Among-river pattern in relative abundance of two salmonid fishes reflects temperature-dependent competition

Temperature is a key driver in determining species distribution and abundance across climatic and geographical gradients. This occurs through direct physiological effects of temperature on performance, but may also result from temperature effects on species interactions. In the current study we experimentally test for temperature-dependent competition between juvenile anadromous Atlantic salmon and brown trout and evaluate the role of temperature in shaping the relative abundance of these species across 88 Norwegian rivers. When the two species were reared in allopatry their somatic growth rates were similar across a range of temperatures (4-16 degrees C). When reared in sympatry, the growth of salmon was greatly reduced at the coldest temperature relative to when reared in allopatry, whereas this was not the case for trout. The effect of interspecific competition on growth was more similar for the two species at warmer temperatures. Thus, interspecific competition effects were strongly asymmetric only at the lowest temperature, with trout outcompeting salmon. The results from the experiment were reflected in qualitative patterns of relative abundance of salmon and trout in natural populations, which shifted from being trout dominated in rivers with low summer temperatures to being salmon dominated in warmer rivers. These results provide an empirical example of how temperature effects on the relative abundance of species can only be understood in light of information about temperature-dependent species interactions.