Comparison of fitness-related life history traits of pure westslope cutthroat trout (Oncorhynchus clarkii lewisi) and hybrids with rainbow trout (O. mykiss)

Background/Question/Methods

In Alberta, the current distribution and abundance of pure native westslope cutthroat trout (WCT, Oncorhynchus clarkii lewisi) has been severely reduced compared to the historical distribution. Although recent rates of decline in Alberta are not known, it is certain that the decline of pure WCT continue due to stochastic events such as catastrophic environmental changes (i.e., climate change) and anthropogenic factors such as overexploitation, habitat degradation, and the introductions of non-native fish species. Out of these, hybridization and genetic introgression with introduced salmonids such as rainbow (RT, O. mykiss) and Yellowstone cutthroat (YCT, O. c. bouvieri) trout has been identified to be the utmost threat to the persistence of native WCT in Canada, including Alberta populations. While there have been numerous genetic studies of WCT in Alberta, population structure, growth rates and life history patterns of headwater populations are not well known. We sampled 72 streams in the South Saskatchewan and North Saskatchewan River Systems in the Rocky Mountains in Alberta. A total of 915 trout samples were collected at 61 of these locations and their genetic status was evaluated using 6 microsatellite loci. Out of these, we chose to study three pure WCT (0%H) and three hybridized (WCT x RT) populations’ (30-63%H) fitness-related life history traits (i.e., longevity, growth rates, condition factors, sex ratios, age and size at maturity and fecundity).

Results/Conclusions

Hybridization (WCT x RT) was detected in 35 of 61 populations (57%). The mean proportion of admixture (pRT) differed significantly among the sampled populations and decreased with increasing elevation. Also, the degree of hybridization (%H) varied across sampled creeks (0%-80%). The comparison of fitness-related life history traits showed that pure WCT fish were significantly older, larger and heavier than fish from hybridized (WCT x RT) populations. The von Bertalanffy length and weight growth curves and equations for pure WCT and hybridized (WCT x RT) populations were, however, comparable and didn’t differ significantly. Pure WCT populations attained generally lower condition factors (K) but higher Gonadosomatic Index values than hybrid fish. WCT populations mean fecundity at length and weight were greater than in WCT x RT populations.  Life history and genetic data are critical for understanding the impacts of RT introductions on WCT populations and findings of this study may be used to determine conservation priorities and management actions designed to maintain viable, productive fisheries such as establishing effective population size or reproductive potential.