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

Abstract

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 5-year 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 size-classes 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 = 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 = 0.02) than did similarly sized migratory fish (mean S = 0.35; SE = 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.