TRAWL DIVING: A METHOD USED BY FISHING GEAR TECHNOLOGISTS TO EVALUATE TRAWLING SYSTEMS

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1 TRAWL DIVING: A METHOD USED BY FISHING GEAR TECHNOLOGISTS TO EVALUATE TRAWLING SYSTEMS Ian K. Workman National Marine Fisheries Service 3209 Frederic Street Pascagoula, Mississippi, U.S.A. Trawl diving has been used by fishing gear technologists since the early 1950s. Starting from very basic origins, it has evolved into a very specialized method of evaluating trawls. Conditions encountered in trawl diving are demanding and require that the trawl diver be in good physical condition and trained in trawl design and performance characteristics. Normally, standard scuba gear is used with special consideration in selection given to safety, simplicity, durability and compatibility with trawl diving. There are two basic techniques that allow diving researchers to study trawls underwater. One is to use a diver's sled to move to and from the trawl. The other way is to descend directly down the towing warp to the trawl. On the trawl, divers are able to measure different aspects of the trawl's performance. performance, observe animal behavior in relation to the trawl or document trawl performance with underwater photography. Trawl diving has primarily benefited the commercial fishing industry, but its greatest value is in the evaluation of biological survey trawls. Limitations to trawl diving are restricted depth and bottom time. INTRODUCTION Trawling is a primary method of fishing and has been used by trawl fishermen to catch fish and other marine organisms for over 400 years (Wardle 1983). The first successful trawl was the beam trawl. Named for the heavy solid beam required for spread, the beam trawl was clumsy to handle and awkward to deploy from the sailing vessels that towed them. In 1894, when steam power became available for fishing vessels, a beamless trawl was introduced (Scott 1894). The beam was replaced by two underwater kites (otter boards) for spread and the trawl was called an otter trawl. No new principles in trawling gear have developed since 1894, but, over the years, the otter trawl has undergone a number of important changes and modifications to improve its efficiency. Until fairly recent times, the process of improving trawl efficiency was very slow. Trawl fishermen selectively developed their gear by testing and either accepting or rejecting changes and modifications based on positive or negative effects on their catch.

2 234 Diving for Science...86 Advances in technology (particularly the invention of the aqualung) made it possible in the early 1950s for fishing gear technologists to directly observe trawling gear under actual fishing conditions. Trawl diving techniques have advanced since that time from the very simple approach of watching as trawling gear was towed by to the specialized techniques of actually riding and working on the gear or observing it from a towed vehicle as it is being fished. Trawl diving techniques have been used by Scottish, Russian, Russian, Australian, and American gear researchers and have led to a number of improvements in trawl gear design, rigging and fishing techniques. Fishing gear technologists at the Pascagoula, Mississippi National Marine Fisheries Laboratory use trawl diving extensively in their studies and have been able to measure, adjust and evaluate the fishing characteristics of a variety of trawls. The specialized equipment and techniques they use to dive on trawls are described here. TRAWL DIVERS AND DIVING EQUIPMENT During evaluations, trawls are towed at normal fishing speeds which range from 2.5 to 5.0 knots depending on the intended use of the trawl. To study the affects of haul (retrieving the trawl) on trawl configuration or the behavior of organisms captured, back (retriev.ing the trawl diver may have to hold onto the net at speeds in excess of 5.0 knots. These conditions are not only rigorous; they can be hazardous and, therefore, require that the trawl diver be proficient and in good physical condition. To further reduce the possible hazards in trawl diving, the trawl diver must be trained in trawl design and performance characteristics. Trawl diving is demanding not only on the divers but also on the equipment they use. Most of the diving gear used for trawl diving is standard, but, in the process of selection, special attention is given to safety, simplicity, durability and compatibility with trawl diving. The regulator of choice is a two stage, single hose, demand design with an adjustment for breathing resistance on the second stage (Scubapro adjustable second stage). Regulators without a breathing resistance adjustment have a tendency to free flow and, if used, have to be modified by masking or closing off some of the ambient ports or possibly the purge button on the second stage. Scuba cylinders are equipped with K-valves and not J-valves to avoid associated malfunctions and possible premature actuation of the reserve mechanism. To prevent an out of air emergency, the divers use submersible pressure gauges. A depth gauge and bottom timer are attached in combination with the pressure gauge to allow the diver to monitor depth and bottom time at the same time that air supply is checked. A combination backpack-buoyancy compensator (stabilizer jacket) is used to hold the scuba cylinder and to provide the diver with a means of making buoyance adjustments. A A stabilizer jacket is preferred over a standard horse-collar buoyance compensator and backpack because it reduces the number of straps and buckles and increases the ease of getting into and out of the gear. A bladderless design such as the Scubapro stabilizer jacket works best because it presents minimum surface area resulting in minimal drag on the trawl diver. To eliminate the possibility of accidental inflation of the stabilizer jacket while the diver is on the trawl, a CO 2 inflator system is not used. Instead, a Scubapro Air II with an inflator hose attached to the first stage of the regulator is used for inflation and also serves as an alternate air source for emergency purposes. For

3 Workman: Trawl Diving Techniques and Tools. 235 added safety, a lanyard is attached between the cylinder valve and the stabilizer jacket to prevent loss if the cylinder band comes undone as a result of a snag on the trawl. Due to increased heat loss associated with trawl diving, trawl divers wear thermal exposure suits over most of the year. In the summer, when the water is fairly warm, a lightweight, full length or shorty style wet suit is worn, while a heavier full length wet suit or dry suit is worn in the winter. A A small dagger type knife with a good serrated edge for cutting webbing is worn on the trawl divers forearm or the inside of his calf and is used to free any diver entanglements in the webbing or to make an opening in the trawl should a diver get caught inside. A folding knife is often carried up the sleeve of a wet suit or in the accessory pocket on the stabilizer jacket to serve as a spare. Normally, surface communications between the divers and the trawling vessel are relayed via hand-held hand-held radio through a diver support boat, while underwater communications involve the release of signal floats from the trawl. The divers standardly communicate with each other using hand signals. When more precise vessel to diver or diver to diver communications are required, a hard wire communications system is used. The communications system consists of a surface transmitter/receiver and two full-face AGA masks equipped with microphones and earphones. A dive sled is used in combination with the communications system to provide points of attachment for the communication wires. A dive sled is often used as a vehicle to transport the divers to and from the trawl. The sled has an overall length of 2.4 m and width of 2.2 m and weighs only 41 kg (Workman and Watson, in prep.). The divers lie side-by-side in a prone position with the pilot to port and passenger, facing either forward or aft, to starboard. Sled controls which attach to two large wing-like control surfaces are operated by the sled pilot. A towline coming from the towing vessel is attached to a towing point located at the forward end of the sled. TRAWL DIVING TECHNIQUES AND TOOLS Techniques used to dive on trawls have been described by High (1967), High and Workman (in press) and Wickham and Watson (1976). Small trawls and trawls pulled from outriggers in relatively shallow water are normally evaluated with the aid of a dive sled. The sled is towed behind the trawling vessel and is positioned above and slightly behind the trawl's headrope before the divers board. The divers, transported in a small diver support boat, are positioned well ahead of the sled close to the downwind side of the sled towrope. When the divers are ready to enter the water, the support boat is turned away from the towrope, and the motor is taken out of gear. Once the divers are in the water and clear of the propeller, the support boat motor is placed in gear, and the support boat is moved to a position slightly behind and to the downwind side of the sled. The divers position themselves 3 to 6 m apart along opposite sides of the towline. The pilot takes the lead position facing the port side of the sled. When the sled reaches the pilot, he grabs the passing control surface or sled frame and trails back along side of the sled. From this position, the pilot slides aboard the sled and assumes a prone position at the controls. The passenger boards in the same way but from the opposite side. When the divers are positioned, the pilot releases the dive control restraints and takes control of the sled. The divers descend to the trawl and, depending on the size of the trawl or objectives of the dive, observe it from the sled or land the sled on the

4 Diving for Science trawl and tie off to the trawl webbing. With the sled tied off, both divers can leave the sled to conduct their work which might consist of measuring different aspects of the trawl, observing animal behavior in relation to the trawl and/or documenting trawl performance with underwater photography. The divers move to different parts of the trawl by grabbing onto meshes or lines and pulling themselves hand-over-hand. At the end of the dive, the divers re-board the sled, release the tie downs and ascent to the surface. The support boat then moves to the sled, and on a signal from the pilot, the motor is taken out of gear. The divers kick free of the sled and swim over to board the support boat. When working off of a stern trawler, the divers either use the sled or descend directly to the trawl. To descend directly to the trawl, the divers are positioned with the diver support boat and enter the water alongside one of the main towing warps. They descend along the towing warp to the trawl otter board and follow the bridles or leglines directly from the otter board to the trawl. On the trawl, they conduct their work, and, at the end of the dive, they let go of the trawl and ascend directly to the surface. When they reach the surface, the divers signal for the support boat to pick them up. Trawl efficiency is determined by measuring the dimensions and observing the performance characteristics of a trawl while under tow. Tools used to measure trawls have to be simple and fairly easy to use. Trawl horizontal spread is measured by two divers using a 3.2 mm diameter stainless steel cable marked in 30.5 to 152 cm increments. The cable is stretched across the mouth of the trawl with one end attached to the first hanging on one wing and the other cable end pulled through a small pulley attached to the first hanging on the opposite wing. The cable is pulled taut across the net by one diver while the other records the spread reading. Vertical height and opening on small bottom trawls is measured at select points with a fiberglass measuring pole marked in 15 cm increments. On larger trawls, vertical opening is measured with a diver's calibrated depth gauge. Door performance measurements are made with an inclinometer to determine tilt and a door angle measuring device (modified protractor) for angle of attack. Water flow in and around a trawl or its parts is measured with an Oceanic current meter rigged for hand-held operation. Water flow patterns are determined with dyes. Solid dye cakes, contained in small mesh bags, are suspended across the mouth of a trawl to determine general flow patterns, and a dye injector device filled with liquid dye is used for select point studies. Underwater photography is an important tool for documenting trawl gear performance. The cameras used include 35 mm Nikonos underwater cameras and 16 mm movie and 8 mm video cameras contained in underwater housings. The cameras are fitted with wide angle lenses to photograph large areas of the trawl or gear at relatively short camera to subject distances. Filming can be done from the sled or the trawl. When filming from the trawl, the diver-cameraman uses a line or harness to snap into the trawl and free his hands for filming. TRAWL DIVER SAFETY In addition to the potential hazards associated with standard scuba diving, a number of unique problems exist in trawl diving. These include such problems as being run over by a trawl or its parts, becoming caught or entangled in a trawl or being tumbled head over heels in the turbulence created by an otter board. There is also the possibility of

5 Workman: Trawl Diving Techniques and Tools. 237 being stung or wounded by animals or objects wept into the path of the trawl diver or the diver becoming pinned between the trawl and an object in its path. To avoid or reduce the potential problems, the trawl diver must be alert at all times and maintain good contact with his buddy. If work is conducted on the inside of a trawl, only one member of a buddy team enters; the other remains in close monitoring distance on the outside of the trawl. Should the inside diver become entangled or trapped in the trawl, the outside diver is there to assist by freeing the entanglement or opening the trawl for the inside diver to exit. If an emergency occurs during a trawl dive, an emergency signal float is released from the trawl and a predetermined emergency plan is set into action. Using a sled increases the possibility of uncontrolled pressure-volume problems such as squeezes (barotrauma) and lung over expansion which could result in an air embolism, surgical emphysema or pneumothorax. To avoid these problems good communications between the pilot and passenger(s) are essential. The pilot must communicate any changes such as ascents or descents before they are initiated. Breathing while on the sled is normal (no breath holding) on descent and level flight but should be modified to shortened inhalations followed by long exhalations on ascent. TRAWL DIVING VALUES AND LIMITATIONS The commercial fishing industry has benefited the most from information gained through trawl diving. Trawl designers and manufacturers have been made aware of weaknesses in a number of design' and' construction techniques, while trawl fishermen have received information to aid them in selecting, rigging and fishing their trawling gear. Possibly the most important application for trawl diving is in the evaluations of biological survey trawls. Survey trawls not only provide scientific researchers with samples for study; they also provide data used to manage stocks of commercially important species. If the performance characteristics of a survey trawl are not known and understood, then, it is virtually impossible to make an accurate stock assessment. Even with a knowledge of trawl performance it is not possible to make a totally accurate assessment without knowing the trawl's catchability coefficient for each species of importance. Trawl diving can be used to determine both trawl performance and catchability coefficients. Limitations to trawl diving are the same as those confronting conventional (static) diving - limited depth and bottom time. Trawl divers can work in depths up to 36 m, but the type and amount of work conducted on trawls is restricted by increasing depth. Trawl diving operations conducted in shallow water (18 m or less) allow ample time for trawl evaluations but may not give a totally accurate picture of how some trawls perform in deep water. The differences in trawl performance between shallow and deep water are not considered to be critical though, and, when accurate measurements are required in deep water, an electronic trawl measuring system can be used in addition to trawl diver evaluations. The use of trade name does not imply endorsement of product by National Marine Fisheries Fi8heries Service.

6 238 Diving for Science...86 LITERATURE CITED High, W.L Scuba diving, a valuable tool for investigating the behavior of fish within the influence of fish gear. FAO FAa Fish. Rep. (62)2,E2, High, W.L., and I.K. Workman (in press). Diving while underway. In: NOAA Diving Manual. Scott, J.R British Patent, Fish and Fishing. No.3, 187. Wardle, C.W Fish reactions to towed fishing gears. Pages in A.G. Macdonald and I. G. Pride (eds.), Experimental biology at sea. London, England. Academic Press. Wickham,D.A., and J.W. Watson, Jr Scuba diving methods for fishing system evaluation. Mar. Fish. Rev. 1192: Workman, I.K., and J.W. Watson, Jr. (in prep.). Construction and operation of a two place diver's sled.