1United Graduate School of Agricultural Science, Kagoshima University, Kagoshima, Kagoshima 890-0065, 2Faculty of Fisheries, Kagoshima University, Kagoshima, Kagoshima 890-0056, Japan
The Ryukyu-ayu Plecoglossus altivelis ryukyuensis, whose population is steadily declining, inhabits only limited areas of the Ryukyu Islands in southern Japan. The period, body size, age and developmental stage at upstream migration of larvae and juveniles of Ryukyu-ayu were investigated in the Yakugachi River in Amami-oshima Island in 1994 and 1996. The upstream migration lasted for four months from late January to late May. The range of standard length was 24.4-46.7mm (average±SD=35.2±3.36), and the range of ages 54-128 days (83.7±15.4). Developmental stages were composed of seven steps from the “whitebait” larvae stage to juvenile stage having comb-like teeth in the jaws. Upstream migrating Ryukyu-ayu were characterized by a smaller size, a younger age and an earlier developmental stage, compared with ayu Plecoglossus altivelis altivelis. Those charactersistics seemed attributable to a shorter period of coastal life before upstream migration.