Different mortality in larval stage of ayu Plecoglossus altivelis by birthdates in the Shimanto Estuary and adjacent coastal waters

Different life stages of ayu Plecoglossus altivelis, including downstream m igrating yolk-sac larvae, larvae and juveniles distributed in the estuary o r coastal waters, and upstream migrating juveniles, were collected in the S himanto River and adjacent coastal waters from October 1996 to June 1997. T he objective of this study was to examine mortality process of larval and j uvenile ayu by comparing the seasonal change in number of newly hatched lar vae with the birth date distributions of the later life stages. For yolk-sa c larvae during the downstream migration, two-peaks in abundance were found in mid-November and between late December and mid-January. In the later li fe stages, November hatched larvae and juveniles were few, while larvae and juveniles hatched between late December and early January were more abunda nt. This indicates a high mortality of the larvae hatched in mid-November 1 996. From October to November 1996, water temperatures in the Shimanto Estu ary and adjacent coastal waters were over 20 degrees C, in which the surviv al rate of larval ayu was considerably low. We propose that the unusually h igh water temperature of the sea in autumn 1996 caused the high mortality o f larval ayu hatched in November.