Modifications of cod pots to reduce Tanner crab bycatch

摘要

Recent declines in stocks of Tanner crab Chionoecetes bairdi in the Gulf of Alaska and Bering Sea and increases in the catch allocations of Pacific cod Gadus macrocephalus to vessels fishing pot gear have led to heightened concerns about Tanner crab bycatch. We conducted a laboratory study to examine the ability of two modifications in standard cod pots to reduce the catch of Tanner crabs compared with the standard pot. We also observed crab responses to a pot rigged with cod socks instead of cod triggers. In one modified pot, a slick plastic board was attached flush to the lower tunnel ramp of a pot (slick-board pot). In another modification, a webbed Z-K excluder horizontally divided the pot tunnel and three loose twine excluders were rigged on the tunnel eye (Z-K excluder pot). The plastic board on the slick-board pot deflected the current and led more crabs (32.4 percent) to approach side panels, rather than the tunnel eyes, compared with the standard pot (4.7 percent) and the Z-K excluder pot (7.9 percent). Both modified pots significantly reduced the reach probability P(r|a): number of crabs that reached the tunnel eye divided by the number that approached the pot. The P(r|a) for the Z-K excluder pot was also significantly lower than that for the slick-board pot. Crabs took significantly more time reaching the tunnel eyes of the slick-board pot and Z-K excluder pot than the standard pot. There was no significant difference on entry probability among the three pots. However, the mean stay duration (crab contact with the tunnel eye) significantly differed among the three pots. As a result, compared with the standardpot, both modified pots significantly reduced the catch probability P(e|a), i.e., the number of entries divided by the number of approaches. The mean P(e|a) +- 95 percent confidence interval (CI) was 0.05 +- 0.03 for the standard pot, 0.01 +- 0.01 for the slick-board pot, and -0.01 +- 0.01 for the Z-K excluder pot. Crabs had a significantly (0.26 + 0.24 95 percent CI) higher entry probability P(e|r)-number of crabs that entered divided by the number that reached the pot-for cod socks than cod triggers and a significantly higher catch probability (0.18 +- 0.21 95 percent CI). Modifications with a Z-K excluder may be suitable for use in the commercial fishery to reduce bycatch of Tanner crabs.