Quantification of the Vital Rates, Abundance, and Status of a Critical, Endemic Population of Bonneville Cutthroat Trout PHAEDRA BUDY,* GARY P. THIEDE, AND PETER MCHUGH U.S. Geological Survey, Utah Cooperative Fish and Wildlife Research Unit, Department of Watershed Sciences, Utah State University, Logan, Utah 84322-5210, USA 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 ¼ 6%), and growth rates were also high (up to 0.50 g/d; mean ¼ 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 (k) appeared to be declining overall; however, 95% confidence intervals of k 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. Populations of native cutthroat trout Oncorhynchus clarkii were once distributed from the Columbia River to the Missouri River, across the Great Basin, and in southern parts of the northern Rocky Mountains. These populations have since declined or been extirpated from much of their historical range because of the combined effects of habitat fragmentation and degra- dation, disease, and hybridization and competition with nonnative salmonids (Behnke 1992; Dunham et al. 2002; Quist and Hubert 2004). Our understanding of the relative influence of these different threats in the overall decline, however, is limited by the overall paucity of even the most basic population and demographic data (e.g., Hilderbrand 2002) and in some cases by difficulties associated with studying small, fragmented populations (e.g., Novinger and Rahel 2003). In addition, when one or more of four demographic parameters (recruitment, survival or mortality, immigration, and emigration) is limiting, the cause of an observed change in abundance or distribution can be difficult to identify without long- term and exhaustive population and demographic data (e.g., Gowan and Fausch 1996; Ham and Pearsons 2000; Peterson et al. 2004). Nevertheless, identification and prioritization of conservation and management actions depend on accurate and precise population assessment (e.g., abundance and trend) and an ability to identify limiting factors and life stages (Williams et al. 2001; Morris and Doak 2002). Further, there is a need to proactively evaluate management options for populations that still have an opportunity for conser- vation (Allendorf et al. 1997; Hilderbrand 2002). Despite the importance of accurate and precise population assessment, the estimation of intraannual and interannual population abundance and the quanti- fication of key demographic factors (e.g., survival) are time consuming and often expensive (e.g., Hankin and Reeves 1988; Lebreton et al. 1992; Al-Chockhachy et al., in review). In addition, population assessment and trend analyses can be further limited by natural variability in fish distribution and abundance (Fausch et al. 2002; Young and Guenther-Gloss 2004), spatial and temporal variability in fish behavior (Zabel et al. 2005), and for many trout species, variable and large- scale fish movement (e.g., Hilderbrand and Kershner 2000a; Fausch et al. 2002; Colyer et al. 2005). Differences in fish behavior, whether natural or in response to sample methodology (e.g., Nordwall * Corresponding author: phaedra.budy@usu.edu Received March 14, 2006; accepted September 7, 2006 Published online April 26, 2007 593 North American Journal of Fisheries Management 27:593–604, 2007 Ó Copyright by the American Fisheries Society 2007 DOI: 10.1577/M06-085.1 [Article]