Carrying capacity as a fundamental issue on stock enhancement : a case study for Japanese flounder juveniles

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The present study was performed by means of experimental release and recapture to reveal ecological traits directing to estimation of carrying capacity for the Japanese flounder juveniles of which number of release has been ranked first among marine fishes. A research team was organized under collaboration among Japan Sea-Farming Association, Kyoto University and Fukui Prefectural university to realize providing hatchery-raised juveniles to be released as required, effective field survey and comprehensive analysis by various methods.Experimental model fields were set up at Wada and Yura beaches located in the western part of Wakasa Bay, the Sea of Japan. Experimental mass releases were conducted at different times, fish sizes and nurseries to see density-dependent feedijalg, growth and mortality. Consequently, feeding status of released juveniles was positively correlated to abundance of prey mysids, particularily to relative mysid abundance in relation to flounder juvenile abundance. Daily growth of released juveniles was negatively correlated to number of released flounder as well as total number of released and wild flounder. Furthermore, some evidences of effects of mass-release on ecological relationships in the nurseries were demonstrated: mass-release into a restricted area rose a prominent reduction in mysid abundance and decreased feeding incidence in wild flounder juveniles.Since Japanese flounder juveniles are characterized as a typical specialized feeder to mysids, reasonable amount of release associated with carrying capacity could be roughly estimated based on gut-content quality and quantity, daily growth and distribution pattern in relation to dynamic changes in mysids, cohabiting fishes and wild flounder juveniles.