Spatial Distribution of Larval Ayu Plecoglossus altivelis in the Shimanto Estuary, Japan

Abstract

Ayu larvae, amphidromous salmonidei fish, were collected at discrete depths in the center of the main current and the waters adjacent to the banks in the Shimanto Estuary during November 1992-February 1993. Yolk-sac larvae smaller than 7.0mm BL were dispersed throughout the estuary. After absorption of yolk, larvae aggregated near the bottom of the estuary, and remained there until reaching 10mm BL. Larvae of 10mm BL and larger were observed to aggregate in the waters adjacent to the banks. Thus, larvae migrate from the near bottom waters in the central channel to the waters adjacent to the banks of the estuary. We suggest that an increase in the specific gravity of larvae after yolk absorption facilitates the aggregation near the bottom. The larval characteristic of sinking to the bottom of the estuary may enhance their retention in the estuary.