日本海西部におけるシイラ漁況の研究-VII : 餌組成と表層生物相との関係について

概要

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The dolphin is distributed at a layer from the sea surface to about 15m deep in the fish-ing grounds under report1). Stomach contents taken from the commercial catch comprise two groups of fishes differing in size, juveniles and medium sized ones being about 10-15cm in body length2), The present study covers investigations of materials taken during June through October, 1955-57 (Fig. 1, Table 1). The finding are summarized as follows. 1) Juveniles found in stomachs of the dolphin dominated among the surface net collections. Quantitative species composition of juveniles taken by the fish resembled that in the sea (Table 2, Fig. 2). 2) In most of the stomachs, medium sized fishes exceeded juveniles in amount. Species com-position of medium sized fishes in the stomachs changed two or three times within each of the three fishing seasons (Fig. 3). During such periods of alternation, medium sized fishes decreased while juveniles increased in amount. 3) Based on the above observations, the following inferences have been drawn. The dolphin mainly feed on medium sized fishes but sometimes on juveniles when the former become scarce. Thus frequency of juveniles found in the stomachs is reversely related with abun-dance of medium sized fishes in the sea.
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