Water velocity preference of ayu Plecoglossus altiveris altivelis was studied using a stream tank. Ayu preferred the velocity of 43.4 •} 9.2 cm/s (mean •} SD) at growing season in summer and 33.2 •} 9.5 cm/s at just before spawning season in fall. The former velocity approximates to the water velocities measured at grazing scar of ayu on cobble (41.9 •} 14.5 cm/s) in growing season and the latter velocity at spawning bed of ayu (26.7 •} 9.1 cm/s), respectively. These results indicate that ayu

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... efer the water velocity of rapid riffle in growing season, and prefer that of flat riffle mainly in autumn. And this down of preferred velocity and habitat change occur for spawning success.