Larval Distribution of Ayu Plecoglossus altivelis in the Surface Layer of Estuary Regions in Toyama Bay

Abstract

The distribution patterns and body size ranges of ayu larvae (Plecoglossus altivelis) were investigated in the estuary regions of Toyama Bay from 1992 to 1998. Ayu larvae commonly occurred in the surface layer above the halocline from October to December, but rarely collected in the layer below the halocline. Occurrence of the larvae was restricted to the 2.5 km area from the coast. Standard length or notochordlength of the larvae collected ranged from 3 to 22 mm including individuals of 9-12 mm size, which were rarely reported in the previous studies. Most of the larvae larger than 10 mm occurred in the shallow waters within 1 km off the coast, especially near the rivermouth. The halocline existed within 1 m depth. It was suggested that the shallow water layer near the estuaries of Shou, Oyabe and Jinzu Rivers is an important habitat for the early larvae of ayu less than 16 mm. Ayu larvae would disperse widely and then with growth migrate to the surf zone through the surface layer of the estuary regions in Toyama Bay.