Ibaraki Fisheries Research Institute, Freshwater Branch Office, Namegata, Ibaraki 311-3512, Japan
We studied the distribution patterns of ayu Plecoglossus altivelis larvae and juveniles in the coastal waters adjacent to the Kuji River estuary to identify their nursery areas. Fish were collected using a purse seine (mesh size of the cod end: 2 mm) in the survey area, which had a depth of about 3 to 18 m. Ayu (13.6-87.7 mm in standard length) were caught mainly between November and April from areas with a depth of less than 10-12 m near the coastline, such as sandy beaches and the Kuji River mouth. The first occurrence of the larvae was found at the offshore zone beyond the breaker zone of sandy beaches. The ayu extended their distribution from shallow to deep waters as they developed, and individuals that were born earlier tended to extend their distribution first. The nursery areas of ayu expanded to areas shallower than approximately 10 m, near the coastline and river mouth. Moreover, the offshore zone beyond the breaker zone of sandy beaches was important for larvae recruited to the nursery areas.