Research update about the effective design of tori-line for Japanese small-scale fleet in the North Pacific
In order to examine the design of the tori-line suitable for Japanese small-scale longline fleet in the North Pacific Ocean, operational experiments were carried out in the North Pacific Ocean by chartered small commercial vessel from the winter to the spring of 2018 to 2019. In order to verify the effect of bycatch reduction by increasing the aerial extent, we compared the attacking behavior to baited hooks and bycatch rate between a control (conventional design) group and a tori-line using light materials. The use of lightweight materials enabled the line to expand the aerial extent more widely than control even with smaller drag power. However, there was no statistically significant effect on the performance of bycatch reduction by using the lightweight line, although there was a tendency to reduce both the behavioral and bycatch risks. From the results, it is possible that the expansion of the aerial extent using the lightweight material is effective for bycatch reduction from this experiment, but the conclusion could not be made due to the insufficient data. During the experiment, it was found that the practical tori-line design would be limited due to the lack of strength of tori-pole for specifications of Japanese small-scale vessels. When considering designs for small-scale fleet, it is also necessary to consider the structural characteristics of the poles.