オホーツク海北部における夏季のニシンの研究-II

Abstract

In a previous paper, the size and age composition of the shoals and several biological characteristics of the herring, Clupea pallasi, in the northern part of the Okhotsk Sea in the summer of 1971 were reported. In the present study, the author attempted to clarify the features of the age composition of the shoals and the relationship between their distribution and hydrographic conditions on the surface layer. Fish samples and data of oceanographic observation were obtained by the research vessel“Oyashio Maru”in the northern part of the Okhotsk Sea in the summers of 1971-1973. The surface gill nets usded were composed of nets of various mesh sizes 30, 35, 42, 48, 55, 63, 72, 82mm, and others, and the results of the analysis of these mesh selection curves suggested that these nets can relatively capture herrings non-selec-tively. The results obtained are summarized as follows: 1) The shoals were separated into 3 groups according to the type of their age composition; Shoal A was composed of fish older than 4 years old (3+), Shoal B was mainly composed of 2-3 year-old fish, and Shoal C was mainly composed of 4 year-old fish. 2) Shoal B was distributed in comparative warm waters (12-14°C), where the temperature and salinity gradient were small. 3) Shoal C was observed in only remarkably low salinity areas (27.1-31.2‰), where the discharges of rivers could have some influence. 4) There is a possibility that the extension of the low salinity waters helped to define the distribution of Shoal A. 5) It is deduced that each shoal generally separates from one another geographi-cally due to the distribution of Shoal B and Shoal C in connection with peculiar water masses, respectively.