| Summary: | Hybridization and introgression with non-native salmonids is one of the greatest factors threatening native cutthroat trout species. Westslope cutthroat trout (Oncorhynchus clarkii lewisi; WSCT) were recently listed under the Canadian Species at Risk Act (SARA) as "special concern" (British Columbia populations) and "threatened" (Alberta populations). I employed a 10 locus-microsatellite DNA assay to investigate levels of hybridization between westslope cutthroat trout and introduced rainbow trout (O. mykiss; RT) at 159 sampling locations in southwestern Alberta and parts of southeastern British Columbia. My results revealed that hybridization is extensive across the region sampled. Admixture levels (qwsct of 0 = pure rainbow trout, 1.0 = pure westslope cutthroat trout) at sampling locations ranged from 0.01 to 0.99. An average qwsct below 0.99 is a criterion that has been used in previous work to designate a population as "hybridized." Landscape genetic analysis using regression trees indicate that water temperature, elevation, distance to the nearest stocking site and distance to the nearest railway were significant components of a model that described 34% of the variation in qwsct across 58 sites for which habitat variables were available. Building on this finding, I explored the role of water temperature, the best predictor of hybridization levels amongst the variables tested, in limiting the spread of admixture by evaluating cold tolerance in both species using critical thermal methods (CTM). Analysis of variance revealed a statistically significant difference between the critical thermal minima (CTMin) of WSCT and RT acclimated to 15 °C (1.0 ± 0.8 °C and 1.4 ± 1.0 °C respectively). The heritability of cold tolerance observed in this study appears to be complex and does not seem to behave in a simple additive manner. The identification of water temperature as a major factor influencing admixture and subsequent test for physiological differences in cold tolerance provide evidence to support a hypothesis that cold water habitats act as a natural barrier to hybridization between WSCT and RT. This information provides insight into the evolutionary history of WSCT and RT and will be useful in assisting conservation efforts aimed at mitigating the wide-spread loss of WSCT to genomic extinction.
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