STREAM CHANNEL EXPERIMENTS ON DOWNSTREAM MOVEMENT OF RECENTLY EMERGED TROUT, SALMO-TRUTTA L, AND SALMON, S-SALAR L .3. EFFECTS OF DEVELOPMENTAL STAGE AND DAY AND NIGHT UPON DISPERSAL

Four experimental stream channels were used to examine the temporal pattern of downstream dispersal of young salmon and trout at four constant water velocities from the time of emergence from the gravel onwards. At low water velocities (c. 8.0 cm s −1) young salmon showed a high rate of dispersal which was maintained until a very low population density was attained. At higher velocities salmon showed a lower rate of dispersal and the final population density was higher. Trout showed a low dispersal rate at low velocity and higher rates at higher velocities. However, the final trout population density was similar at all water velocities because the low dispersal rate observed at the low velocity persisted for a much longer period than did the higher values at the higher water velocities. The results do not support the view that young salmonids have a narrow and definable period of sensitivity to water velocity. Dispersal rates were significantly higher by night than by day (P <0.001) at all velocities for both fish species. Copyright © 1991, Wiley Blackwell. All rights reserved