Interspecific hybridization in a large‐river population of Yellowstone Cutthroat Trout: A 20‐year programmatic evaluation

ObjectiveHybridization between native and nonnative fishes represents a global threat to biodiversity. Understanding how hybridization changes in response to management actions is critical to evaluating the efficacy of conservation efforts.MethodsWe quantified changes in levels of hybridization between Yellowstone Cutthroat Trout Oncorhynchus virginalis bouvieri and Rainbow Trout Oncorhynchus mykiss in the South Fork Snake River watershed, where a multipronged approach has been implemented to protect the evolutionary distinctiveness of one of the last remaining large‐river populations of Yellowstone Cutthroat Trout.ResultOver a 20‐year period, we observed an increase in the number of sample reaches without hybrids in the South Fork Snake River watershed; however, contrasting patterns were noted in main‐stem and tributary reaches. Through time, hybrid abundance increased at main‐stem reaches of the South Fork Snake River below Palisades Dam but decreased in tributaries. Efforts to reduce hybridization in spawning tributaries, including both suppression and selective passage weirs, were effective at preventing the expansion of hybridization in resident and migratory populations. Multimodel inference was used to understand factors affecting levels of hybridization, and year, sampling reach, and the interaction thereof was identified as the best‐fit model but explained only a small percentage of the overall variation, suggesting that other factors not captured in our model were driving patterns in hybridization.ConclusionChanges in hybridization in the South Fork Snake River watershed are likely the result of multiple processes, namely management actions to reduce Rainbow Trout and hybrids in tributaries, as well as demographic changes in Rainbow Trout in the main‐stem river below Palisades Dam. Our results suggest that Yellowstone Cutthroat Trout populations in the South Fork Snake River watershed have not experienced widespread interspecific hybridization with Rainbow Trout but that proactive management will be necessary to ensure long‐term conservation.