Long-term genetic outcomes of a mixed-source introduction of Westslope Cutthroat Trout

Introductions are an increasingly important conservation tool, but few long-term studies have examined how source population selection influences outcomes. We examined the long-term (approximately 4 generations) genetic outcomes of a large-scale, mixed-source introduction of Westslope Cutthroat Trout in Cherry Creek, Montana. We tested whether source population contribution deviated from neutral expectations (a proxy for fitness), and whether this correlated with source population characteristics. The two donor stocks with the lowest genetic variation had reduced contributions to the final population, whereas those with the highest genetic variation exceeded neutral expectations by 152%–198%. Source contribution was strongly associated with genetic variation ( R 2 = 0.6). Nearly every fish in the final study year had mixed ancestry, and mean observed heterozygosity reached its highest recorded level, surpassing any one source. Our results suggest that using multiple sources and sources with higher variation may improve the outcomes of conservation introductions across multiple generations.