An assessment tool for estimating effects of entrainment at hydropower facilities on adfluvial fish populations

The growing demand for hydropower has influenced the connectivity of freshwater ecosystems. Entrainment through turbines has been identified as one factor which can potentially affect fish populations within and downstream of reservoirs and, in some cases, large numbers of entrained fish are recorded. There is a need to understand and assess species-specific population-level effects of entrainment by considering population growth rate, maturation age, and fecundity. Here, we used field-derived data, life stage-structured population models and Monte Carlo simulations to estimate the influence of the entrainment rate on adfluvial kokanee ( Oncorhynchus nerka ) and bull trout ( Salvelinus confluentus ) populations. Sensitivity and elasticity analyses suggested that entrainment of early life stages have a relatively large effect on long-term population growth. Populations downstream of hydropower facilities may have a relatively high growth rate if fish entrained from upstream survive and downstream habitats are conductive to supporting population growth. Given the same entrainment rates on early life stages, kokanee generally had a lower probability of population decline than bull trout. However, the risk of population decline for kokanee increased more rapidly than bull trout with increasing entrainment rate. Our study provides a framework and assessment tool that could help identifying species-specific critical life stages that require further investigation and management attention to mitigate the negative effect of entrainment on fish population. The relationship between entrainment rate and the probability of population decline could be used to inform threshold setting for acceptable entrainment rate based on management goals. Importantly, field surveys and long-term monitoring will be important to verify model assumptions and reduce the uncertainties in modeling outcomes because of the stage/age-, population-, and reservoir-specific entrainment-related parameters and different facility designs and sizes.