Catch Rates with Variable Strength Circle Hooks in the Hawaii-Based Tuna Longline Fishery

摘要

The Hawaii-based deep-set longline fleet targets bigeye tuna [Thunnus obesus (Lowe, 1839)] and infrequently takes false killer whales [FKW , Pseudorca crassidens (Owen, 1846)] as bycatch. From 2004 to 2008 with 20%–26% observer coverage, nine mortalities of and serious injuries to FKW were documented in the deep-set fishery in the Hawaii EE Z, yielding a mean take estimate of 7.3 animals yr−1. Weak hook technology can utilize the size disparity between target and other species to promote the release of larger non-target species. Four vessels tested the catch efficacy and size selectivity of 15/0 “strong” circle hooks (4.5 mm wire diameter) that straighten at 138 kg of pull in comparison with 15/0 “weak” (4.0 mm) that straighten at 93 kg of pull. Vessels alternated hook types throughout the longline gear and maintained a 1:1 ratio of strong and weak hooks. Observers monitored a total of 127 sets of 302,738 hooks, and randomization tests were applied to test for significant differences in catch for 22 species. There were no significant catch differences for bigeye tuna; however, there may be limitations to these inferences because trials were not conducted during spring when larger bigeye tuna are available to the fishery. There were no significant differences in mean length of 15 species. Observers collected 76 straightened hooks, of which six were control and 70 were weak hooks. There was one observation of a FKW released from a stronger circle hook. Overall, there was no statistical reduction in catch rates of bycatch species.