The shear flow environment of juvenile salmonids

The structure and function of streams is of critical importance to salmonid populations. The structure of streams is related to the amount and complexity of cover. Drift feeding salmonids use cover for a variety of reasons, but a primary reason is as velocity shelter. Fish choose focal positions near cover with low average velocities near stream regions with fast water velocities.; Of all the variables that influence stream dwelling salmonids current velocity is considered to be of prime importance. Velocity is typically reported as average values, but riverene flows are turbulent for the vast majority of cases. Turbulence arises from shear on the bed and around large roughness elements, both regions preferentially occupied by young salmonids. The association between turbulence, cover and salmonid habitat occupancy provides an opportunity to use turbulence to better define habitat requirements.; A combination of laboratory and field studies were conducted using high and low frequency velocity meters to document the turbulent velocity fluctuations at the focal point of young salmonids, their response to turbulent flow, and the relation between habitat complexity and habitat use. It was found that fish select focal points characterized by low turbulence, but that turbulence is a good predictor of the occurrence of young salmonids. These results appeared to be contradictory. Fish selected low turbulence focal positions, but selected focal positions near turbulence producing cover.; This contradictory response was explained through the introduction of the turbulent attraction and avoidance (TAA) hypothesis. This hypothesis states that fish use turbulence produced from shear to navigate to locations in proximity to cover. Once there, focal positions are chosen that minimize exposure to turbulence produced from the cover.; Another application of this work is in the field of hydraulic habitat modeling. Existing models do not predict potential focal positions or rearing densities with any consistency. Hydraulic habitat models are an important tool, and their use will likely increase. Therefore improvements in the ability of these models to realistically portray salmonid habitat use is critical, and the use of turbulence related habitat descriptions have the potential to improve the efficacy of these models.