Effect of meso-scale oceanographic features on the larval growth and survival of small pelagic fishes in the Kuroshio-Oyashio transitional regions.

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In this study, we aimed to explore how and to what extent meso-scale oceanographic features affect larval growth and survival of small pelagic fishes such as anchovy in the Kuroshio and Kuroshio-Oyashio transitional regions, where major spawning and nursery habitats were located. For this purpose, focusing on the meso-scale eddies caused by frontal disturbances of the Kuroshio and Kuroshio Extension, we made intensive field surveys in addition to detailed analyses of existing data. Numerical bio-physical models for evaluating the effect of meso-scale eddies on the biological production in relation to larval growth/survival were also developed.It has been revealed that warm water regions associated with the northward expansion of the Kuroshio water as seen in the northern edge of the Kuroshio front apparently play an important role in biological production and larval growth/survival of anchovy and other small pelagic fishes. In particular, meso-scale eddies caused by frontal disturbances in these regions contribute greatly to the enhancement of larval food production in addition to entraining and accumulating small-sized larvae into the edge of the eddies. On the other hand, larval growth is appreciably low in less productive Kuroshio water to the south and cold water associated with Oyashio water to the north. Therefore, possibility of the encounter of larvae with meso-scale eddies could determine the extent of larval survival in the offshore water.