Shark catch in a pelagic longline fishery: Comparison of circle and tuna hooks

The effects of circle hooks on blue shark Prionace glauca catch in a pelagic longline fishery were assessed in fishing experiments on two research vessels in the western North Pacific off the coast of Japan from May to September 2005. We used conventional tuna hooks (standard Japanese hook size; 3.8 sun) and two sizes of circle hooks (4.3 sun and 5.2 sun) for each fishing operation and compared catch rates, size compositions and mortalities of blue shark between hooks. One vessel used stainless steel wire leaders and the other vessel used nylon-monofilament leaders. Total numbers of blue shark caught were 755 and 2598 for the respective vessels. Mean catch rates (per 1000 hooks) of blue shark for the 3.8 sun tuna hook, the 4.3 sun circle hook and the 5.2 sun circle hook were 40.5, 37.9 and 36.1, respectively, for one vessel, and 81.6, 95.2 and 93.9, respectively, for the other. Catch rates did not differ significantly between the three hook types on either vessel (P = 0.48 and 0.43, two-way ANOVA). Proportions of dead individuals for the 3.8 sun tuna hook, the 4.3 sun circle hook and the 5.2 sun circle hook were 0.03, 0.02 and 0.05, respectively, for one vessel, and 0.10, 0.11 and 0.11, respectively, for the other. The proportion of dead individuals was not significantly different between the three hook types on either vessel (P = 0.31 and 0.70, χ2-test of independence). Mean estimated pre-caudal lengths of blue shark caught by each hook type were between 133 and 135 cm for one vessel and 193 and 194 cm for the other. The difference in mean length between hook types was insignificant for one vessel, but significant for the other (P = 1.00 and 0.03, ANOVA). These results indicate that the circle hooks used in this study had little impact on catch rate and mortality of blue shark. We also discuss the possible relationships between hook type, leader material, hooking location, and catch rate of sharks.