Use of the average and fluctuating velocity components for estimation of volitional rainbow trout density

The relationship between discharge and fish density in streams is commonly accepted to be curvilinear, progressing from low discharge and low fish density to a peak and then declining to high discharge and low fish density. Discharge is related to water velocity; in general, as discharge increases velocity increases. Habitat complexity is an important aspect of salmonid ecology; more complex habitats support higher fish densities. Habitat complexity also influences the flow of water through the process of flow separation and turbulence production. More complex habitats generally exhibit higher levels of turbulence even though both may have similar average velocities. We tested whether the physical link between habitat complexity and turbulence was correlated with fish density. Turbulent kinetic energy and average velocity for three different levels of discharge and habitat complexity were measured in a flume. Turbulent kinetic energy was found to predict fish density, whereas average velocity did not. We concluded that the physical link between habitat complexity and turbulence production correlates with fish density and that turbulent kinetic energy captures the combined effect of discharge and cover, whereas average downstream velocity only measures the effect of changes in channel discharge.