オホーツク海北部における夏季のニシンの分布および生物学の特徴について

Abstract

Though many biological studies have been made on the commercial-sized herring, Clupea harengus pallasi, in the northern part of the Okhotsk Sea in summer, knowledge on the dis-tribution and biological characteristics of the juvenile in the same period is inadequate even today. The authors studied informations obtained from the measurements of about 2400 samples. The samples were obtained by surface gill nets, varying in mesh size from 30mm to 82mm, used for capturing juvenile salmon, and the samplings were conducted on the research vessel “Oyashio Maru” in the northern part of the Okhotsk Sea from August 17 to September 3, 1971. The results obtained are summarized as follows: 1) The shoals, composed of middle and large-sized fish (20-33cm) were found in the area off the Koni Peninsula, the small-sized fish (12-15cm) in the area off the Okhotsk, and the medium-sized fish (18-26cm) in the area off the Ayan. The area, where each shoal appears, seems to vary yearly. 2) The fish of 2-3 age were observed in comparative warm waters (13-14°C) in 1971, 12-13°C in 1970 (Unpublished data). 3) Samples of the 4 age, caught in the area off the Ayan, were smaller in their average fork length, and heavier in the ratio of body weight to fork length, and higher in the mean maturity index {gonad weight×102/(body weight-gonad weight)} than those from the area off the Koni Peninsula. There is a possibility that the fish in the area off the Ayan may not belong to the Okhotsk population.