NORMAN L. PEASE
U.S. Bureau of Commercial Fisheries
Exploratory Fishing and Gear Research Base
Pascagoula, Mississippi 39567, U.S.A.
The Gear Research Unit of the Exploratory Fishing and Gear Research Base, Pascagoula, Mississippi, has been developing an electrical shrimp trawl since 1961. A battery-powered model, completed in 1964, was found to be inadequate. A re-examination was then made in the eastern Gulf of Mexico of shrimp's responses to various electrical fields. It was observed that shrimp responded most rapidly when burrowed in a mud substrate and the minimum electrical field was found to be 3.0 V at 4 to 5 pulses/sec. With these data, the staff designed and built a new prototype electro-shrimp trawl system. The system, when operating, receives, from the vessel's generator, alternatíng current (AC) which is transmitted by an electrical cable to an electronic pulse generator on the trawl door, where it is converted to direct current (DC) and is then released at a specified pulse rate to an electrode array on the trawl. Fishing trials on the north Gulf shrimp grounds indicated the system could catch, during daylight, 96 to 109 percent of the shrimp caught at night by a conventional trawl. In the south-eastern Gulf the catch dropped about 50 percent. Two possible reasons suggested for the smaller catch are: 1) a reduction of the electrical field because of a change in substrate from mud to a sand-shell composition and 2) also the possibility that shrimp burrow deeper in a sand-shell substrate.
REALISATION ET ESSAI EN MER D'UN DISPOSITIF DE CHALUTAGE ELECTRIQUE DE LA CREVETTE
Le Service de recherche sur les engins de pêche de la Base de pêche exploratrice et de recherche sur les engins de Pascagoula (Mississippi) travaille depuis 1961 à la mise au point d'un chalut à crevettes électrifié. Un modèle à accumulateurs, achevé en 1964, n'a pas donné de bons résultats. On a alors procédé, dans la partie est du golfe du Mexique, à de nouvelles études des réactions des crevettes à divers champs électriques. La réaction la plus rapide a été observée lorsque les animaux étaient enfouis dans un substrat vaseux et pour un champ électrique minimal de 3,0 volts à 4–5 impulsions par seconde. Sur la base de ces données à été conçu et construit un nouveau prototype de chalut à crevettes électrifié. Le dispositif fonctionne de la façon suivante: le générateur du bord envoie un courant alternatif, qu'un câble électrique transmet à un générateur d'impulsions électronique installé sur le panneau de chalut, où il est converti en courant continu, envoyé avec une certaine fréquence d'impulsion à une série d'électrodes placées sur le filet. Sur la base d'essais de pêche effectués sur les fonds à crevettes de la partie nord du golfe, l'engin pourrait capturer, en plein jour, de 96 à 109% des quantités de crevettes pêchées la nuit au chalut de type classique. Dans le secteur sud-est du golfe, les prises ont diminué d'environ 50%. Deux explications sont suggérées: 1o diminution du champ électrique due à la difference de substrat, le fond n'étant plus vaseux, mais sablo-coquillier; 2o possibilité que les crevettes s'enfouissent plus profondément dans un tel substrat.
PROYECTO Y ENSAYO PRACTICO DE UN SISTEMA ELECTRICO DE PESCA AL ARRASTRE DEL CAMARON
El Grupo de Investigaciones de Equipo de la Base de Investigaciones de Equipo y Pesca Exploratoria de Pascagoula, Mississippi, estudia desde 1961 un arte de arrastre eléctrico para la pesca del camarón. En 1964 se terminó un modelo accionado por baterías que no dio resultados. Posteriormente, en la parte oriental del Golfo de México se estudiaron de nuevo las reacciones del camarón a diversos campos eléctricos, observándose que reaccionaba más rápidamente cuando estaba enterrado en un substrato de fango, y que el campo eléctrico mínimo era de 3,0 V a 4 ó 5 impulsos por segundo. Con estos datos, el personal proyectó y construyó un nuevo prototipo de sistema eléctrico de arte de arrastre camaronero. Cuando está en funcionamiento, el sistema recibe del generador de la embarcación una corriente alterna que se transmite por un cable eléctrico a un generador de impulsos electrónico en la puerta del arte, en el que se transforma en corriente continua y que, a una velocidad de impulso especificada, se descarga a un conjunto de electrodos en el arte. Los ensayos de pesca en los bancos de camarones del norte del Golfo indican que durante el día, el sistema puede capturar el equivalente del 96 al 109 por ciento del camarón que se pesca de noche con un arte ordinario. En la parte sudoriental del Golfo, la captura se redujo a cerca del 50 por ciento. Dos razones que posiblemente explican esta disminución son: (1) una reducción del campo eléctrico debido a que el substrato era de arena y conchas en lugar de fango; (2) la posibilidad de que el camarón se entierre más profundamente en un substrato de arena y conchas.
Preliminary studies to determine the possibility of developing an electrical system for catching shrimp began in 1961 at the Exploratory Fishing and Gear Research Base, Pascagoula, Mississippi. The objectives were to provide a system which would give the commercial shrimp fleet the capability of working continuously on a 24-hour basis rather than the 12 hours per day the vessels currently work. This is because during daylight most shrimp burrow in the bottom, whereas at night they come out to forage for food. Therefore, commercial fishing for nocturnal species of shrimp is restricted to night trawling. It was theorized that a more effective use of manpower and equipment would be realized and production costs could be reduced proportionately if the shrimp could be caught during their inactive period. Two phases of the early work were presented to the Second World Fishing Gear Congress in 1963. Fuss (1964) reported on the burrowing behaviour of pink shrimp, Penaeus duorarum Burkenroad and Wathne (1964) reported on their response to low voltage electricity. Following this work, experiments were conducted to determine the components for a working electrical trawl model. Electrical parameters were determined but staff limitations at that time did not permit in-house designing or fabricating the necessary electronic components, therefore bids were requested from several industrial electronic companies (Wathne and Holt, 1964). The electronic unit provided by the selected company in 1964 was a compact, battery-powered pulse generator that was designed to be attached to a trawl door. This unit produced an electrical field of about 0.5 V at 2 pulses/sec. Fishing trials were encouraging from the operational aspect but catch results were considered poor by commercial shrimp fleet standards. The electrical field was not adequate to force shrimp out of the bottom. Therefore it became necessary to examine closely shrimp's responses to various electrical fields.
A project was initiated to determine the optimum electrical requirements necessary to electrically stimulate shrimp burrowed in the substrate (Klima, MS). Because of the difficulty in obtaining accurate in situ measurements, underwater motion picture photography was selected to document these responses. During the first phase, laboratory studies determined the threshold voltages of shrimp placed at various angles to the electrodes. In the second phase, the escape reaction of burrowed shrimp from different type substrates was recorded for various combinations of voltages and pulse rates. Over 1,000 shrimp were individually stimulated and photographed on 16 mm motion picture color film by SCUBA divers in the eastern Gulf of Mexico. The minimum power requirements necessary to force shrimp out of the substrate were determined by analyzing these films. These requirements were 3.0 V at 4 to 5 pulses/sec.
The Gear Research Unit staff designed and fabricated a prototype electro-shrimp trawl system that produces an electrical field immediately in front of a shrimp trawl. The electrical field causes an involuntary “kicking” response in shrimp that forces them out of the substrate and into the path of the oncoming trawl. The system has four primary components: 1) the power control panel aboard the vessel, 2) a power cable from the vessel to a trawl door, 3) an electronic pulse generator mounted on one of the trawl doors, and 4) an electrode array.
3.1 Electrical Power Panel
The power panel consists of a variable transformer, two voltameters, two lights, and an on-off switch. The vessel's generator furnishes 110 V AC to the panel. Voltage from the panel to the pulse generator is controlled with a variable transformer and is monitored on one of the voltameters. The second voltameter shows the voltage output of the pulse generator. One light indicates when the power is on between the panel and the pulse generator and the other light flashes synchronously with the pulse rate output of the pulse generator.
Fig. 1 The polyvinyl chloride housing for the electronic pulse generator is shown mounted on a trawl door. The small galvanized pipe housing immediately above contains a 225-W resistor which was located separately to dissipate the heat it created.
3.2 Electrical Power Cable
A 4-conductor, American wire gauge No. 12, neoprene covered cable about 300 m long was used during the testing of the system. Two of the conductors were to supply the AC to the pulse generator and the other two monitored the pulse rate from the pulse generator. This cable provided ample power to the system.
3.3 Electronic Pulse Generator
The pulse generator converts a.c. received from the vessel to DC. It stores the electricity in a bank of capacitors and releases the DC to the electrodes at a predetermined pulse rate. The electronic components are mounted on a Bakelite1 board and are encased in a cylindrical watertight polyvinyl chloride (PVC) housing 20.3 cm nominal diameter (Fig. 1). The electrical power connections in and out of the housing are made with watertight connectors.
During initial field trials of the electrical components, heat generated within the housing did not disseminate into the surrounding water and caused premature failure of several components. The insulating qualities of the PVC apparently caused the excessive heat build-up. Two approaches to eliminate this problem were considered. The first was to design and build a new housing from material which would conduct the internal heat to the water. The second approach was to remove from the PVC housing the major heat creating source, a 225 W resistor, and place it in a separate galvanized iron pipe housing. In the interests of expediency, the second approach was adopted. In addition, cooling oil was used inside the PVC housing to help dissipate the heat. The resistor was housed separately and was then wired to the adjacent pulse generator. The unit functioned satisfactorily after the change was made.
1 Trade names referred to in this publication do not imply endorsement of commercial products
3.4 Electrode Wires
Five electrode wires the width of the trawl opening were secured so that they would hang in even bights about 61.0 cm apart, parallel to and ahead of the trawl footrope (Fig. 2). The last one was placed immediately ahead of the trawl footrope. The current from the pulse generator was released directly to each electrode to create the electrical field. Experiments were conducted with several kinds of wire. The problem was to find a wire that could withstand the rigors of being dragged over the bottom and carry an electrical current. The last wire which we tried had 6 strands, 3 strands of insulated stainless steel wire for strength and 3 strands of exposed copper wire to carry the current. This wire performed satisfactorily during our field trials.
The final phase of the project was to determine the ability of the system to catch burrowed shrimp during daylight from various commercial shrimp grounds in the Gulf of Mexico. In the northern Gulf the major producing areas are located intermittently from northwest Florida to Texas where the predominant commercial species of nocturnal shrimp is the brown shrimp, Penaeus aztecus Ives. In the shrimp grounds off south Florida, the pink shrimp, P. duorarum, replaces the brown shrimp in this ecological niche.
Two locations in the northern Gulf and one in the south Florida grounds were selected for the fishing trials. The first of these areas ran confluently along the Alabama and Mississippi Continental Shelf. Several river systems flow into large bay areas providing favourable inshore conditions for shrimp nursery grounds. Offshore, in the 9 to 27 m depth range, the obstruction-free substrate is a soft black mud. It was in the latter area that the fishing trials were held. The second area was the coastal shelf off Freeport, Texas, where environmental conditions are similar to those found in the Alabama-Mississippi area, however the shelf is broader and provides more dragging area. On the Tortugas shrimp grounds in southern Florida, the substrate is a calcareous, sand-shell composition. Species of gastropods, echinoderms, and sponges, which are found here, are sometimes collected in the net and create problems while dragging. Approximately 130 km to the north of the Tortugas are the Sanibel shrimp grounds where brief trials were held. The bottom there is composed of broken shell with small corals and the pen shell, Atrina rigida, thriving profusely. These conditions prevented us from making any significant catches.
Fig. 2 A shrimp trawl stretched open showing configuration of electrode array in front of trawl footrope.
The United States Bureau of Commercial Fisheries research vessel GEORGE M. BOWERS was used for the experimental fishing. It is a 22.6 m (LOA) wood-hull shrimp vessel, which is rigged for double trawling with two 8.0 m outrigger booms. A two-drum trawling winch, which is powered by a mechanical power take-off from the main engine, holds 734 m of trawl wire. Standard 12.2 m flat Gulf of Mexico shrimp trawls (Bullis, 1951) constructed of 5.1 cm stretched-mesh nylon with polydacron hanging lines were used. One of the trawls was rigged with a single tickler chain, which was secured to a bracket on the lower rear of each of the 2.1 m trawl doors. The other trawl had the electro-shrimp trawl components.
To test the effectiveness of the electric trawl, both trawls were dragged simultaneously day and night so we could make comparisons of the individual trawl catch rates. The significant figures were the comparison of the nighttime catches with the non-electric trawl and the daytime catches with the electric trawl. Upon reaching the strimp grounds, a series of night drags would be completed in a given area and the catch results recorded. Then, during daylight, another series of drags would be completed in the same area. This procedure was repeated during the entire cruise. By comparing the results, it was possible to assess the fishing ability of the electric trawl. All drags discussed here were 1 h long.
During fishing trials to test the system, 134 double-rigged drags were completed. These were divided among the three fishing locations. In the two northern Gulf areas, daytime average catches with the electric trawl were 8.1 kg/h and 7.0 kg/h (Table I). Nighttime average catches with the non-electric trawl were 7.4 kg/h and 7.3 kg/h, respectively. A comparison of these averages shows that during the day the electric trawl caught 109 percent and 96 percent of the night non-electric trawl catch. On the Tortugas shrimp grounds the daytime average catch with the electric trawl was 8.6 kg/h and the nighttime average catch with the non-electric trawl was 17.0 kg/h, a ratio of 50.1 percent.
The electric trawl catch in all areas fished at night was consistently less than the non-electric night trawl catch. The reason for this was not determined.
Average shrimp catches from comparative fishing trials made with electric and non-electric shrimp trawls
|Electro-trawl||Non-elect. trawl||Electro-trawl||Non-elect. trawl||Electro-trawl||Non-elect. trawl|
|DAY||No of drags||19||19||18||18||29||29|
|NIGHT||No of drags||10||10||20||20||23||23|
Tests with this equipment on commercial shrimp grounds demonstrated that the system will catch commercial quantities of shrimp during the day when they are not available to conventional gear. We did not experience any unusual or awkward gear handling problems. The electrodes did not become entangled, and the pulse generator housing was neutrally bueyant and did not appreciably affect the trawl door to which it was attached. The electric cable, which was handled separately, could be improved for industry application. For example, an armored electric cable, which could also serve as a trawl warp, would eliminate the extra handling now necessary.
The fishing results demonstrated that the electrical system was capable of increasing a vessel's shrimp catch. The ratio of day to night catches is apparently related to the type of substrate in the area being fished, the latter probably is related to the ability of electricity to penetrate the different substrate compositions. During behavior studies, shrimp emerged faster and jumped higher from mud than from any other type of substrate. During fishing trials the highest catch ratio was made from a mud substrate. When gear trials were moved to the Tortugas areá, the catch ratio, as noted earlier, dropped almost 50 percent. The difference in substrate, from mud in the northern grounds and a sand-shell in the southeastern grounds, is believed partly responsible because the sand-shell substrate retarded the penetration of the electrical field. Williams (1958) reported that pink shrimp, P. duorarum, burrow deepest in a sand-shell substrate. On the Sanibel shrimp grounds, the substrate was entirely of broken shell. This created severe conditions for the electrical field and contributed to the low catch in this area. Additional research should be conducted to study the relation between a sand-shell substrate and an electrical field. From the information gathered during this project, it is suggested that manufacturing and distribution of this system by private enterprise will be beneficial to the fishing industry.
Bullis, H.R., 1951 Gulf of Mexico shrimp trawl designs. Fishery Leafl.Fish Wildl.Serv.U.S., (394):16 p.
Fuss, C.N., Jr., 1964 Shrimp behaviour as related to gear research and development. 1. Burrowing behaviour and responses to mechanical stimulus. In Modern fishing gear of the world, London, Fishing News (Books) Ltd., vol.2, pp. 563-6
Klima, E.F., Studies on electrical stimulated behavior of shrimp which led to the development of the electro-shrimp trawl system, (MS.) 30 p.
Wathne, F., 1964 Shrimp behaviour as related to gear research and development. 2. Shrimp reaction to electrical stimulus. In Modern fishing gear of the world, London, Fishing News (Books) Ltd., vol.2, pp. 566-70
Wathne, F. and J.K. Holt, 1964 How the electrical shrimp trawl was developed and what the tests show. Fish Boat, 9(3):56–8
Williams, A.B., 1958 Substrates as a factor in shrimp distribution. Limnol.Oceanogr., 3(3):283–90