Effects of vertical distribution and soaking time of tuna longline fishing hooks on catches in North Pacific

We investigated the relationship between vertical distribution and soaking time of hooks and catches collected in the North Pacific (150°W−164°W, 30°N−37°N) from September to December 2018 by the Chinese Longline Fisheries Observer Programme. The results show that the depth ranges of 11 main species were different. The species with the shallowest and deepest depths were Katsuwonus pelamis and Prionace glauca, respectively. Except for Prionace glauca and Alepisaurus ferox, the mean depth and depth distributions of bycatch species were significantly different from those of the targeted Thunnus obesus. With the increase of soaking time, the total catch rate firstly increased and then decreased. Besides, it was higher in 10~14 h, which reached the maximum value at 10^th hour 1.44 ind·(1 000 hooks·h)⁻¹. The catch rate of bigeye tuna was higher in 10~16 h, which reached the maximum value at 11^th hour 0.45 ind·(1 000 hooks·h)⁻¹. The results provide references for the fishing strategy and catch per unit effort (CPUE) standardization as well as formulating effective management measures to reduce the bycatch species.