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 gr ayling Thymallus thymallus in experimental flumes with channelized (monoton ic configuration and low amount of stones) or restored (highly heterogenous substrate) streambed structures at low and high flows and at different tim es of year. Both brown trout and grayling used higher water velocities in s ummer high flows. Both species also occupied higher-velocity microhabitats in channelized than in restored flumes, but for grayling this difference wa s significant only in summer. Overall, brown trout used lower water velocit ies than grayling. In winter and in high-flow treatments, brown trout were more susceptible to downstream displacement in channelized compared with re stored flumes. For grayling, the effect of flume type occurred only in wint er when fish were located further downstream in channelized flumes. Both sp ecies were more aggregated in winter. Brown trout also exhibited a differen t spatial pattern among the flume types, being more aggregated in channeliz ed flumes. The flume type did not affect the spatial pattern of grayling, b ut they were more contiguously distributed in high flows. Because both brow n trout and grayling used lower velocities in winter, the potential for int erspecific competition may increase in winter. The spa tiotemporal differen ces in fish habitat preferences suggest that assessment: of the physical ca rrying capacity of a stream in relation to species-specific habitat require ments is a prerequisite for effective management of brown trout and graylin g populations. Preference curves indicated that the habitat requirements we re narrowest in winter and that the crucial habitat factor was the availabi lity of flow refuges; thus, the curves helped identify the timings of, and the key factor underlying, a potential habitat bottleneck in northern borea l streams.