Assessing the population genetic structure of introduced rainbow trout (Oncorhynchus mykiss) in the Lake Tahoe Basin: a case for understanding 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, RBT) represents one of the most successful introduced species, often establishing naturalized populations that become entrenched. The cutthroat trout (Oncorhynchus clarkii, CT), a close congener, is particularly susceptible to RBT introductions, resulting in competition and hybridization. Introgressive hybridization has led to rangewide population declines and loss of CT genetic variation. The Lake Tahoe basin is part of the historic range of the Lahontan CT (O. c. henshawi, LCT). LCT were extirpated from the basin by the 1940s due to overfishing and introduction of non-native salmonids. Today, the basin supports robust naturalized populations of RBT, posing a threat to LCT recovery and reintroduction. Here, we characterize population genetic variation of naturalized RBT in a subset of tributaries to assess potential homing of RBT to specific streams for spawning, thereby informing conservation and management as LCT reintroduction proceeds. Diploid reproductively viable RBT were stocked annually into Lake Tahoe until the mid-2000s by California and Nevada fish and wildlife agencies, planting the same commonly raised hatchery strains over time. Since 2007, however, California no longer plants RBT, and triploid RBT comprise the bulk of RBT planted by Nevada. Despite dispersal from stocking locations across all regions, our analyses revealed variation in population differentiation among tributaries, with individuals from spatially proximate populations grouping more closely than others 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 in a single tributary (McKinney Creek). These results illustrate the extent of differentiation within and among streams and could inform possibilities for and implications of RBT removal and LCT reintroduction.