Predicted effects of flow diversion byRun-of-Riverhydropower on bypassed stream temperature and bioenergetics of salmonid fishes

Many anthropogenic disturbances impact stream ecosystems by changing flow and temperature regimes. The emerging industry of small Run-of-River (RoR) hydropower reduces streamflow in bypassed reaches, with largely unknown consequences for water temperatures and fish growth. We used empirical and simulated data from two small RoR regulated streams in British Columbia (Canada) to quantify changes in water temperatures in bypassed reaches and assess the potential impacts to resident rainbow trout (Oncorhynchus mykiss) growth using bioenergetics models under a range of consumption scenarios. We found increases in mean monthly water temperature in bypassed reaches due to flow diversion of 0.5-0.8 degrees C (0.17-0.19 degrees C/km). Bioenergetics models using those temperatures predicted increases in annualO. mykissgrowth (compared to natural temperatures) if consumption was unlimited (+200-450%), increases (+15-42%) if consumption was scaled with higher metabolic demand, and small reductions (-5 to 7%) if consumption remained constant. If food availability was reduced by 25%, annual growth was predicted to decline by 45%. Empirical estimates of annual growth of fish sampled indicate modest reductions in annual growth less severe than those modelled by our Scenario 2. Our results highlight that increases in water temperature induced by flow diversion for small RoR hydropower could be large enough to have consequences forO. mykissgrowth, but the impacts depend on how and when RoR hydropower affects food supply and consumption.