The Evolutionarily Significant Unit Concept and the Role of Translocated Populations in Preserving the Genetic Legacy of Lahontan Cutthroat Trout

The Lahontan cutthroat trout Oncorhynchus clarkii henshawi, a listed subspecies of cutthroat trout O. clarkii, has been extirpated from over 90% of its historic waters. The 1995 U. S. Fish and Wildlife Service Recovery Plan identified three evolutionarily significant units, or distinct population segments (DPSs), of Lahontan cutthroat trout based upon morphological, genetic, and ecological data. Teams composed of federal and state agency and university biologists were organized to formulate recovery strategies that were specific to each DPS. In this context, fish native to each DPS are considered exclusively for recovery activities. Lahontan cutthroat trout populations in the Willow-Whitehorse drainage of the Coyote Lake basin, Oregon, were initially identified as part of the Northwestern DPS, but recent genetic analyses suggest that these populations may warrant a fourth DPS designation. The Willow-Whitehorse Lahontan cutthroat trout are found in seven within-basin streams and have been translocated into multiple out-of-basin streams. Genetic analysis of seven populations translocated into out-of-basin streams of the Steens Mountain (Alvord Lake basin), Oregon, in the 1970s and 1980s support inclusion of these streams in any DPS-based recovery strategy for the Willow-Whitehorse Lahontan cutthroat trout. Overall gene diversity per locus was similar among translocated and source populations (Steens Mountain, 0.096-0.908; Willow-Whitehorse, 0.039-0.903). However, 135 of 300 alleles identified at 20 microsatellite loci were found in the Steens Mountain populations but not in the contemporary Willow-Whitehorse populations. The Steens Mountain populations are found in small habitats subject to the effects of genetic drift. Lahontan cutthroat trout from these out-of-basin streams should be included in conservation plans.