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

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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.