MFR PAPER 1325

A Trawl Design: Employing Electricityto Selectively Capture Shrimp

WILBER R. SEIDEL and JOHN W. WATSON, Jr.

nificant when juvenile fishes are dis­cardcd on thc inshore nursery grounds.

One solution to eliminate the fishingpressure on demersal fin fish hy shri mptrawlers is to develop trawling gear thatcaptures only shrimp while avoidingfish. As a conservation measure. theNMFS Harvesting Technology Groupat Pascagoula. Miss .. has undertakenthe development of selective shrimptrawling gear to reduce the finfish har­vest by the shrimp Reets in the south­eastern United States.

WEBBING PANEL TYPESEPARATOR TRAWLS

ABSTRACT-A shrimp trawl designed to selectively capture shrimp and elimi­nate the incidental capture of bottomfish and other marine organisms is described.The purpose of this trawl is to conserve the fish presently lost during shrimptrawling. Normal shrimp trawl bycatch is eliminated by completely closing theentrance to the net with a webbing panel. Shrimp pass into the trawl through thebottom ofthe net which is constructed of large mesh webbing. Effective control andseparation of shrimp and fish utilizes the behavioral difference of the animals whenexposed to an electrical field. Shrimp are forced up through the bottom mesh of thenet by the electrical field located under the net, while the fish are frightened awayfrom the path of the net by the electricity.

INTRODUCTION

The catch per unit effort in the Gulfof Mexico industrial bOllomfish fisheryhas steadily declined since 1975. Thevast abundance of these fish in thenorthern Gulf of Mexico may be near­ing an end. Juhl et al. (1976) reported amarked drop in the availability of bot­tom fish on traditional fishery groundsand hypothesized that the uecline maybe due to heavy fishing by shrimp ves­sels, which discard a large tonnage ofthe demersal tin fish.

The latest National Marine FisheriesService (NMFS) figures indicate thatthe magnitude of shrimp effort and thecorresponding discard of fish is consid­erable. Total production of ground fishfor 1976 by the commercial fisherieswas 45,350 t. In 1975 the esti mated

Wilber R. Seidel and John W. Wat­son, Jr. are with the PascagoulaLaboratory, Southeast FisheriesCenter, National Marine FisheriesService, NOAA, Pascagoula, MS39567. This is Contribution No.78-22P from the Southeast FisheriesCenter, Pascagoula Laboratory.

Sep/ell1ber /978

discard rate by the U. S. shrimp Reetwas 330,000 t (S. Drummond, South­east Fisheries Center, PascagoulaLaboratory, Pascagoula, Miss., pers.commun.). The NMFS shrimp Reetdiscard survey data for 1975 shows that15,400 t of Atlantic croaker, Mirropo­gon undu/atus, spot, Leiostomusxanthurus, and sand seatrout, Cynos­rion arenarius, were discarded in theinshore waters (0-10 m) off Louisiana,Mississippi, and Alabama. Data col­lected by the Mississippi Gulf CoastResearch Laboratory indicates thatlarge numbers of juvenile Atlanticcroaker, Gulf menhaden (Brevoortiapatronus), spot, and sand seatrout pre­sent in the Mississippi Sound weregreatly reduced because of intensivetrawling when the shrimp seasonopened in June 1976.

It is apparent that the discarding offish by the shrimp Reets causes greaterthan normal fish mortality, and can be amajor factor contributing to the declinein the Gulf ground fish stocks. Thesediscards will certainly contribute to afurther reduction in fish stock abun­dance and may be particularly sig-

The initial effort to develop a simpleand economical trawl. which would besimilar to a conventional shrimp trawl,was to attempt separation using web­bing panels inside the trawl to sortshrimp from fish (Seidel, 1975; Watsonand McVea, 1977). Webbing panelswere strategically placed in conven­tional shrimp trawls to use water flowthrough the net to force shrimp throughthe panels into the trawl bag. Fish wereled along the panel into an escapechute, which allowed them to escapethe trawl unharmed (Fig. I). The panelseparator trawl is a simple approach tothe discard problem and, if successful,could easily be adopted by the shrimpindustry.

The Harvesting Technology Grouphas been developing and evaluatingwebbing panel type separator trawls inthe Gulf of Mexico since 1975. Thedevelopment work has resulted in aseparator trawl that functions relativelywell under most conditions in theoffshore shrimp fishery (10-30 fath­oms). Comparative trawling tests be­tween the separator and standardshrimp trawls indicate that an average60-80 percent of Ihe fin fish captured ina standard trawl can be eliminated by aseparator trawl.

A major problem with the webbingpanel type separator trawl, however, isthe Joss of shrimp. No matter what thespecific design, some portion of shrimpentering the trawl mouth will pass outthe escape exit along with the finfish.Shrimp loss can vary greatly dependingon conditions encountered. On themajority of offshore fishing grounds

2/

o ······CHUTE EXIT

····ESCAPE CHUTE

"'CHUTE SLIDE

...........·::·.>·:'.SEPARATOR PANELS

/Figure I. .-Webbing panel type shrimp separator (rawl design.

tem, and particularly the electrodearray design, have significantly im­proved performance. Recent be­havioral studies of shrimp reactions tothe electrical stimulus and field experi­ments with the trawl established thefirst known. accurately defined catchefficiency for any trawl (Watson,1976). Further studies of fish behaviorto electrical fields made possible thecontrol of fishes by electrotaxis (K lima,1972; Seidel and Klima. 1974). Thisexpertise and technology. which hasbeen established within the HarvestingTechnology Group. can be utilized todevelop a shrimp separator trawl sys­lem which potentially could eliminatethe capture of all fish on any shrimpground, and would certainly offer asolution to the difficult problem ofseparating fish and shrimp on the in­shore grounds. A general concept of thetrawl design follows.

where most of the fish are larger and notheavily concentrated, the separatortrawl shrimp catch can average as littleas 10 percent less than the standardshrimp trawl. When heavy concentra­tions of fish are encountered (500­1,000 pounds/hour) or when the fish aresmall in size, the shrimp loss may reach50-60 percent. The smaller fish alsotend to gill in the separator panel andblock the panel, thereby reducing theamount of shrimp that can pass throughto the bag. Large concentrations of fishalso cause more shrimp to be carried outthe fish exit without contacting theseparator panels. One other factor af­fecting the efficiency of the shrimpseparator trawl is the towing time. As itincreases, so does the rate of shrimploss because accumulating fish clog theseparator panels.

Limitations of the panel typeseparator trawl make general accep­tance of the trawl by the commercialshrimp fleet doubtful. Results indicategood potential for limited applicationon many offshore shrimp grounds, butit is not likely to be universally ac­cepted. The greatest potential destruc­tion of ground fish stocks by shrimptrawlers occurs in the inshore shrimpfishery where fish concentrations are

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more dense and the fish sizes generallysmaller. Under these conditions, wherea shrimp separator trawl is mostneeded, the conventional webbingpanel type separator trawl is the leastefficient and would be unacceptable byshri mp fishermen. There is, therefore,an urgent need for a more effective ap­proach to the fish discard problem, par­ticularly by the inshore shrimp fishery,even though the solution may be morecomplicated and somewhat more ex­pensive to use.

RECENT ADVANCESIN ELECTRICAL

FISHING TECHNOLOGY

The Harvesting Technology Grouphas been involved in applying electricalfishing techniques since the mid­1960's. This work resulted in the de­velopment of an electric shrimp trawlsystem (Seidel, 1969). Although notwidely accepted at present by thefishing industry, the electric shrimptrawl system is an effective harvestingdevice and can increase shrimp produc­tion day or night. Recent advances bythe Harvesting Technology Group withthe electric shrimp trawl system havegreatly improved its capability. Im­provements to the electrical pulser sys-

SELECTIVE ELECTRICSHRIMP TRAWL

A diagrammatic view of a selectiveshrimp trawl using electricity is shownin Figure 2. The most prominent andunusual feature of this concept is thatthe mouth of the trawl is completelyclosed with a webbing panel, and 6-8feel of setback is provided to give themouth panel a pronounced slope (Fig.3). The fish barrier closing the mouth ofthe trawl is made of webbing and allowswater to flow through the trawl, butprevents fish from entering the net. Alarge (12- to l8-inch) mesh bottompanel is placed in the net and the trawl isadjusted to fish 12-24 inches from thebottom. The electrode array is attachedto the footrope of the trawl and trailsback under the net to create an electricalfield under the net. The electrodes areconstructed of I-inch diameter ropecovered with conducting material toprovide a large diameter electrode andthus increase the electrical current flowinto the water. A capacitor dischargeelectrical pulsar, similar to that de­scribed by Seidel (1969) and Watson(1976), is used to create the electricalfield, which controls the behavior of theshrimp. Shrimp are forced to jump upthrough the large mesh bottom panel,while fish stimulated by the field are

Marine Fisheries Review

~_lARGI MISH _OHOM

Figure 2. -Selective electric shrimp trawl design showing placement of large mesh bOllomentrance. fish barrier, and electrodes.

Figure 3. --Selective electric trawl diagram depicting method of shrimp separation employ­ing an electrical field.

inducing electrotaxis in twelve coastal pelagicand bOilom fishes. J. Fish Res. Board Can.29: 1605-1614.

Seidel. W. R. 1969. Design, construction, andfield testing of the BCF electric shrimp-trawlsystem. Fish. Ind. Res. 4:213-231.

____ . 1975. A shrimp separator trawl forthe southeast fisheries. Proc. Gulf Caribb.Fish. Inst.. 27th Annu. Sess .. p. 66-76.

____ , and E. F. Klima. 1974. In situ ex­peri ments with coastal pelagic fishes to estab­lish design criteria for electrical fish harvestingsystems. Fish. Bull.. U.S. 72:657-669.

Watson, J. W., Jr. 1976. Electrical shrimp trawlcatch efficiency for Penaetls dtlorarum andPenaetls a,tectls. Trans. Am. Fish. Soc.105:135-148.

____ . and C. McVea, Jr. 1977. Develop­ment of a selective shrimp trawl for the south­eastern United States penaeid shrimp fisheries.Mar. Fish. Rev. 39( 10): J8-24.

LITERATURE CITEDJuhl. R., S. B. Drummond, E. J. Gutherz. D. M.

Roithmayr. J. A. Benigno, and J. A. Butler.1976. Oceanic resource surveys and assess­ment task status report. U.S. Dep. Commer..NOAA, Natl. Mar. Fish. Serv., Pascagoula,Miss., 50 p.

Klima, E. F. 1968. Shrimp-behavior studies un­derlying the development of the electricshrimp-trawl system. Fish. Ind. Res. 4: 165­181.

1972. Voltage and pu Ise rates for

shrimp trawl into the shrimp fishing in­dustry, and would greatly enhance thecapability for the effective conservationand management of our shrimp and fishresources.

herded ahead of the trawl (Fig. 3).Other fish are simply directed awayfrom the trawl by the closed mouth.This approach is unique and differsfrom other separator trawl designs inthat the fish never enter the trawl; there­fore, they do not have to be separatedfrom the shrimp catch. The applicationof electricity to sort shrimp from fish ispossible because of the differences inbehavioral reactions of fish and shrimpto an electrical field.

Klima (1968) studied the reaction ofpenaeid shrimp to electrical fields. Hefound that the optimum voltage leveland pulse rate needed to induce a"jumping" reaction (Watson, 1976) inshrimp was 3.0 V (measured across 10cm distance parallel to the electric field)at a pulse rate of 4-5 pulses per second.To induce electrotaxis in fish, pulserates of 15-50 per second at voltagelevels of 1.5-3.0 V across 10 cm arerequired (Klima, 1972: Seidel andKlima, 1974). The slower pulse ratesrequired to control the shrimp do notinduce electrotaxis in fish, but producea fright response instead. In addition,the reaction direction of shrimp to theelectric field is vertical while the frightreaction of fish is horizontal or 90° dif­ferent from the shrimp's movement.These factors, together with optimizingthe electric field, will effectively selectshrimp for capture and at the same timedrive fish away from the electrified areaunder the trawl.

The electrical separator trawl con­cept is technically feasible and repre­sents a very effective approach to solv­ing the shrimp fleet discard problem.The use of an electrical field providesdirect control of the shrimp and fishrather than relying on their passive be­havior as with panel type separatortrawls. The technology and expertiseexist to accomplish the developmentand introduction of this selective

MFR Paper 1325. From Marine Fisheries Review, Vol. 40, No.9, September1978. Copies of this paper, in limited numbers, are available from 0822, UserServices Branch, Environmental Science Information Center, NOAA, Rockville,MD 20852. Copies of Marine Fisheries Review are available from the Superin­tendent of Documents, U.S. Government Printing Office, Washington, DC20402 for $1. 10 each.

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