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The effects of size asymmetries and habitat diversity on interspecific interactions in juvenile salmonids (Oncorhynchus sp.) Young, Kyle Accipiter

Abstract

Sympatric coho salmon (Oncorhynchus kisutch) and steelhead trout (O. mykiss) fry tend to occupy pools and riffles, respectively, in coastal streams in northwestern North America. Coho fry emerge earlier, are larger, competitively dominant, and displace steelhead from deep low-velocity pools into shallow high-velocity riffles. The species' morphologies (coho-laterally compressed, steelhead-fusiform) match their habitat associations, suggesting the species are adapted to different hydrologic conditions. Where the three species occur together, cutthroat trout (O. clarki) tend to occupy intermediate depths and velocities. I conducted a series of experiments in artificial stream channels in order to examine the effects of size asymmetries and physical habitat structure on interspecific interactions between coho, steelhead, and cutthroat. A reciprocal transplant experiment using coho and steelhead revealed that in the absence of interspecific competition both species grow faster in pools than in riffles. Steelhead grew faster than coho in both habitats. There was no evidence of a growth rate trade-off between the species across the two habitat types. The pattern suggests steelhead have a higher intrinsic growth rate than coho, but that the species' growth rates vary similarly across habitats. Next, I investigated how relative size and species identity affected competitive ability and fighting behavior. In dyadic contests between coho and steelhead, competitive ability depended mostly on the relative size of individuals. Both species dominated when given a size advantage, and neither dominated when the species were size matched, suggesting no difference in innate competitive ability. Fighting behavior was determined mainly by species identity. Coho were more likely to use aggressive chases, whereas steelhead were more likely to use passive displays. The difference in fighting behavior is that predicted to evolve under asymmetric competition if the species' optimal behaviors depend on different trade-offs between the chance of winning a contest and the cost of escalation. I conducted habitat selection experiments under two competitive scenarios- coho larger (natural asymmetry) and species size matched (asymmetry removed)- in order to determine the relationship between asymmetric competition, density dependent habitat selection, and niche overlap. The size asymmetry increased the per capita effect of coho on steelhead habitat selection by increasing the proportion of steelhead using riffle habitat across a wide range of species' densities. Asymmetric competition reduced niche overlap between the species at both the habitat and microhabitat scale. The competitive asymmetry affected how individuals arranged themselves within habitat type more than the distribution of individuals among habitat types. In a final experiment, I used cutthroat trout to test the hypothesis that habitat diversity increases species diversity by reducing the negative effects of interspecific competition. In a three species community, cutthroat trout grew faster in the diverse habitat than in homogeneous pool or riffle habitats. Coho and steelhead grew as fast in the diverse habitat as in either homogeneous habitat. The experiment provides partial support for the competition hypothesis, is consistent with observational evidence linking habitat diversity to species diversity in natural salmonid communities, and one of the first to control variation along other niche axes when investigating the relationship between habitat diversity, interspecific competition, and species diversity. Together, these experiments improve our understanding of the effects of asymmetric competition on behavior and habitat selection among interference competitors. They also illustrate the potential importance of size asymmetries and habitat diversity in structuring juvenile salmonid communities.

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