Can hydropeaking by small hydropower plants affect fish microhabitat use?

Despite the numerous benefits of hydropower, this renewable energy source can have serious negative consequences on freshwater fish, as a result of short-term artificial flow fluctuations downstream, often known as hydropeaking. Alterations in species habitat use are expected to occur, as a result of variations in the physical environment. However, such assessments conducted at the microhabitat scale, and stratified by season and ontogeny, have rarely been assessed, yet they are fundamental to improve our mechanistic understanding of hydropeaking influences on fish. The goal of this study was to assess fish microhabitat use and availability of leuciscids at upstream undisturbed (2) and hydropeaking-affected (2) river sites located downstream from two small hydropower instalments (SHP), Douro basin, NE Portugal. Fish surveys (juveniles and adults) were performed in spring and summer by electro-fishing followed by the establishment of river transects to acquire use and availability data, respectively. A multivariate approach was then employed to analyse both datasets. Cover and depth were found to be the most important variables driving microhabitat use of species at both the reference and hydropeaking sites. Fish exhibited similar patterns of non-random microhabitat use between the reference and the hydropeaking sites, mainly occupying deeper and more sheltered ones than those available. Overall, seasonal and size-related patterns in species microhabitat use were similar between the reference and hydropeaking sites, with the species showing seasonal patterns in microhabitats use from spring to summer, but, in most of the cases, revealing no size-related difference between both types of sites. This work showed that artificial peak-operations by SHP had negligible effects on fish microhabitat use downstream from SHP when compared to the reference sites, and that the high resilience of the hydropeaking sites appears to be related to the amount of cover habitat and the availability of undisturbed substrates, which provide conditions that still support similar driving patterns of fish habitat use at sites fragmented by SHP.