Catch Rates with Variable Strength Circle Hooks in the Hawaii-Based Tuna Longline Fishery
The Hawaii-based deep-set tuna longline fleet targets bigeye tuna (Thunnus obesus) and infrequently takes false killer whales (FKW, Pseudorca crassidens) in inicdental catch. In the most recent five years (2004-2008), with 20-26% observer coverage, nine mortalities and serious injuries of pelagic FKW were documented in the deep-set longline fishery in the Hawaii exclusive economic zone, yielding mean take estimates of 7.3 animals per year. 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 303 lb (138 kg) of pull in comparison with 15/0 "weak" (4.0 mm wire diameter) that straighten at 205 lb (93 kg) of pull. Vessels alternated hook types throughout the lonline gear and maintained a 1:1 ratio of strong and weak hooks. Observers monitored a total of 127 sets of 302,739 hooks, and randomisation tests were applied to test for significant differences in catch by hook type for 22 individual species. There were no significant catch differences among hook types for bigeye tuna; however there may be limitations to these statistical inferences because longline 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 among hook comparisons. Observers collected 76 straightened hooks, of which 6 were control and 70 were weak hooks. There was one observation of the false killer whale released from a stronger circle hook, thereby reducing the potential for serious injury. Overall there was no statistical reduction in catch rates of bycatch species.