Particle size composition of spawning gravel of Ayu （Plecoglossus altivelis） in Lake Biwa basin.

Abstract

The purpose of this study was to describe the particle size composition of spawning gravel of ayu （Plecoglossus altivelis） in 10 rivers flowing into Lake Biwa. We sampled surface gravel of their spawning ground （loose-stone riffles） near the mouth （＜2 km from the mouth） of each river, and examined particle size distribution of the gravel. The average and median values of the particle size ranged from 5.2 to 28.4 mm and 4.8 to 28.1 mm, respectively, with the percentage of particles ＜32 mm being 57-100%. These results were generally consistent with previous reports on characteristics of spawning habitat of ayu. However, the particle size in five study rivers on the eastern side of the lake was larger, which may be related with recent concern that spawning habitat of ayu is degrading in those rivers.