In the present paper is described the state of those yellowtails having been caught which usually come migrating to the waters around Hokkaido and is also discussed the relation between this fish stock and another migrating to the Tsushima Islands, based upon the daily records of yellowtails which were caught in trap-nets along different coasts of Japan during the five years from 1953 to 1957.
Larger yellowtails (> class 7, as shown in Fig. 1) are caught in shoals during the months from July to November, especially in autumn, in trap-nets off the western coast of Hokkaido (Fig. 1 and Table 1), and it is sometimes so with the Pacific coast of the Toshima Peninsula in Hokkaido and the Japan Sea coast of Aomori Prefecture (Fig. 1), but it is not so with the other regions of Japan, excepting the Tsushima Islands which are located at the gate of the Japan Sea, as described in details in our previous paper (MITANI. 1959). The writer has herein taken into consideration the relationship between those two stocks, one being a stock of yellowtails caught off Hokkaido and the other, a stock of the same fish caught off the Tsushima Islands.
The courses of the Tsushima Current in the Japan Sea are semidiagrammatically shown in Fig. 2. As is seen in Fig. 2, in the Japan Sea the contact zone is remarkably formed, extending from off the eastern coast of Korea to off the western coast of Hokkaido across the central part of the Sea, which is the habitation for such fishes as cods, saury, sardines, mackerels, pampanos and even some squids. Most of these are favourite preys for the yellowtail. Judging from the current conditions and habit of fish, yellowtails, which come back to the Korean Straits after spawning in the East China Sea in spring, migrate north looking for food probably along the contact zone where plenty of food for them is found. Some of the fish may reach the western region off Hokkaido. Accordingly, the Hokkaido Ground is presumed to be connected with the Tsushima Ground, with the contact zone lying across the Japan Sea intervening between them. In other words, those two stocks of yellowtails which have been referred to in the preceding paragraph probably belong to the same population.
