Studies on mitigation measures and gear optimization for reduction of incidental catch of seabirds and sea turtles in tuna longline fisheries

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In order to develop mitigation measures for reduction in incidental catch of seabirds in tuna longline fisheries, the hook sinking speed of branch lines used by Japanese tuna longline vessels was investigated, and an underwater setting device was experimentally manufactured. Also, to avoid sea turtle by-catch, a new method in which all the branch lines are set at almost the same depth with instaling the midwater lifting-up floats to the main line was invented. The results are shown as follows.(1)In the first year, the branch lines used by Japanese tuna longline vessels was investigated, and the composition and the feature of branch lines were clarified. The hook sinking speed in ten types of branch lines was examined. In addition, to improve sinking speed of the hook, two new techniques were tested : material of 10m line close to the hook was altered from nylon monofilament to fluorocarbon ; and weight of 10-60gw was added to the hook. The average sinking speed of the hooks from 0 to 1 … More 0 m in depth varied from 0.161 to 0.228 m/s in the ten types of branch lines. However, no hook in the ten types was not able to sink in depth of over 10m before the fishing vessel at vessel speed of 10 knots for casting was 150 m away from the point of casting line, that is, effective area of bird scaring streamer (Tori-pole). The sinking speed of the hook with fluorocarbon line was 1.6 times larger than that with nylon monofilament, and the hook with additional weight of 45gw sank almost two times faster than conventional one. (2)In this fiscal year, First of all an underwater setting device was manufactured for preventing incidental catch of seabirds. By the sea trial, it was confirmed that the fishhooks were always released into the water depth of 1.3 m. On the other hand, the setting shape of the tuna longline in the sea with flow was clarified through the sea trial and theoretical analysis. (3)To avoid sea turtle by-catch, a new method using the midwater lifting-up floats for tuna longline was invented. From the experimental results, it was possible to set all the hooks at almost the same depth by giving adequate buoyancy to some points in the main line.As mentioned above, in this research, the purpose was achieved and the reduction in incidental catch of seabirds can be expected by using the improved branch line and by using the developed underwater setting device. Moreover, by introducing a new method using the midwater lifting-up floats in operation, it is also possible to avoid sea turtle by-catch. Less