Density-Dependent Effect of Chum Salmon, Oncorhynchus Keta, on Growth and Reproduction

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Since the 1970's the number of chum salmon returned to Japan increased exponentially due to artificial propagation. In recent years, coastal catch of Japanese chum salmon depends by one fourth on Iwate chum salmon and by two third on Hokkaido chum salmon. Incremental ratio of the number of chum salmon returned to Iwata prefecture was higher than other areas in Japan.The aim of this study is to examine the effect of such enhancement of chum salmon stock on biological characteristics of this stock itself.We investigated age and size at maturity of three chum salmon populations in southern Iwate prefecture since 1975. Size decrease was observed since the early 1980's. Also mean age at maturity tended to become higher and condition factor reduced simultaneously.To estimate the growth during ocean resident years, we measured length of scale radii and number of circuli at each ocean resident year. Age 4 fish were subsampled for scale measurement. Though annual variation of length of scale radii and unmber of circuli were observed for each ocean resident year, stunted growth at first ocean year was obvious in recent ten years.Negative correlations were detected between mean fork length and number of adults returned to Iwata prefecture, and between first ocean growth and number of Iwate juvenile chum salmon released. Positive correlation existed between mean age of return and number of adults returned to Iwata prefecture. From these results existence of density dependent growth of first ocean growth was suggested.