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 inhabits very limited areas of the Ryukyu Islands in southern Japan. A field study was carried out to clarify the body sizes, day ages, and number of upstream migrating individuals of Ryukyu-ayu at the Kawauchi and Yakugachi Rivers flowing into Sumiyo Bay in Amami-oshima Island in 1994. The upstream migration from the sea lasted for four months from late January to late May at both rivers. The fish which migrated to the river ran into the upper reaches and kept growing there. The average migrating speed in the lower and middle reaches was slow, 220 (133-300)m/day. The smaller size of fish (mean body length: 34.0 mm) and the number of riffles to the lower and middle reaches influenced their migrating speed. The speed in the upper reaches was slower, 125-128 m/day, while fish grew larger (mean body length: 73.3 mm). The condition factor and growth rate of the fish in the upper reaches were remarkably higher than those in the lower and middle reaches. These results show that the fish in the upper reaches might have migrated upward by repeatedly moving and staying.