Evaluating the effects of selective passage of migratory Westslope Cutthroat Trout on nonnative admixture

Objective: Hybridization with nonnative Rainbow Trout Oncorhynchus mykiss is a primary threat to the persistence of Westslope Cutthroat Trout O. clarkii lewisi. Managers concerned with conserving migratory populations of Westslope Cutthroat Trout in the presence of Rainbow Trout often face the predicament of tolerating the spread of hybridization, intentionally isolating Westslope Cutthroat Trout populations, or suppressing Rainbow Trout source populations. Methods: Selective passage of migratory Westslope Cutthroat Trout above existing barriers is a management approach with the potential to limit hybridization, while minimizing the population impacts of barriers in populations with preexisting low levels of hybridization. We took a before-after–control-impact approach to evaluate a phenotype-based selective passage management program for migratory Westslope Cutthroat Trout in the Jocko River, Montana. Result: Of the 364 genotyped individuals selectively passed upstream of a barrier, 82% had a proportion of Rainbow Trout admixture < 0.01 and 98% had < 0.10. Over 14 years (2005–2019), there was no significant increase in hybridization at sites (n = 12) upstream of the barrier, but metrics increased within some of our control (no barrier, n = 8) sites. This increase was greatest at a site just downstream from the barrier, suggesting that blocked fish might have spawned opportunistically in this adjacent tributary. Conclusion: We demonstrate that phenotypic traits were useful in implementing a selective Westslope Cutthroat Trout passage program that allowed managers to promote the migratory life history without increasing nonnative admixture in this watershed.