Demographic Characteristics, Population Structure, and Vital Rates of a Fluvial Population of Bull Trout in Oregon

—Identification of the factors limiting inland salmonid populations, such as those of the threatened bull trout Salvelinus confluentus in the Pacific Northwest, can be particularly challenging due to substantial gaps in our understanding of population demographics, population structure in the presence of multiple life history forms, and vital rates. We implemented a large-scale mark–recapture program over a 5year period using an innovative combination of active and passive techniques to estimate (1) bull trout age and growth by size-class, (2) the proportion of the population exhibiting resident and migratory behavior, and (3) survival rates (S) for different life stages and life history forms (resident and migratory). Our results suggest that bull trout reached sexual maturity at a relatively small size (200 mm) and young age (3–4 years) and that large individuals (.600 mm) can reach ages greater than 12 years in this fluvial population. Using active and passive mark–recapture methods, we found that large bull trout (.420 mm) were predominantly migratory in behavior (72% were migratory) and that there was considerable variability among other sizeclasses in the proportion exhibiting migratory behavior. Survival rate varied significantly across size-classes and study years. Juvenile bull trout (120–170 mm) exhibited the lowest annual S on average (0.09) and the highest interannual variability (coefficient of variation 1⁄4 0.60) in S among size-classes. Fish exhibiting migratory life history patterns generally had higher S than did resident fish; small, juvenile residents had a significantly mean S (0.15; SE 1⁄4 0.02) than did similarly sized migratory fish (mean S 1⁄4 0.35; SE 1⁄4 0.04). Collectively, our results highlight important differences across life history forms within and across populations; these factors must be considered when designing future recovery and management strategies for any single bull trout recovery unit or across larger geographic areas. The design of sound recovery and management strategies for fish populations requires an understanding of life stages that limit overall population growth and persistence (Wilson 2003; Legault 2005). Identification of limiting life stages often involves the use of population models (Stearns 1992), which require explicit demographic and vital rate information. However, obtaining this information can be temporarily and monetarily challenging and extremely difficult when populations exhibit multiple life history forms, low abundance, and high variability in demographic processes (Al-Chokhachy 2006; Homel and Budy 2008). Nevertheless, this information is necessary for providing a framework to assess the relative effects of various management options, such as harvest practices (e.g., Crowder et al. 1994), restoration efforts (e.g., Hilderbrand 2003), and management scenarios (e.g., Marschall and Crowder 1996). A sound understanding of population dynamics, demographics, and vital rates is critical to planning effective conservation strategies for bull trout Salvelinus confluentus, a species of char that is native to the Pacific Northwest and Canada and that has been listed as threatened under the Endangered Species Act in the United States since 1998 and as a species of special concern in Canada since 1995. Across their native range, bull trout have exhibited substantial declines in population abundance and distribution as a result of habitat degradation and fragmentation (Fraley and Shepard 1989; Rieman and McIntyre 1995; Ripley et al. 2005) and the introduction of nonnative species (Leary et al. 1993). Bull trout are known to exhibit multiple life history forms including anadromous, fluvial, and adfluvial; multiple forms can coexist within a single population (Rieman and McIntyre 1993; Nelson et al. 2002; Homel and Budy 2008). As in many other salmonid populations (e.g., Bonneville cutthroat trout Oncorhynchus clarkii utah; Colyer et al. 2005), the migratory component of many bull trout populations has declined significantly (Nelson et al. 2002). As a result, bull trout exist only as subpopulations across the range of their former distribution (Rieman et al. 1997). Bull trout are also known to be generally associated with complex habitats (Muhlfeld and Marotz 2005; Al-Chokhachy and Budy 2007) and to occur in naturally low densities (Rieman and McIntyre 1993). These attributes, in conjunction with the diverse life history strategies, can result in * Corresponding author: [email protected] Received November 20, 2007; accepted May 26, 2008 Published online December 4, 2008 1709 Transactions of the American Fisheries Society 137:1709–1722, 2008 Copyright by the American Fisheries Society 2008 DOI: 10.1577/T07-247.1 [Article]