Abstract
Bull Trout Salvelinus confluentus is a federally threatened species in the conterminous United States. Although some populations are stable or increasing, Bull Trout in the United States Fish and Wildlife Service's designated Coeur d'Alene Core Area in Idaho have experienced substantial declines in abundance. Today, the remaining extant population in the Coeur d'Alene Core Area returns to the headwaters of the St. Joe River to spawn. The population has been monitored annually since 1992 using spawning ground surveys, but little is known about early life stages in the system. The objective of our research was to evaluate the distribution and abundance, age and size structure, habitat associations, and outmigration characteristics of juvenile Bull Trout in the upper St. Joe River basin. In 2022–2023, we sampled 200 stream reaches on the mainstem St. Joe River and four tributaries (Heller, Medicine, Sherlock, and Wisdom creeks). We sampled 1,529 Bull Trout varying in length from 29−257 mm in total length (TL; mean ± SD; 108 ± 44 mm). Population estimates suggested there were 1,841 (95% CI = 1,188−2,494) juvenile Bull Trout in the study area in 2022 and 2,388 (1,646−3,130) in 2023. Regression models indicated that abundance was positively related to canopy cover, amount of large substrate, amount of large woody debris, and amount of gravel, and negatively related to water temperature. We tagged 1,142 fish with passive integrated transponders (PIT) and detected 163 (14%) of these fish using a stationary tag array that was operational during June or July through October. Peak autumn outmigration occurred in October of both sampling years. Of the fish that were detected moving, age varied from 1−4 years, but age‐1 and age‐2 fish were more commonly detected moving downstream compared to other age classes. This study provides important information on the ecology of juvenile Bull Trout that can be used to guide conservation and recovery efforts in montane ecosystems. Furthermore, juvenile Bull Trout in the upper St. Joe River basin were present at similar densities and experienced growth rates similar to more robust adfluvial populations (e.g., Lake Pend Oreille, Idaho), thereby indicating that factors contributing to the low abundance of adults are not likely occurring in the headwaters of the St. Joe River. Thus, conservation efforts may be more successful if they are focused on the migration corridor and Coeur d'Alene Lake.