Comparisons of habitat availability and habitat use by an allopatric cohort of juvenile Atlantic salmon Salmo salar under conditions of low competition in a Norwegian stream

Allopatric Atlantic salmon parr occupied a wider and lower range of mean water velocities than have been previously reported for parr in sympatry. With regard to depth, substrate and cover no substantial differences were found. Different fish sizes did not affect habitat use by parr significantly, neither did habitat use change during the season. Habitat availability strongly influenced observed habitat use thus, invalidating use of habitat suitability curves based on observations of habitat occupancy by fish only. The allopatric parr consistently selected habitats different from the available habitat, i.e. had preferences also in the absence of intraspecific interactions between age classes. Habitat conditions not used by the fish were more readily identifiable than habitats used. The parr rarely used shallow (≤ 10 cm) and deep (> 60 cm) stream areas, low mean water velocities (< 10 cm s^-1), fine substrate (sand and finer) and stream areas without cover. However, the calculated habitat preferences were also affected by habitat availability. Multivariate analysis did not identify any of the habitat variables as substantially more important than the others, but suggested that the importance of variables may vary with type of habitat studied.