サケ,マスの鉛直的な日周移動の研究-I : 表層刺網による罹網傾向について (1)

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It is presumed that the salmon in the North Pacific Ocean come up to the surface at dusk and swim near the surface until dawn, and then submerge again to midwater. Therefore, if the hourly variation of netting by the gill-net during night is known, it will explain the phenomenon of the vertical movement of salmon. With the above idea, the authors carried out the experiments by the hourly interval fishing method in 15 different localities in the North Pacific Ocean and the Okhotsk Sea from early June to early August in 1965. Experiments were performed on the training ship “Hokusei Maru” of the Faculty of Fisheries, Hokkaido Univ. and the research boat “No. 5 Kosho Maru” (Fig. 1). The set of the test net consisted of 15-20 “tan” (unit) of amilan multi-F. nets which have 100, 106, 112 and 115 mm mesh sizes. These test nets were commonly used for the pink salmon and the mesh sizes were used according to the fishing season and sea area respectively. Fishing operations in each station were repeated in 4-10 series in one night as shown in table I. In this paper, they discuss the tendencies of netting as observed in the vertical movement of pink salmon and chum salmon. The results obtained are as follows; 1) Two remarkable peaks of catch ratio of pink salmon are found in one night. They are found at 0.2 point (about 1.5 hours) time scale after sunset and before sunrise. The ascending curve after sunset and descending curve before sunrise indicate the characteristic of the vertical movement of pink salmon. The catch ratio toward midnight shows the mean value (Fig. 3). 2) The tendency of the catch ratio of chum salmon in one night resembles that of pink salmon, although the peak of the catch ratio after sunset is not remarkably similar. If the suitable mesh sizes of net had been used, the tendency of catch ratio would have shown more resemblance (Fig. 4).
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サケ,マスの鉛直的な日周移動の研究-I : 表層刺網による罹網傾向について (1)

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