| Summary: | Homing of presmolts and adults of Atlantic salmon Salmo salar was demonstrated within the Alta River by hydroacoustic tagging and analysis of growth differences. In spite of small nonsystematic differences between spawning habitats used by sympatric populations of Atlantic salmon and brown trout Salmo trutta , no interspecific superimposition of redds of the two species was observed. Atlantic salmon inhabiting streams that are cold in winter spawn early, whereas Atlantic salmon in warm winter streams spawn late. Enhancement programs involving fish transplantations should take into account the thermal regimes of the donor and target streams. As a consequence of interspecific competition, presmolt Atlantic salmon are found at higher water velocities, farther from the banks, and in deeper water than other sympatric salmonids. In a Norwegian river partially dammed by weirs, brown trout became the dominant species in the weir basins, where water velocity was reduced, but the proportion of Atlantic salmon remained unchanged in the areas of the river between the weirs, where water velocity was unchanged. Different competitiveness by salmonid species under varying physical conditions should be considered when the effects of human activities in salmon streams are analyzed.
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