長崎県男女群島に来遊するブリの産卵群について

Abstract

In the present paper are considered some characteristics of the shoals of the yellow-tail, Seriola quinqueradiata T. & S., which are migrating to Meshima Fishing Ground around the Danjo Islands in the East China Sea off Nagasaki, Japan. The author has based these characteristics on the close examination of the gonads of the fish caught in April and May, 1958. By the way, yellow-tails are then in season in that locality. The region around the Islands is oceanographically interesting to us for its high temperature with a narrow annual range and its high transparency ranging from 15 to 30m, as is seen from Table 1. The author can safely conjecture that the fish which migrate to the fishing ground just referred to between mid-April and early May are ready to spawn, judging from the fully ripe gonads they have, though most of the fish are medium-sized ones ranging from 70 to 85cm in fork length, or from 4.9 to 9.7kg in body weight. Their eggs spawned in the region may go up north carried by the Tsushima Current and grow up to be the resources in the Japan Sea. But larger fish more than 85cm in length, or over 9.7kg in weight, which are caught early in the season in the same fishing ground, are found to be ‘immature’, so far as the maturity grade of their gonads is concerned. They may migrate down to the south for spawning, and the greater part of the eggs spawned there may drift with the Kuroshio Current, adding finally to the resources in the Pacific. As for smaller fish ranging from 60 to 70cm in length, or from 3.4 to 4.9kg in weight, they do not spawn in the same fishing ground, but go up north after mid-May. Their eggs spawned there may bid fair to increase the resources in the Japan Sea. The body-size composition and maturity grade of the fish which come migrating to Meshima are diagrammatically shown in Fig. 2.