Stadies on the Distribution and Biological Characteristics of the Herring in the Northern Part of the Okhotsk Sea in Summer | 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的特徴について

1) 1971年8-9月にオホーツク海北部で行なわれたさけ・ます幼魚採集用の流網に混獲されたニシンを標本とし，各水域毎の魚群の組成ならびに4才魚の生物学的特徴について検討した。2) コニー半島沖は中-大型魚，オホーツク沖は小型魚，アヤン沖では中型魚がそれぞれ主体をなしている。集群水域は年により変動するが，一般的傾向としては水域による魚群組成の相違が現われる。3) 2-3才の若令魚は12-14℃の高温な水帯を中心として分布する。4) アヤン沖の4才魚はコニー半島沖のものに比べて体長は小さいが，肥満度ならびに成熟度指数が高い。アヤン沖ではオホーツク群以外の系群が含まれている可能性がある。

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 distribution 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seems to vary yearly. 2) The fish of 2-3 age were observed in comparative warm waters (13-14℃) in 1971, 12-13℃ in 1970 (Unpublished data). 3) Samples of the 4 age, caught in the area off the Ayan, were smaller in their average fork length, and heavier in the ratio of body weight to fork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×10^2/(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.