Annual Stock Fluctuations of Ayu, Plecoglossus altivelis altivelis, in the Coastal Waters of the Central Wakayama Prefecture, Japan

Abstract

The annual stock fluctuation of ayu, Plecoglossus altivelis altivelis, was investigated through their larval and juvenile stage from November to May in the 1999, 2000 and 2001 year groups both in the coastal waters and in the Hidaka River of the central Wakayama Prefecture. The catches of juvenile ayu in coastal waters and the number of ayu migrating upstream the Hidaka River increased with the year group from 1999 to 2001. Thus the 2001 year group was considered to be a bumper year. Ayu larvae in the 2001 year group occurred abundantly in the surf zones around the mouth of the Hidaka River, and their sizes were larger than those of the other two year groups. These findings suggest that the amount of larval recruitment and the following larval growth rate in surf zones should correspond to the level of the stock of ayu population. We suppose that high larval mortality of ayu in the sea could occur before recruitment to surf zones, and extent of the mortality would be an important determinant for the year-class strength.