Assessing the population genetic structure of introduced rainbow trout (Oncorhynchus mykiss) in the Lake Tahoe basin: implications for hybridization potential during the reintroduction of native Lahontan cutthroat trout (O. clarkii henshawi)

Hybridization with introduced or invasive species is a major threat and driver of population declines in native salmonids. The rainbow trout (Oncorhynchus mykiss) is a major introduced and invasive species, often establishing entrenched naturalized populations. Here, we characterize population genetic variation of naturalized rainbow trout in a subset of Lake Tahoe tributaries to assess potential homing of rainbow trout to specific streams for spawning, thereby informing native Lahontan cutthroat trout (Oncorhynchus clarkii henshawi) reintroduction. Diploid, reproductively viable rainbow trout of commonly raised hatchery strains were stocked into Lake Tahoe from the late 1800s until the early 2000s by California and Nevada fish and wildlife agencies. Since 2007, however, triploid individuals comprise the bulk of rainbow trout planted. Despite dispersal from stocking locations across regions, our analyses revealed variation in population differentiation among tributaries, with individuals from spatially proximate streams clustering across multiple population genetic analyses. Although subtle, we detected evidence for genetic differentiation among tributaries from the southern, western, and northern regions, including surprising structure involving a single tributary. These results illustrate the extent of differentiation within and among streams and could inform possibilities for and implications of rainbow trout removal and Lahontan cutthroat trout reintroduction into its historical habitat.