding the catenary effect, the force triangle produced by an 18"3 m (60-ft) tether (see Fig. 1) pulled at waist height (about 76.2 cm or 30 in) produced a force equal to 99% of the applied force in a direction parallel to the pond bottom and with a vertical force equal to only 4% of the applied force.

Measurements taken during harvesting operations indicated that even in moderately silted ponds, pulling forces rarely exceed 311-4 N (70 lb) and are usually less than 133.4 N (30 lb). Thus, the lifting forces on the lead line of the net should not exceed 13.3 N (3 lb) and is more likely to be about 5"6 N (1.25 Ib). Thus, less weight was needed to keep the bottom lead line sealed against the pond bottom and the resultant lighter bottom line was less likely to dig into the pond-bottom mud.

In an effort to reduce harvest labor and to make it easier to harvest extremely silty ponds, a low cost prototype boom system was used to replace the harvest crew in the pond (see Fig. 2).

The boom was constructed of light weight perforated 3-8 cm (1} in) square telescopic engineering tubing with a trade name of Telespar ® (Unistrut Corporation, Honolulu, Hawaii). The horizontal boom and vertical arm were given rigidity by the use of tension members made of 0"3 cm (,~ in) diameter (7 × 19) galvanized vinyl-coated cables. The boom has a total length of 4"6 m (15 ft) and extends apoproximately 3-0 m ( 10 ft) to the side of the prime mover. At the pond-end of the boom, a

F x : FCos g : (o -9g) (F)

f loa te r line Fy : F'Sin e = (0 .04) (F)

~ ) ~ ( ~ 6Of t t e t h , r

\

\\ ~:X Fy \ . .

mudl ine ~ ? Fx '

W = 1-751bs

Fig. 1. Net pulling force triangle.

A mechanized seine harvest system _tor fresh water pru wns

telescoping vertical arm section extended down into the pond and was adjusted to reach approximately 15-2 cm (6 in) from the bottom. A tether was attached to the vertical arm end and extended horizontally back to the net's bot tom line. Most of the forces acting on the vertical arm are in the horizontal direction parallel to the pond bottom. These forces were transferred to the prime mover by the cable running from the bot tom of the vertical arm to a short horizontal boom attached to the front of the prime mover.

Net design

The net was of the bot tom seine or beach seine type with a center mounted harvest bag. The bag could be side mounted or not used at all.

short boom / v /

\ \

tether

[ J c ~ l

J

~ /~port cable

te lescoping

~ / v ~ r t i c a l a r m

Fig. 2. L o w cost prime mover mounted boom system. {Drawing not to scale.~

6 7-. M. Losordo. J.-K. Wang, M. Z Brooks, C. K Lain

The primary "wings" (seine-nets to each side of the center bag) were 18-3 m (60 ft) long and 1-8 m (6 ft) deep. The net has 3.8 cm (11 in) stretch (1-9 cm or ~ in square) knotted eye mesh made from No. 9 multi- filament nylon twine. The netting was hung in such a way that 45-7 m (150 ft) of netting made 30-5 m (100 ft) of seine, and was tied to the floater line and lead line every fourth mesh. The float line was made of 1-0 cm (~ in) diameter braided polypropylene with 11 oz buoyancy (SB-6) floats attached ever2,' 38.1 cm (15 in) on center. The bottom line or lead line were l'0 cm (~ in) braided polypropylene; however, it has a 28 strand (each strand is a 1"0 cm or ~ in diameter spun-polyester rope) synthetic "mudline' attached to the lead line to prevent the net from cutting into the mud. The lead line has 55 g split lead weights (No. 8SPL- 1) attached eveN' 61-0 cm (24 in).

Four auxilia~" seine wings can be added to the primal" wings as needed to increase the length of the seine when used in wider ponds. The auxilia~ wing was 9"1 m (30 ft) long and of identical construction as the primary wings except for the addition of a 38-1 cm (15 in) deep net "skirt' attached below the mudline with a light chain on the bottom line of the skirt. These skirts, running the full length of the auxiliary wings, ,,,,'ere designed to prevent losses of prawns which would normally escape when the mudline inadvertently lost contact with the bottom or berm of the pond during the pulling operation.

Fig. 3. Harvest bag with zipper attachment.

A mechanized seine harvest system fbrffeshwater prawns 7

The harvest bag was 4-6 m (15 ft) long by 1.5 m (5 ft) wide and was tapered in depth from 1.8 m (6 ft) at its opening to 0-9 m (3 ft) at the back. (The taper allowed for the product to be carried down the pond rather than througja the bottom mud.) To make the bag, 4.4 cm (1~ in) stretch (2.2 cm or ~ in square) No. 9 multifilament nylon net was sewed to 1.0 cm (3 in) braided polypropytene rope with 311.85 g ( l l-oz) floaters attached every 30-5 cm ( 12 in). The bag had no top for easy access to the product.

All components were attached to each other through fish net zippers produced by YKK (USA) Inc. (Cincinnati. OH). The nylon zippers were sewn onto the vertical perimeter ropes of the seine wings and the open- ing of the harvest bag (Fig. "t a,. The zippers were also used to provide a closing mechanism for the net system as one half of a zipper was sewn two meshes above the mudline over the entire len~h of the prima W and auxiliary wings (see Fig. 4).

HARVEST OPERATION PROCEDURES

The multistrand synthetic mudline depended largely upon the absorp- tion of water for negative buoyancy and should be immersed in water for at least 4 h prior to the first harvest of the day.

Fig. 4. Multistrand mudline with zipper for closing procedure.

8 T.M. Losordo. J.-K. Wang, M. J. Brooks'. C. E Lain

The net system consisted of five pieces: two primary wings, two auxilia~, wings, and the harvest bag. The components were unloaded ,and spaced around one end of the pond before a harvest. The bundles of net were then untied and stretched out on the bank and joined at the zippers, and the tethers were attached to the mudline at the point where the au,,dliary and pr imal , wings connect. The tethers were then adjusted in length to either 7"6 m (25 ft) or 18"3 m (60 ft) for pulling bv harvest boom (Fig. 7) or harvest crew with harnesses (Fig. 5). respectively. The set-up procedure required approximately 10 to 15 min before each harvest. The harvest crew members then took up their positions as indi- cated by Fig. 6(a) (Fig. 6 represents a prime mover and one harness operation for demonstrative purposes).

When harvesting using two prime movers outfitted with harvest booms, the tethers were attached to the harvest boom's vertical arm, which was moved forward at a rate of approximately 6.4 m (21 ft) rain -t. The two harvest crew members in the pond attached the ends of the auxiliary' seine wing mudline to their ankle (closest to the berm), attached the floater line to the boom and walked directly behind the boom along with the prime mover (Fig. 7). These crew members walked along the perimeter of the pond, holding their leg and net end close to the water line of the pond and using their free hands to move grass and other berm obstructions away from the water line (Fig. 8). These crew members also directed tile forward speed of the prime mover according to the efforts required to maintain a good seal with the pond berm.

Upon reaching the end of the pond, on the side where closing would occur, the tether was detached from both the net and the boom. The mudline was firmly pressed into the bottom silt from the toe of the pond

Fig. 5. Harvesters' positions without prime mover.

,-1 m e c h a n i z e d s e i n e harve.5 " s"c~gem for F~e_vhwa~er pra wv, s ~a

tha-v~-3t '..-r"~w r~,gmD:2r"

i " i . . . . . . i " ' !

]

,~7=,,-~- i i / : 2 ~

6Oft t~t,~er, / ?

i ~ ~¢p'6Oft pr,imar,y I ~ , .~_? ' ; is~ln¢ wing ] L _ 4 _ ~ ~_ _( ~.: . _ ~ _ ~ ! -J

ta ) ~ ~ i 'har'vcst bag , , , j j

Fig. 6.

~ t

i , i i ~-~ N

(b)

i " ~ - _ " t ~ "~

,!

; I l l ~-x2

i

t

L ;

' . i ~ . . . . . . . . .

Harvcs t i ng o p e r a t i o n with o u r p r ime movcr . : a pul l ing the net, (b) closing, o p e r a t i o n . (c) z ipp ing thc nct ' s mud l ines toge ther .

Harvester's position with prime mover and boom. Fig. 7.

10 1-. M, Losordo, J.-K. ~2~ng, M. J. Brooks, C E Larn

Fig. 8. Pulling the net,

up to the top of the berm. The two harvest crew members from this side then moved to the other side of the pond to assist in moving the net system around the end of the pond to the point where the other net wing was set (Fig. 6(b)). This operation can be done with the prime mover pulling the net. However, if there seems to be a large quantity of prawns in the net and/or an extremely hard or un-silted bottom, the closing operation should be done entirely by hand to minimize mudline lifting and associated losses of prawns.

When the net ends were brought together at the point of closing, the harvest crew took up positions as indicated in Fig. 6(c). One person remained on the pond bank, one stood behind the harvest bag in the pond, and two members remained inside, the perimeter of the net and performed the net zipping operation. The zippers that were attached just above the mudlines were brought together, joined and zippered closed. When the two members zipping moved away from the bank, both crew members at each end of the net sytem exerted a pulling force on the net as the net wings were zipped together. The lead person pulled the net's mudlines together clearing both debris and prawns from between the mudlines. The second person performed the actual zipping of the net together (Fig. 9). The zipping operation usually requires 7-10 min. Time can be taken at this point to allow for self-sorting of the under market size prawns through the net's mesh.

A mechanized seine harvest s}'stem fbr fi'esh water prawns I !

Fig. 9. Zippingthe mudlines together.

Fig. 10. Removing the net from the pond.

Upon completion of the zipping procedure the net can be gradually removed from the water while keeping the prawns back in the water (Fig. 10). As the net's wing sections were completely removed, the prawns were moved into the harvest bag which is detached from the wings by unzipping and left in the water for a period of approximately 10-15 min to again allow the animals to self sort. While this was occurring the net components were gathered and loaded for transport to the next harvest

12 7-. M. Losordo. J.-K. Bang. M. J. Brooks, C. ~. Lain

site. The entire harvest operation, not including hand sorting, required from i i to 1~ h.

After the self-sorting period, the product can be dip netted from the harvest bag and either hand sorted at pond side or transported directly to the farm live holding facility.

SYSTEM E\L4,LUATION

The harvesting procedure, as a described here, was developed during 15 harvest operations at the AmOrient Aquafarm 54-ha commercial prawn production facility, by University personnel. Following the develop- mental period, the net and boom system ,,,,'as moved to the Kahuku Prawn Co. (KPC) 12"5-ha farm for demonstration and field evaluation.

The net and boom system was evaluated using two test parameters: ha~'est yield versus the previous five harvest yields using the farm's standard net harvesting operation, and the estimated harvest efficiency as measured by the mark and capture method developed by Lain and Wang (1985).

Table t represents the results of the har~'esting operations completed in seven ponds at the KPC farm. The numbers in the parentheses are the

TABLE l Comparison of Harvest Yields

Pond Previous harvests, lbs ( '% o f total yiehl) UH net 7otal no. system yieht,

1 2 3 4 5 lbs

21 156 186 109 (162) (19"3) (113~

13 lS0 122 181_1 (16'1) (10.9) (161

15 99 113 171 (11'4) (13"0) (19-7

16 120 133 139 (11'3) (12-6) (13-1

4 64 64 111 (8.4) (8.4) (14-6

19 175 144. 244 (13.4) (ll.O) (18-7:

14 73 142 199 (7.1! (13-8) !119"3

101 (1t)'5)

202 ( 1 8 ' t 168 (19.3 212 { 20-0 157 (20.7? 319 (24-4 173

I6-8;

143 ( 14-87 199

17-8) 124

14'3) 195 (18.4) 149 ( 19-6 196 ( 15.0

247 (23-9

270 :28.0) 233 (2t).9) 194 (22"3)

259 24.5) 214 (28.2)

227 (17.4) 198

19.2',

965 100 ) 116 1oo) 869

(1oo) 1058 (100) 759

(1oo) 1305 (I00) 1032 (lOO)

,A mec/tattic, ed seine harve.;t ~>'s:em for fresh water prawns I 3

30 [

~.~ 20 t

>

10

= P o n d 4 . = P o n d 1 3

, = P o n d 1 4

' . = P o n d 1,5

? = P o n d 1 6 • = P o n d 1 9

,~ = P o n d 21

0 I 2 3 4 5 uH net Hacvest sequence

Fig. 11. Comparison of harvest yields.

percentages of the total of the six harvest yields that the single harvest yield represents. When displayed in graphic form in Fig. 11, a difference is clearly seen between the previous harvest operations and the yields of the new net and boom system (UH net).

The evaluation of the estimated harvest efficiency was performed in six ponds. Two of the operations were carried out by University person- nel while the remaining four tests were performed using the farm's regular harvest crew operating the new system. The results of the harvest efficiency estimation are presented in Table 2. The mean harvest effi- ciency as indicated bv the six field trials was 63"5%.

During the testing and evaluation period, a number of areas for improvement in the net system design were noted• A second net system is currenty being fabricated with the following modifications.

The new seine-net wings will be 90 ft long, eliminating the need for auxiliary wings. Each wing will have a continuous 90 ft zipper above the

14 T. M. Losordo, J.-K, Warlg. M. J. Brooks, C, ~: Larn

T A B L E "~

H a r v e s t E f f i c i e n c y E s t i m a t e

Pond no. Harvest yield Est imated harvest Harvest crew Ib~ e~7(iency i ':'~,~

13 2 3 3 63 U H

4 214 61 U H

9 I93 66 K P C

19 ~ - - ~., / ~ .~ K P C

14 198 - 73 K P C

16 179 63 K P C

mudline (versus the current three 30 t~ zippers) to facilitate an easier closing procedure. To avoid prawn losses over the floater line, the floaters will be spaced closer together. The outside 60 ft of each wing will have the floats spaced every 12 in on center. The remaining 30 ft of the wing will have floats spaced at 9 in on center. The seine-net eye size will be increased to 1¼ in stretch mesh and the center mounted harvest bag will be eliminated. The two 90 ft wings will be zipped directly at the center, a harvest bag could be added if a variation in mesh sizes is required for prawn self sorting.

SUMMARY AND RECOMMENDATIONS

A 'third generation' mechanized harvest net system has been successfully developed and tested for the Hawaiian prawn industry. The seine-net and boom system was capable of routinely catching 63% of the available market size prawns in heavily silted and eroded production ponds. While doubling previously reported har~'est efficiencies, the net also signifi- cantly reduced both the time and effort required by the traditional harvesting method currently employed by the industry.

ACKNOWLEDGEMENTS

This study has been supported by the Aquaculture Development Program, Department of Land and Natural Resources, State of Hawaii (Grant No. 16589) and the University of Hawaii National Seagrant Program (Grant No. A/R- 15).

A mechanized seine harvest system ~or freshwater prawns 15

The authors wish to acknowledge the contributions of time "and facili- ties made by Kahuku Prawn Co., AmOrien t Aquafarm and Prawns of Hawaii, Inc. in the testing of the net system.

Gratitude is extended to Lisa Brown, Sally Higa and Joanne Kurosawa for efforts in the development of this paper.

R E F E R E N C E S

Fujimoto, M., Fujimura, T. & Kato, K. (1977i. An idiot's guide to prawn ponds. In: Shrimp and Prawn Farming in the ~t~'stern Hemisphere, eds J. A. Hanson and H. L. Goodwin, Dowden, Hutchinson & Ross, Inc., Stroudsburg, Pennsylvania.

Gibson, R. -17. & Wang, J. K. (1977). An alternative prawn production systems design in Hawaii. Sea Grant Technical Report UN[HI-SEAGRANT-TR- 77- 05. University of Hawaii.

Lain, C. Y. & Wang, J. K. (1985). Estimating harvesting efficiency of freshwater prawns. Presented at the Summer Meeting of the American Society of Agri- cultural Engineers, East Lansing, Michigan, 23-26 June 1985.

Peterson, J. J. (1982). Freshwater prawn harvesting: a comparison of three methods. Proc. World Mariculture Sot., 13, 104-19.

Polovina, J. & Brown, H. (1978). Population dynamics and production economics. Proc. L~'brld Maricultttre Soc,, 9,393-d~04.

Rietow, Allan, S., President, Prawns of Hawaii, Kilauea, Hawaii. Personal com- munication.

Shang, Y. C. (1981). Freshwater Macrobrachium rosenbergii production in Hawaii. practices and economics. Sea Gratzt Miscellaneous Report. UNIHI- SEAGRA NT-MR-81-07, University of Hawaii.

State of Hawaii (1978). Aquaculture Development for Hawaii: Assessments and recommendations. Aquaculture Planning Program, Honolulu, Hawaii.

State of Hawaii (1983). Unpublished farm survey, Department of Land and Natural Resources, Aquaculture Development Program, prepared bv Kendrick K. F. Lee.

Wallace, J.. Manager, AmOrient Aquafarm, Kahuku. Hawaii. Personal com- munication.

Williamson, M. R. and Wang, J. K. { t982). An improved harvesting net for fresh- water prawns. Aquaclzltttral Engineering, I (2), 81-91.