Toyama Prefectural Fisheries Research Institute, Namerikawa, Toyama 936-8536, Japan
Juveniles of Ayu Plecoglossus altivelis were distributed from the surf zones to the offshore shallow waters in Toyama Bay, but the range of their occurrence was restricted within ca. 3km from the coast. Upstream running of the juvenile into the Shou and Jinzu Rivers started in April when the river water temperature reached 10°C, and lasted until late May. The size distributions of the sea-run juveniles were similar between the Shou and Jinzu Rivers. The body size of the early run in early April was larger than that of the late run. This is probably due to accumulation of larger individuals in the estuaries waiting until the riverwater temperature reaches ≥10°C. Comparison of condition factors between river- and sea-collected specimens of the same size range revealed that juveniles, especially specimens over 6cm SL, could grow better in the river than in the sea.