Water Velocity, Turbulence, and Migration Rate of Subyearling Fall Chinook Salmon in the Free-Flowing and Impounded Snake River

We studied the migratory behavior of subyearling fall Chinook salmon Oncorhynchus tshawytscha in free-flowing and impounded reaches of the Snake River to evaluate a hypothesis that suggests velocity and turbulence are the primary causal mechanisms of downstream migration.  The hypothesis states that impoundment should reduce velocity and turbulence, and migratory behavior of juvenile salmon would be altered due to their reduced perception of these cues.  At a constant flow (m³/s), both velocity (km/d) and turbulence (velocity SD) decreased from riverine to impounded habitat as cross-sectional areas increased.  We found evidence for the hypothesis that subyearlings can perceive velocity and turbulence cues and respond to these cues by varying their behavior.  The percentage of the subyearlings that moved faster than the average current speed decreased as fish transitioned from riverine reaches with high velocities and turbulence to upper reservoir reaches with low velocities and turbulence, but then increased again to riverine levels as the fish moved further down in the reservoir where velocity and turbulence remained low.  Migration rate (km/d) decreased in accordance with longitudinal reductions in velocity and turbulence as predicted by the hypothesis.  Variation in migration rate was better explained by a repeated-measures regression model containing velocity (AIC = 1,769.0) than a model containing flow (AIC = 2,232.6).  We conclude that subyearling fall Chinook salmon can respond to changes in water velocity and turbulence, which work together to affect migration rate.