Downstream (seaward) migration of Ryukyu-ayu (Plecoglossus altivelis ryukyuensis) larvae after hatching was investigated in the Yakugachi River flowing into Sumiyo Bay, Amamioshima Island, southern Japan. Larvae collected near the spawning ground and in brackish water had notochord lengths of 4.5-5.9 and 4.5-24.4 mm, respectively, larval densities in the brackish water being greater. During day time, larvae were found only in the bottom layer, but at night time were also evident in the surface layer, such behavior probably acting so as to prevent the larvae from drifting away from the brackish water area.
