Quantification of the Vital Rates, Abundance, and Status of a Critical, Endemic Population of Bonneville Cutthroat Trout

Abstract

Most subspecies of cutthroat trout Oncorhynchus clarkii are imperiled or extinct due to the combined effects of habitat degradation and interactions with exotic species. To quantify abundance and vital rates and evaluate trends, we selected a large population of Bonneville cutthroat trout O. clarkii utah from the Logan River of northern Utah, a river characterized by high-quality and connected habitat. Over a 5-year period, we completed a comprehensive population assessment, including depletion-based abundance estimates and a mark–recapture study (1,050 tagged fish) of site fidelity, growth, and survival. Population density exceeded 1,500 cutthroat trout/km at high-elevation sites; this is substantially higher than most other reported densities of inland, stream-type cutthroat trout. Fish demonstrated extremely high rates of site fidelity on average (92%; SE = 6%), and growth rates were also high (up to 0.50 g/d; mean = 0.09 g/d). Cormack–Jolly–Seber survival rates (fish ≥ 100 mm) increased with age-class (group effect) and condition (individual covariate) and ranged from over 64% at high-elevation sites to approximately 30% at lower-elevation sites. Population growth rates (λ) appeared to be declining overall; however, 95% confidence intervals of λ frequently overlapped 1.0, indicating high variability that limited conclusions about future status. Both survival rate and fish density were consistently lower at sites where Bonneville cutthroat trout were sympatric with exotic brown trout Salmo trutta. The continuity, connectedness, and large size of this habitat fragment of the Logan River have clearly contributed to the persistence of this population. Our results provide important conservation and recovery benchmarks for identifying rangewide limiting factors of cutthroat trout. We recommend a precautionary approach to management of this endemic and important population; potential options include habitat protection or restoration and the removal of exotic brown trout.