Abstract
Many fish schools were caught owing to the effect of blocking and leading their courses by the set-net leader-net. The behaviour of the fish schools was investigated in the set-net fishinggrounds around the coast of Japan using a scanning sonar for determining the function of theleader-net. The leader-net was effective in blocking off the course of fish school, although itsmesh size was large enough for the fish to pass through. Only 8 percent of the fish schools passedthrough the leader-net, while 76 percent of fish schools encountered the leader-net and moved along it. There were three times as many fish schools moving offshore than toward the coast along the leader-net. The leader-net influenced the fish schools in the area which extended 60m from it. The encountering angle of the fish school's course against the leader-net shows that they had already tended to move offshore. The blocking and leading effect of the leader-net is regarded as being attributed to a tactic movement.
