Dispersal of land-locked larval Ryukyu-ayu, Plecoglossus altivelis ryukyuensis, in the Fukuji Reservoir, Okinawa Island
The dispersal of larval land-locked ryukyu-ayu, Plecoglossus altivelis ryukyuensis , which originated from an amphidromous form on amami-oshima island, was investigated from November 2000 to april 2001 in the freshwater Fukuji reservoir. a total of 232 larvae were collected at three stations at different distances from the mouth of the San- numata river. They were most abundant at the station closest to the San-numata river (Station1; 76.3% of total catch) and their distribution in the reservoir was not uniform. although larvae tended to disperse throughout the sampling area during the sampling period, their abundance decreased significantly as the distance from the river increased. The body length (BL) of larvae ranged from 5.1 to 23.0 mm. Yolk-sac larvae were collected only at Station1, and larvae smaller than 10 mm BL accounted for 80.2% of the all larvae collected at Station1, whereas they accounted for 56.3% of the all larvae collected at Station2. The body size and estimated age based on body length were highly similar between the largest larva in the reservoir (23.0 mm, about 60 days) and the smallest ascending larva (24.2 mm, about 63 days). These results indicate that larvae inhabit the area close to the river mouth after drifting, and staying there until they begin upstream migration without experiencing long-distance dispersal.