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

本研究采用放生和再捕的实验方法，揭示放生数量居海洋鱼类首位的日本比目鱼幼鱼的生态特性，并据此估算其承载能力。日本海洋养殖协会、京都大学和福井县立大学合作组建了一个研究小组，以实现根据需要提供孵化场饲养的幼鱼，并通过各种方法进行有效的实地调查和综合分析。在日本若坂湾西部的和田海滩和优乐海滩建立了实验模型场。实验在不同的时间、鱼的大小和苗圃进行了大量放生，以观察密度依赖性摄食、生长和死亡率。因此，放生幼鱼的摄食状况与捕食的mysid丰度呈正相关，尤其是与比目鱼幼鱼的相对mysid丰度呈正相关。放生幼鱼日生长量与放生比目鱼数量、放生比目鱼总数和野生比目鱼总数呈负相关。此外，还论证了大量放养对苗圃生态关系的影响：大量放养在限定区域增加了野生比目鱼幼鱼的mysid丰度，显著降低了它们的取食率。由于日本比目鱼幼鱼是典型的食饵类动物，因此可以根据肠道内容物的质量和数量、每日生长和分布规律以及与比目鱼、同栖鱼和野生比目鱼幼鱼动态变化的关系，大致估算出与承载能力相关的合理放放量。

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.