Ecological consequences of hybridization between native westslope cutthroat (Oncorhynchus clarkii lewisi) and introduced rainbow (Oncorhynchus mykiss) trout: effects on life history and habitat use

In the Upper Oldman River, Alberta, Canada, hybridization between introduced rainbow trout (RT; Oncorhynchus mykiss) and native westslope cutthroat trout (WCT; Oncorhynchus clarkii lewisi) has produced a genotypic gradient with rainbow trout alleles undetectable (<1%) at high elevations and increasing in abundance downstream. Few F, hybrids were found, and genotype frequencies suggest strong backcrossing of hybrids to pure parental populations at both ends of the gradient. The increasing prevalence of RT alleles downstream is accompanied by a life history gradient such that RT alleles were positively associated with growth rate and negatively associated with survivorship. Although physical barriers extended the distribution of pure WCT downstream, several streams held genetically pure WCT in their uppermost reaches, even when no barriers were present. For example, RT alleles were undetectable in headwaters of Dutch Creek, which had no barriers but was very cold (average summer temperature <7.3 degrees C). We propose that RT alleles affect the choice of habitat, as well the metabolic characteristics that determine their life history and competitive abilities. Factors such as climate change, clear-cut logging, and anthropogenic enrichment that can lead to increased temperature and (or) productivity might be expected to result in an upstream shift of RT alleles.