New Insights into the Ecology of Adfluvial Bull Trout and the Population Response to the Endangered Species Act in the North Fork Lewis River, Washington

Abstract

Like many other salmonids, Bull Trout Salvelinus confluentus migratory life history expressions are becoming increasingly rare. A critical step in effectively refining management and conservation strategies is a robust assessment of the effectiveness of such strategies and key biological information used in monitoring and recovery planning. To address this need, we integrated long-term monitoring data to evaluate how the demographics (abundance) and vital rates (survival) of a Bull Trout population shifted in response to their listing under the Endangered Species Act (1998). In addition, we employed mark–recapture data to investigate Bull Trout population dynamics and assessed the life history characteristics (aging, growth, spawning migrations, and iteroparity) to improve our understanding of Bull Trout ecology and diversity. Our abundance and survival data, which extends to the early 1990s, illustrated positive responses in survival and abundance following the protection of Bull Trout under the Endangered Species Act. Over this period, we found high interannual variability in both survival and abundance, and adult survival (average = 0.45; SE = 0.04) was surprisingly lower than that of subadult individuals (average = 0.66; SE = 0.04), suggesting limitations at this important life stage. Our mark–recapture data also suggested that the attributes driving the Bull Trout population trend (i.e., recruitment to the adult stage or adult survival) has varied through time, with declining trends in the relative contribution of recruitment. Bull Trout spawning migrations varied with body size, and a considerable portion of smaller adults (<650 mm) did not spawn each year. Additionally, most spawning individuals made only one spawning migration, while <13% made three or more spawning migrations. Our results provide important insights into the effectiveness of the Endangered Species Act listing and new information regarding the life history patterns of adfluvial Bull Trout and can serve as a template to consider factors potentially limiting this and other native salmonid populations.