Spatial distribution of juvenile Atlantic salmon (Salmo salar) in a subarctic river:: size-specific changes in a strongly seasonal environment

We examined seasonal variation in the spatial distribution and habitat preference of juvenile Atlantic salmon (Salmo salar) in a subarctic river characterized by near-zero water temperatures for 7-8 months a year. Size-specific differences in habitat use were pronounced in summer but disappeared at the onset of winter, when the habitats preferred by small (less than or equal to6 cm) and larger (7-17 cm) salmon overlapped heavily. Small salmon favoured low-velocity habitats during all seasons, and in summer and autumn they mainly occupied shallow microhabitats along stream margins. In winter and spring, small salmon preferred deeper habitats farther away from the shoreline. Larger salmon favoured deeper habitats in all seasons, but they only occupied slowly flowing areas at low water temperatures. Since all salmon size classes were closely associated with deep and low-velocity habitats in winter and spring, this time of year may represent a size-selective bottleneck for Atlantic salmon populations. Therefore, management schemes aiming to enhance salmonid habitats in boreal streams must rest on the knowledge of season- and size-specific habitat requirements of salmonid species.