National Research Institute of Far Seas Fisheries, Fisheries Research Agency, Shimizu, Shizuoka 424-8633, Japan
This review classifies and explains the bycatch mitigation measures in tuna longline fishery based on their functional mechanism across bycatch species. In longline fishery bycatch mortality occurs through a common process that typically follows these stages: 1) fishing operations overlap with the distribution of non-target animals; 2) non-target animals perceive the baited hook (or hooked fish) as food and approach it; 3) contact with the fishing gear; 4) become hooked or entangled; and 5) die. Mitigation measures block one or several stages of this process to reduce the bycatch and resultant mortality by avoiding the spatio-temporal overlap (time and area closure, and modification of gear configuration and operational pattern), by diminishing perception and attraction of baited hooks (night setting, offal control, and blue-dyed bait), by interrupting the physical contact between non-target animals and fishing gear (bird-scaring line, line weighting, underwater and side setting, and deterrent methods using aversive stimuli), by preventing hooking (hook and bait modification), and by reducing post-hooking mortality (safe release with rescue devices). Since there is no panacea for every occasion, combined use of complementary methods that fit the actual situation of fishing operation enhances the effectiveness of mitigation. The experience and innovation of fishermen are important in developing and improving the mitigation measures.