A genetic assessment of natural barriers for isolating a proposed Greenback Cutthroat Trout reintroduction area

Objective: Native inland trout conservation efforts rely on physical barriers to exclude nonnative salmonids from target habitats. We used genetic techniques to evaluate a series of natural waterfalls for their potential to serve as barriers to prevent nonnative salmonids from entering a proposed reintroduction area for federally threatened Greenback Cutthroat Trout Oncorhynchus virginalis stomias. Methods: Genetic samples were collected from nonnative Brook Trout Salvelinus fontinalis at 11 sampling reaches above and below natural waterfalls (height: ~1–3 m under base flow conditions) along a 33-km segment of Colorado's upper Cache la Poudre River near the outflow of the proposed reintroduction area. To evaluate whether upstream movement of Brook Trout is restricted by any of these waterfalls, we characterized longitudinal trends in genetic diversity along the river corridor and examined patterns of genetic differentiation and population structure in relation to waterfall locations using a panel of microsatellites. Result: We found no evidence that the waterfalls served as complete movement barriers for nonnative Brook Trout based on genetic clustering analyses, estimates of population differentiation, and longitudinal genetic patterns. Our multilocus assessment did not identify alleles restricted to downstream reaches, and the river segment was genetically homogenized. Conclusion: Our evaluation suggests that the existing waterfalls do not fully prevent upstream movement by nonnative Brook Trout, and thus barrier modification would be needed to establish an isolated Greenback Cutthroat Trout population in the proposed wilderness area.