EVALUATING THE MANAGEMENT AND CONSEQUENCES OF HYBRIDIZATION BETWEEN NONNATIVE RAINBOW TROUT AND NATIVE WESTSLOPE CUTTHROAT TROUT

The introduction of nonnative fish is a major driver in the decline of native fish species. Nonnative rainbow trout (Oncorhynchus mykiss; RBT) introduced into the native range of westslope cutthroat trout (O. clarkii lewisi; WCT) have led to the introgressive hybridization between these two species. This widespread hybridization is a primary threat to the long-term persistence of WCT as it can cause population-level genomic extinction. Since there are no set management solutions for hybridization, there is a need to evaluate the different conservation approaches to ensure the persistence of WCT populations. Additionally, beyond propagule pressure, the array of drivers that form hybridization landscape patterns are equivocal. This study focused on evaluating management actions and furthering our understanding of the potential mechanisms providing resistance to the spread of hybridization. We conducted a Before-After- Control-Impact study to evaluate the accuracy of selective passage of phenotypic WCT above barriers and the resulting impact on hybridization in the Jocko River Watershed, Montana. Our results showed phenotypic-based passage was generally successful; of the fish passed above the barrier, 82% had a proportion of RBT admixture \u3c 0.01. We saw no significant increase in hybridization metrics in the above barrier populations over 9-14 years, while populations below the barrier had a significant increase in RBT admixture. Second, we validated the use of otolith microstructure to estimate hatch date in WCT with hatchery origin WCT. We than evaluated the effect of RBT admixture on age-0 Oncorhynchus hatch date and growth in the Rock Creek and Rattlesnake Creek Watersheds. Within sites, there was high variation in hatch date and individual growth rates. In the two streams where WCT were present, they had a significantly higher growth rates than hybrids. Our findings show promise for using barriers to manage the spread of RBT hybridization while maintaining the migratory WCT life history. We add support to previous research that found selection against RBT alleles is occurring at the early life stage, which provides valuable information on the potential mechanisms limiting the spread of RBT hybridization