Effects of circle hooks on pelagic catches in the Hawaii-based tuna longline fishery
Sixteen vessels within the deep-set Hawaii-based tuna longline fleet tested the catch efficacy, fish size selectivity and survival on longline retrieval of large-size 18/0 circle hooks vs. Japanese style tuna hooks, size 3.6 sun and vs. size 9/0 “J” hooks. Vessels alternated hook types throughout the longline gear and maintained a 1:1 ratio of circle hooks to their existing tuna or J-hooks. Observers monitored a total of 1393 sets; 1182 sets were circle hooks vs. tuna hooks and 211 sets were circle hooks vs. J-hooks. The 18 most-caught species were analyzed representing 97.6% of the total catch by number. Two statistical methods were used to assess differences in catch (randomization test) or catch rate (generalized linear mixed models (GLMMs)). There were no significant catch or catch rate (catchability) differences among hook types for bigeye tuna (Thunnus obesus), the primary target species, with either statistical method. However, GLMMs indicated that catch rates on circle hooks were significantly lower for 16 and 8 species compared to tuna and J-hooks, respectively. There were no significant differences in mean length of bigeye tuna among hook comparisons. Caught condition at retrieval varied considerably among the 18 species. Large circle hooks had greater effects on catch rates than on fish size selectivity and fish survival. We contend that reduced catch rates are a function of 18/0 circle hook shape, where the minimum width (4.9 cm) was 57% and 25% wider than the Japanese tuna (3.1 cm) and J-hook (3.9 cm), respectively. In contrast to tuna hooks, large circle hooks have conservation potential for use in the world's pelagic tuna longline fleets for some highly migratory species, with catch rate reductions of 29.2–48.3% for billfish species and 17.1–27.5% for sharks.