NPRB 1117 Cooperative research to develop new trawl footrope designs to reduce mortality of southern Tanner and snow crabs (Chionoecetes bairdi and C. opilio) incidental to Beri...

Alaska’s Bering Sea is home to some of the world’s most productive groundfish and crab stocks and the fisheries that depend on them. Their spatial overlap creates problems when bottom trawls affect crabs in their paths, either through capture and discard or damage to crabs that remain on the seafloor. We worked with Bering Sea bottom trawlers to develop and test changes to trawl footropes to minimize crab mortality, by reducing damage to crabs that are not caught, and maintaining low crab capture rates. A previous NPRB project (0711) showed that crab mortality from herding cables ahead of trawls could be greatly reduced with simple modifications to raise them above the seafloor. This project did the same for trawl footropes, the part of the trawl located between the leading edge of the trawl net and the seafloor. Such gear changes allow crab mortality to be reduced without restricting trawling to the few areas of the Bering Sea without crabs.

The mortality of crabs encountering bottom trawls has long influenced management of both Bering Sea groundfish and crab fisheries. NPRB Project 711 (Rose et al. 2013) provided estimates of mortality rates for crabs that encounter, but escape, trawls and showed how these mortalities could be greatly reduced by slightly raising the trawl components (sweeps) that affect most such crabs. The current project applied similar methods and resources to design and test changes to trawl footropes, the other major trawl component that crabs encounter, to reduce the remaining mortality of crabs due to trawling. This project continued cooperative participation of a group of Bering Sea trawl captains in generating new footrope design concepts, reviewing research plans for practicality and relevance to their commercial fishery, and communicating results to their peers. Three principles were identified for reducing footrope effects on crabs: 1) lower weight-in-water, 2) less seafloor contact area, and 3) higher and wider spaces between contact points. Mortality rates for crabs following contact with footropes incorporating these footrope modifications were all substantially lower than an earlier design tested under NPRB project 711. Acceptable crab bycatch and fish escapement rates resulted from the field experiment. The Bering Sea bottom trawl fleet has adopted usage of the improved footropes.

Go to http://projects.nprb.org/#metadata/d3d4915c-786d-42d0-9fcf-ffef4a24c708/project and email rick.towler@noaa.gov

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Video and physical samples were viewed and assessed for degree of trawl interactions with benthos.