Skip of the routine habitat in an amphidromous migration of ayu

Ayu Plecoglossus altivelis is a typical amphidromous fish with ontogenic requirements for habitats among lower reaches of a river, the marine coastal area, and middle reaches of a river at the immature adult stage. Mature fish that had a small body size like ordinary young were found around the spawning ground. In comparison with standard adults, the migratory histories of the dwarf adults were examined using otolith microchemistry analysis. Sr:Ca ratio estimated that the dwarf fish were exposed to saline or brackish water after once moving into the river, whereas the standard-size fish migrated upstream.