Translocated Populations and Their Role In Lahontan Cutthroat Trout Recovery Strategies: a Tale of Three Watersheds

Lahontan cutthroat trout (Oncorhynchus clarkii henshawi, LCT) is listed under the U.S. Endangered Species Act. This subspecies has been lost from over 90% of its historic range. In some cases, single populations of the native strain are all that remain in a watershed. Beginning in the early 1900s fish from throughout the Lahontan basin were transplanted into out-of-basin locations. As the recovery of listed subspecies of cutthroat trout proceeds transplanted populations may play an important role in the restoration of strains to their native watersheds from both a demographic and genetic perspective. Here I compare genetic variation present in transplanted populations whose founders originated from the Truckee and Carson rivers watersheds in the western basin genetics management unit (GMU) and the Willow-Whitehorse river drainage in the northwestern GMU to the remaining extant native populations in these river systems. Morrison and Bettridge creeks in the Pilot Peak Mountains of Utah have populations of an LCT strain extirpated from the Truckee River watershed in the 1940s. These populations are significantly differentiated from Independence Lake (F_ST = 0.2862, P = 0.003), the single extant native population in the Truckee River basin, which is also differentiated from museum preserved samples of this strain collected prior to extirpation (F_ST = 0.139, P = 0.003). The Independence Lake population has a high level of heterozygosity (H_E = 0.737) but is a very small population (< 200 adults). Although heterozygosities in the Morrison and Bettridge creek populations are lower than Independence Creek they are still relatively high (H_E = 0.548, 0.582 respectively). Four populations from the Carson River drainage found in the Mokulemme and Owens rivers, California, clearly show the effects of small founder size with low levels of average heterozygosity and allelic richness (H_E = 0.244; R_S = 2.38) compared to the three extant in-basin populations (H_E = 0.5158; R_S = 4.233) from which they were also significantly differentiated (G_ST = 0. 0.352, P = 0.002). Fourteen of the 104 alleles identified at the 10 microsatellite loci were unique to the out-of-basin populations. Genetic analysis of seven populations transplanted into the Steens Mountains, Oregon show high levels of heterozygosity (average H_E = 0.535-0.740), allelic richness (R_T per locus 1.20-6.29) (average H_E = 0.497-501; R_T = 1.00-4.92), and unique alleles (N = 193/286) compared to their Willow-Whitehorse river source populations. These examples suggest that out-of-basin populations vary in their usefulness in developing recovery strategies.