Microhabitat Use by Age‐0 Brown Trout and Grayling: Seasonal Responses to Streambed Restoration under Different Flows

We examined microhabitat selection by age‐0 brown trout Salmo trutta and grayling Thymallus thymallus in experimental flumes with channelized (monotonic configuration and low amount of stones) or restored (highly heterogenous substrate) streambed structures at low and high flows and at different times of year. Both brown trout and grayling used higher water velocities in summer high flows. Both species also occupied higher‐velocity microhabitats in channelized than in restored flumes, but for grayling this difference was significant only in summer. Overall, brown trout used lower water velocities than grayling. In winter and in high‐flow treatments, brown trout were more susceptible to downstream displacement in channelized compared with restored flumes. For grayling, the effect of flume type occurred only in winter when fish were located further downstream in channelized flumes. Both species were more aggregated in winter. Brown trout also exhibited a different spatial pattern among the flume types, being more aggregated in channelized flumes. The flume type did not affect the spatial pattern of grayling, but they were more contiguously distributed in high flows. Because both brown trout and grayling used lower velocities in winter, the potential for interspecific competition may increase in winter. The spatiotemporal differences in fish habitat preferences suggest that assessment of the physical carrying capacity of a stream in relation to species‐specific habitat requirements is a prerequisite for effective management of brown trout and grayling populations. Preference curves indicated that the habitat requirements were narrowest in winter and that the crucial habitat factor was the availability of flow refuges; thus, the curves helped identify the timing of, and the key factor underlying, a potential habitat bottleneck in northern boreal streams.