Migrating behaviour of fish schools in set-net fishing grounds

Abstract

The migrating behaviour of fish schools such as jack mackerel Trachurus japonicus, spotted mackerel Scomber australasicus, and Japanese anchovy Engraulis japonicus was investigated in set-net fishing grounds by means of sonar image records. The spotted mackerel and Japanese anchovy schools moved at an average speed of 18-19cm/s, while the jack mackerel schools moved at 29cm/s. Schools of all species changed their direction of movement at an average veering angle of 19 ?? 36°. However, the veering angle of Japanese anchovy schools showed a tendency to be larger than that of the other two species. The average duration of straight movement of the spotted mackerel and Japanese anchovy schools was 2 ?? 2.4min, while that of the jack mackerel schools was 2.9min. This confirms a frequent veering of fish schools in the set-net fishing grounds. The jack mackerel and Japanese anchovy schools tend to move along the isobathic route with a depth change ratio of 0.6 ?? 0.8m/min between the first and final image. In case of the spotted mackerel schools the larger depth change was observed up to 6m/min.