Age and Growth of Ayu Larvae Plecoglossus altivelis Collected in the Nagara, Kiso and Tone River during the Downstream Migration

Abstract

Ayu larvae, amphidromous salmononidei fish with a 1 year life span, were aged with otolith daily ring in order to reveal the ecological aspects of their downstream migration from the spawning ground. The age of larvae ranged from 0 to 12 d, body length from 4.6 to 8.8 mm and sagittal otolith diameter from 20.5 to 48.0 pm. Mean age of larvae collected in the Nagara, Kiso and Tone River was 4.5, 1.8 and 4.0 d, respectively, and mean body length was 7.2, 6.8 and 6.9 mm. Age, body length and otolith diameter had tendency to increase in the lower reaches of the rivers, whereas yolk volume decreased towards rivermouth. Age, otolith diameter and yolk volume did not differ significantly between two samples collected up and down of a dam near the rivermouth of the Tone River, although body length was larger in the downstream, 7.4 mm, than in the upperstream, 6.9 mm. The fish density was about 4 times higher at the upperstream of the dam than at the downstream, suggesting that the drifting larvae were pooled in the dam. © 1991, The Japanese Society of Fisheries Science. All rights reserved.