Studies on response processes of ocean ecosystem to environmental changes and effect on the replacement of dominant pelagic fish species

研究的重点是下列与小型中上层鱼类补充过程有关的未解决问题。(1)（2）水温和摄食环境对仔鱼生长的影响;（3）仔鱼生长速率和与捕食者接触概率对仔鱼死亡率的影响;（4）小型中上层鱼类替代机制的改进模型。为此，对黑潮和黑潮-俄亥俄过渡区进行了深入的实地调查、室内实验和历史资料分析。主要结果如下。(1)利用历史资料和黑潮沙丁鱼幼鱼的补给率，在黑潮大弯曲路径期间，黑潮沙丁鱼幼鱼的滞留面积增大。但在此期间，恩舒-那达海产卵场的仔鱼滞留率有所增加。研究发现，黑潮路径的两种模式的替换影响了沙丁鱼和黑潮之间优势地位的替换。(2)通过对黄颡鱼幼鱼耳石和胃内容物的分析，明确了水温和饵料密度对黄颡鱼生长率的重要性，以及变态时间对黄颡鱼补充率的影响。(3)20世纪70年代和80年代，黑潮-俄亥俄过渡区春季叶绿素浓度处于较高水平。结果发现，这对应于Oyashio地区向南扩张的强化时期，也对应于日本沙丁鱼更好地招募的时期。(4)在20世纪70年代末和80年代初，黑潮-俄亥俄过渡区的春季水华时间推迟了约一个月。这一事实支持了中上层鱼类和食物浮游生物之间匹配-不匹配的重要性。(5)指出在未成熟期过度捕捞会抑制鲭鱼资源的恢复和生长。

The research were focussed on the following unresolved problems relating to the recruitment processes of small pelagic fish. (1) effect of larval transport and retension, (2) effect of water temperature and feeding environment on larval growth (3) effect of larval growth rate and probability contacting with predator on the larval morality (4) improvement models on the replacement mechanism of small pelagic fish species. For this purpose, intensive field survey on the Kuroshio and Kuroshio-Oyashio transition regions, laboratory experiment, historical data analysis, were conducted. Main results are as followings.(1) By using historical data and supplying rate of sardine larvae in the Kuroshio, its retension area is increased during the period of large meander path of the Kuroshio. However, retension rate of anchovy larvae in the spawning ground in the Enshu-Nada Sea is increased during the period. It was found that this replacement of two modes of the Kuroshio path affect on the replacement of dominance between sardine and anchovy. (2) Based on the analysis of otolith and stomach content of anchovy juvenile, importance of water temperature and food density on the growth rate, the timing of metamorphosis on the recruitment rate were made clear. (3) Chlorophyll concentration in the Kuroshio-Oyashio transition region in spring were in high level during 1970s and 1980s. It was found that this corresponds to the intensified period of southward expansion of the Oyashio area and also to the period of better recruitment of Japanese sardine. (4) Timing of spring bloom in the Kuroshio-Oyashio transition region was delayed by about one month during later 1970s and earlier 1980s. This fact supports the importance of match-mismatch between pelagic fish and food plankton. (5) It was pointed out that overfishing of Japanese common mackerel in the unmatured stage suppressed its recovery of the stock and elongation of the anchovy stock.