Competition between stream-dwelling cutthroat trout (Oncorhynchus clarki) and coho salmon (Oncorhynchus kisutch): Effects of relative size and population origin

We conducted competitive trials between stream-dwelling, juvenile cutthroat trout (Oncorhynchus clarki) and coho salmon (Oncorhynchus kisutch) in laboratory stream channels to examine the effects of relative size and population origin on cutthroat foraging and agonistic behavior. Two experiments were conducted: one-on-one trials (pairs of cutthroat and coho) and serial removal trials (groups of three cutthroat/coho pairs). Each experiment was run using two distinct populations of cutthroat: allopatric cutthroat that had been historically isolated from coho by a barrier falls and sympatric cutthroat that naturally cooccurred with coho. Competitive ability and dominance were indexed by relative (proportional) foraging success and aggression. In the one-on-one trials, allopatric cutthroat were stronger interspecific competitors (versus coho) than sympatric cutthroat, and size-matched cutthroat outperformed size-impaired cutthroat. Within cutthroat/coho pairs, allopatric cutthroat outperformed coho when size matched, but not when size impaired, whereas coho outperformed sympatric cutthroat when given a size advantage, but not when size matched. In serial removal trials, both populations of cutthroat outperformed coho. These results suggest that size is perhaps equally important as species identity in determining competitive dominance between sympatric populations of cutthroat trout and coho salmon.