Seasonal Changes in Size Composition of Skipjack Catches in Izu Islands.

Abstract

The skipjack catch in the Izu Islands region were classified into 2 groups with a body weight of small (B. W.≤2.4kg) and large (B. W.>2.4kg) size. Here, the small size was dominant, showing 82% of the total catch during the period from 1982 to 1989. Both the large and the small sizes migrate to Izu Islands region seasonally. In this region the skipjack can be caught throughout the year. During 8 years of fishing operation, the size composition of skipjack taken by Japanese skipjack pole and line fleets differed according to the fishing season. The percentage of catches for the large size was high in the autumn and winter, while a high percentage of the small size occurred from spring to summer. Due to increments in the modal size of skipjack, the small and the large size taken in spring and summer tend to differ in body size from that caught from autumn through winter.