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An excluder device to reduce halibut bycatch by cod trawlers

(Oct 2006) MCAF's involvement on fishing gear modifications to reduce halibut bycatch in the Gulf of Alaska cod trawl fishery began when members of the Kodiak-based Alaska Dragger's Association contacted John Gauvin to see if the excluder devices he had worked on in the flatfish fisheries in the Bering Sea would work for their fishery. Halibut bycatch during the fall cod fishery in the Central Gulf of Alaska has increased in recent years, partly because the halibut population near Kodiak is increasing and also due to regulations intended to protect sea lions.

While most fishermen prefer to fish for cod in the winter and spring when the fish are tightly schooled, part of their season was shifted to the fall because of regulations to protect sea lions. In the fall, however, the cod are more dispersed, resulting in longer tows and increased bycatch of halibut, which is closely monitored and can result in closure of the fishery if bycatch limits are reached.

In the Bering Sea, rigid grates have been used to effectively separate halibut from smaller flatfish. The larger halibut can't pass through the grate and are allowed to escape while still catching nearly all of the smaller target flatfish such as yellowfin sole.

A different approach was needed to exclude halibut from a cod trawl, however. With their large heads, cod would also be unable to pass through the grate. Additionally, the rigid flatfish grates are not flexible which is a problem for small Gulf trawl vessels where deck space is limited and any device needs to be able to be rolled up on the net reel.

MCAF's Gauvin met with Dr. Craig Rose of the Alaska Fisheries Science Center's Resource Assessment and Conservation Engineering (RACE) Division. Dr Rose has used underwater video to observe fish behavior as they swim through a net, a key factor in improving the selectivity of trawls. This video showed that halibut might be able to escape through slot-shaped portals along the side of the trawl, known as the intermediate, but if set at the right size, market sized cod might not be able to escape.

Some prototype slot panels were fabricated by Gauvin and Dr. Rose using fiberglass rods but when shown to fishermen, they suggested the rods would be too rigid to be wound on a net reel. Instead, they suggested using pieces of a heavy but flexible cable known as "third wire." Each design also incorporated baffles intended to slow the water through the net to make it easier for the halibut to escape.

Using an exempted fishery permit intended for such experimental work, the two designs were pre-tested on the grounds using an underwater video camera to observe the results. The video showed that the original design appeared to optimize halibut escapement but was prone to clogging. The alternative design also appeared to allow a good number of the halibut to escape while avoiding the tendency to clog.

Based on the pre-testing, the partners decided go with the industry alternative. Some initial testing of the "third wire" slots using paired tows, one with the device and the other without were conducted in the late summer of 2006 but the cod were too dispersed at the time to produce meaningful results. Testing will resume next year with the slots made of wire, fiberglass, some different type of construction, or maybe a design that is completely different. "What's really important is that fishermen and scientists are putting their heads together to solve the problem of halibut bycatch in the cod fishery," Gauvin said