Understanding the effects of river regulation on Atlantic salmon fry: The importance of channel morphology

River regulation influences hydrological regime, channel hydraulics, and habitat quality for freshwater biota. Environmental flow assessments typically require river managers to incorporate the requirements of different freshwater species and life stages in designing discharge regimes. However, these requirements can often conflict or be poorly quantified. There is therefore increasing interest in adaptive management whereby the ecological effects of changing flow regime can be monitored and assessed. This study investigated the effects of flow regulation on Atlantic salmon fry below an impoundment. The flow regime incorporated a compensation flow with an atypical seasonal profile where higher discharges were released in the summer than autumn and a spring high flow (freshet) to facilitate downstream migration of smolts through the dam. Saturation stocking and electrofishing were combined with hydraulic and habitat models to predict the effects of discharge on habitat quality and standardized fry densities. The discharge at which the highest standardized fry densities were observed varied depending on reach morphology, particularly channel roughness. Low fry survival was consistent with predictions of consistently low habitat quality during the spring freshet. Fish locations observed during electrofishing were not always consistent with contemporary flows and may reflect the influence of antecedent flows and high site fidelity. The combination of saturation stocking, hydraulic, and habitat models provides a valuable approach for assessing the effects of regulation. It is recommended that future freshets operate at night, that uninterrupted periods of high flow are avoided during the spring, and that compensation flows reflect natural seasonal patterns.