Different Mortality in Larval Stage of Ayu Plecoglossus altivelis by Birth Dates in the Shimanto Estuary and Adjacent Coastal Waters

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  • Different Mortality in Larval Stage of Ayu <i>Plecoglossus altivelis</i> by Birth Dates in the Shimanto Estuary and Adjacent Coastal Waters

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Different life stages of ayu Plecoglossus allivelis, including downstream migrating yolk-sac larvae, larvae and juveniles distributed in the estuary or coastal waters, and upstream migrating juveniles, were collected in the Shimanto River and adjacent coastal waters from October 1996 to June 1997. The objective of this study was to examine mortality process of larval and juvenile ayu by comparing the seasonal change in number of newly hatched larvae with the birth date distributions of the later life stages. For yolk-sac larvae during the downstream migration, two-peaks in abundance were found in mid-Novem-ber and between late December and mid-January. In the later life stages, November hatched larvae and juveniles were few, while larvae and juveniles hatched between late December and early January were more abundant. This indicates a high mortality of the larvae hatched in mid-November 1996. From October to November 1996, water temperatures in the Shimanto Estuary and adjacent coastal waters were over 20°C, in which the survival rate of larval ayu was considerably low. We propose that the unusually high water temperature of the sea in autumn 1996 caused the high mortality of larval ayu hatched in November.

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