オホーツク海北部における夏季のニシンの研究-IV

Abstract

As a method of pursuit investigation of fish group chiefly composed of 4 year-old fish (“Ayan group”) in 1973, scale patterns of herring taken from the different areas in the northern part of the Okhotsk Sea in 1974 and 1975 were examined. The results of analyzing the scale ratio Ri/fork length relationships showed that the fish with scale pattern of Ayan type had mixed with those of Tauisk type in the summers of the fifth and the sixth year of life. In addition, the fish with r2/r3 ratios larger than 0.80, which can be used as an indicator of presence of Ayan group in 1973, were included in samples at frequencies (26-40%) greater than could be expected. The mixing area of these two scale types almost coincides with the distribution area of the older fish group of the Okhotsk Population in each year. These results indicate that the majority of 4 year-old fish found in the area of Ayan in 1973 had recruited to the adult stock of the Okhotsk Population during the fifth year of life, at the latest.