Effects of specifications of branch line on sinking characteristics of hooks in Japanese tuna longline

To develop mitigation measures for reduction in incidental catch of seabirds in tuna longline fisheries, the hook sinking speed in ten types of branch lines used by Japanese tuna longline vessels was investigated. Also, to improve sinking speed of the hook, two new techniques were tested: material of 10 m line close to the hook was altered from polyamide monofilament to fluorocarbon; and a weight of 15-60 gw was added to the hook. The average sinking speed of the hooks from 0 to 10 m in depth varied from 0.16 to 0.23 m/s in the ten types of branch lines. However, assuming the speed of the fishing vessel for casting is 10 knots, no hook of the ten types sank deeper than 10 m within 150 m from stern, that is, the effective area of the bird-scaring streamer (Tori-pole). The sinking speed of the hook with fluorocarbon line was 1.6 times faster than that with polyamide monofilament. The hook with additional weight of 45 gw sank almost two times faster than the conventional one.