延縄による混獲防止策の開発

摘要

Experimental fishing operations using research vessels and captive experiment were con-ducted to develop and test mitigation techniques to reduce incidental mortality of sea turtles in longline fisheries. We review some results from our field and laboratory experiments. Circle hook and bait type have been tested in the western North Pacific. Circle hooks reduced the hooking and/or deep hooking rates of sea turtles compared to conventional tuna hooks. Use of circle hooks had little effects on the catch rates of tuna, billfish and sharks. Fish bait showed lower catch rate of sea turtles than squid bait, because sea turtles are likely to swallow the whole squid bait due to tough muscle texture. The results of satellite telemetry showed that loggerhead turtles spent most of their time within the shallow surface layer of the water column. Although shallow longline has higher risk of catching sea turtles, many of the hooked sea turtles are retrieved alive. To improve post-hooking survival of sea turtles, simple and practical de-hook-ing devices were developed in collaboration with fishers.