COMPETITION BETWEEN STREAM-DWELLING CUTTHROAT TROUT (ONCORHYNCHUS-CLARKI) AND COHO SALMON (ONCORHYNCHUS-KISUTCH) - EFFECTS OF RELATIVE SIZEAND POPULATION ORIGIN

We conducted competitive trials between stream-dwelling, juvenile cutt hroat trout (Oncorhynchus clarki) and coho salmon (Oncorhynchus kisutc h) in laboratory stream channels to examine the effects of relative si ze 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 p airs). Each experiment was run using two distinct populations of cutth roat: allopatric cutthroat that had been historically isolated from co ho by a barrier falls and sympatric cutthroat that naturally cococurre d with coho. Competitive ability and dominance were indexed by relativ e (proportional) foraging success and aggression. In the one-on-one tr ials, allopatric cutthroat were stronger interspecific competitors (ve rsus coho) than sympatric cutthroat, and size-matched cutthroat outper formed size-impaired cutthroat. Within cutthroat/coho pairs, allopatri c cutthroat outperformed coho when size matched, but not when size imp aired, 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.