MFR PAPER 1325 nificant when juvenile fishes are dis­ cardcd on thc inshore nursery grounds. One solution to eliminate the fishing pressure on demersal fin fish hy shri mp trawlers is to develop trawling gear that A Trawl Design: Employing Electricity captures only shrimp while avoiding fish. As a conservation measure. the to Selectively Capture Shrimp NMFS Harvesting Technology Group at Pascagoula. Miss .. has undertaken the development of selective shrimp trawling gear to reduce the finfish har­ vest by the shrimp Reets in the south­ WILBER R. SEIDEL and JOHN W. WATSON, Jr. eastern United States. WEBBING PANEL TYPE SEPARATOR TRAWLS The initial effort to develop a simple ABSTRACT-A shrimp trawl designed to selectively capture shrimp and elimi­ and economical trawl. which would be nate the incidental capture of bottomfish and other marine organisms is described. similar to a conventional shrimp trawl, The purpose of this trawl is to conserve the fish presently lost during shrimp was to attempt separation using web­ trawling. Normal shrimp trawl bycatch is eliminated by completely closing the bing panels inside the trawl to sort entrance to the net with a webbing panel. Shrimp pass into the trawl through the shrimp from fish (Seidel, 1975; Watson bottom ofthe net which is constructed oflarge mesh webbing. Effective control and and McVea, 1977). Webbing panels separation ofshrimp and fish utilizes the behavioral difference ofthe animals when were strategically placed in conven­ exposed to an electrical field. Shrimp are forced up through the bottom mesh of the tional shrimp trawls to use water flow net by the electrical field located under the net, while the fish are frightened away through the net to force shrimp through from the path of the net by the electricity. the panels into the trawl bag. Fish were led along the panel into an escape INTRODUCTION discard rate by the U. S. shrimp Reet chute, which allowed them to escape was 330,000 t (S. Drummond, South­ the trawl unharmed (Fig. I). The panel The catch per unit effort in the Gulf east Fisheries Center, Pascagoula separator trawl is a simple approach to of Mexico industrial bOllomfish fishery Laboratory, Pascagoula, Miss., pers. the discard problem and, if successful, has steadily declined since 1975. The commun.). The NMFS shrimp Reet could easily be adopted by the shrimp vast abundance of these fish in the discard survey data for 1975 shows that industry. northern Gulf of Mexico may be near­ 15,400 t of Atlantic croaker, Mirropo­ The Harvesting Technology Group ing an end. Juhl et al. (1976) reported a gon undu/atus, spot, Leiostomus has been developing and evaluating marked drop in the availability of bot­ xanthurus, and sand seatrout, Cynos­ webbing panel type separator trawls in tom fish on traditional fishery grounds rion arenarius, were discarded in the the Gulf of Mexico since 1975. The and hypothesized that the uecline may inshore waters (0-10 m) off Louisiana, development work has resulted in a be due to heavy fishing by shrimp ves­ Mississippi, and Alabama. Data col­ separator trawl that functions relatively sels, which discard a large tonnage of lected by the Mississippi Gulf Coast well under most conditions in the the demersal tin fish. Research Laboratory indicates that offshore shrimp fishery (10-30 fath­ The latest National Marine Fisheries large numbers of juvenile Atlantic oms). Comparative trawling tests be­ Service (NMFS) figures indicate that croaker, Gulf menhaden (Brevoortia tween the separator and standard the magnitude of shrimp effort and the patronus), spot, and sand seatrout pre­ shrimp trawls indicate that an average corresponding discard of fish is consid­ sent in the Mississippi Sound were 60-80 percent of Ihe fin fish captured in erable. Total production of ground fish greatly reduced because of intensive a standard trawl can be eliminated by a for 1976 by the commercial fisheries trawling when the shrimp season separator trawl. was 45,350 t. In 1975 the esti mated opened in June 1976. A major problem with the webbing It is apparent that the discarding of panel type separator trawl, however, is Wilber R. Seidel and John W. Wat­ fish by the shrimp Reets causes greater the Joss of shrimp. No matter what the son, Jr. are with the Pascagoula than normal fish mortality, and can be a specific design, some portion of shrimp Laboratory, Southeast Fisheries major factor contributing to the decline entering the trawl mouth will pass out Center, National Marine Fisheries in the Gulf ground fish stocks. These the escape exit along with the finfish. Service, NOAA, Pascagoula, MS 39567. This is Contribution No. discards will certainly contribute to a Shrimp loss can vary greatly depending 78-22P from the Southeast Fisheries further reduction in fish stock abun­ on conditions encountered. On the Center, Pascagoula Laboratory. dance and may be particularly sig- majority of offshore fishing grounds Sep/ell1ber /978 2/ tem, and particularly the electrode array design, have significantly im­ proved performance. Recent be­ havioral studies of shrimp reactions to the electrical stimulus and field experi­ ments with the trawl established the first known. accurately defined catch o ······CHUTE EXIT efficiency for any trawl (Watson, 1976). Further studies of fish behavior ····ESCAPE CHUTE to electrical fields made possible the control of fishes by electrotaxis (K lima, "'CHUTE SLIDE 1972; Seidel and Klima. 1974). This expertise and technology. which has ...........·::·.>·:'.SEPARATOR PANELS been established within the Harvesting Technology Group. can be utilized to develop a shrimp separator trawl sys­ lem which potentially could eliminate the capture of all fish on any shrimp ground, and would certainly offer a solution to the difficult problem of / separating fish and shrimp on the in­ shore grounds. A general concept of the Figure I. .-Webbing panel type shrimp separator (rawl design. trawl design follows. SELECTIVE ELECTRIC SHRIMP TRAWL where most of the fish are larger and not more dense and the fish sizes generally heavily concentrated, the separator smaller. Under these conditions, where A diagrammatic view of a selective trawl shrimp catch can average as little a shrimp separator trawl is most shrimp trawl using electricity is shown as 10 percent less than the standard needed, the conventional webbing in Figure 2. The most prominent and shrimp trawl. When heavy concentra­ panel type separator trawl is the least unusual feature of this concept is that tions of fish are encountered (500­ efficient and would be unacceptable by the mouth of the trawl is completely 1,000 pounds/hour) or when the fish are shri mp fishermen. There is, therefore, closed with a webbing panel, and 6-8 small in size, the shrimp loss may reach an urgent need for a more effective ap­ feel of setback is provided to give the 50-60 percent. The smaller fish also proach to the fish discard problem, par­ mouth panel a pronounced slope (Fig. tend to gill in the separator panel and ticularly by the inshore shrimp fishery, 3). The fish barrier closing the mouth of block the panel, thereby reducing the even though the solution may be more the trawl is made of webbing and allows amount of shrimp that can pass through complicated and somewhat more ex­ water to flow through the trawl, but to the bag. Large concentrations of fish pensive to use. prevents fish from entering the net. A also cause more shrimp to be carried out large (12- to l8-inch) mesh bottom RECENT ADVANCES the fish exit without contacting the panel is placed in the net and the trawl is IN ELECTRICAL separator panels. One other factor af­ adjusted to fish 12-24 inches from the FISHING TECHNOLOGY fecting the efficiency of the shrimp bottom. The electrode array is attached separator trawl is the towing time. As it The Harvesting Technology Group to the footrope of the trawl and trails increases, so does the rate of shrimp has been involved in applying electrical back under the net to create an electrical loss because accumulating fish clog the fishing techniques since the mid­ field under the net. The electrodes are separator panels. 1960's. This work resulted in the de­ constructed of I-inch diameter rope Limitations of the panel type velopment of an electric shrimp trawl covered with conducting material to separator trawl make general accep­ system (Seidel, 1969). Although not provide a large diameter electrode and tance of the trawl by the commercial widely accepted at present by the thus increase the electrical current flow shrimp fleet doubtful. Results indicate fishing industry, the electric shrimp into the water. A capacitor discharge good potential for limited application trawl system is an effective harvesting electrical pulsar, similar to that de­ on many offshore shrimp grounds, but device and can increase shrimp produc­ scribed by Seidel (1969) and Watson it is not likely to be universally ac­ tion day or night. Recent advances by (1976), is used to create the electrical cepted. The greatest potential destruc­ the Harvesting Technology Group with field, which controls the behavior of the tion of ground fish stocks by shrimp the electric shrimp trawl system have shrimp. Shrimp are forced to jump up trawlers occurs in the inshore shrimp greatly improved its capability. Im­ through the large mesh bottom panel, fishery where fish concentrations are provements to the electrical pulser sys- while fish stimulated by the field are 22 Marine Fisheries Review herded ahead of the trawl (Fig. 3). ~_lARGI MISH _OHOM Other fish are simply directed away from the trawl by the closed mouth. This approach is unique and differs from other separator trawl designs in that the fish never enter the trawl; there­ fore, they do not have to be separated from the shrimp catch.