Abstract
Bull Trout (BLT) Salvelinus confluentus was listed as threatened in the conterminous United States in 1998 under the Endangered Species Act. Although some BLT populations in Idaho are stable or increasing, BLT in the U.S. Fish and Wildlife Service’s designated Coeur d’Alene Core Area (CDACA) have experienced substantial declines in abundance. Today, the remaining extant population of BLT in the CDACA return 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 the early life stages of BLT in the system. An understanding of juvenile BLT population demographics and dynamics is a crucial first step towards identifying factors limiting BLT abundance in the CDACA. Therefore, the objectives of my research were to evaluate the distribution and abundance, age and size structure, habitat associations, and outmigration characteristics of juvenile BLT in the upper St. Joe River basin, Idaho. In 2022–2023, I sampled 200 stream reaches on the mainstem St. Joe River and four tributaries (Heller, Medicine, Sherlock, and Wisdom creeks). In total, 1,529 BLT were sampled varying in length from 29 mm to 257 mm (mean ± SD; 108 ± 44 mm). For all BLT greater than 70 mm, scales were removed for ageing, a small portion of the anal fin was removed for genetic analysis, and a passive integrated transponder (PIT) tag was injected into the body cavity. Fish movement was tracked throughout the basin using five stationary PIT tag arrays. Additionally, habitat surveys were conducted at a subset of study sites (n = 62) to evaluate dominant habitat associations. Population estimates suggested there were 1,841 (95% CI; 1,187.8−2,494.2) juvenile BLT in the study area in 2022 and 2,388 (1,645.8−3,130.2) in 2023. Regression models indicated that BLT abundance was positively related to canopy cover, amount of large substrate, amount of large woody debris, and the amount of gravel, and negatively related to temperature. Bull Trout that received PIT tags varied in length from 71 mm to 257 mm (mean length ± SD; 125 ± 39 mm) in the upper St. Joe River basin (USJRB). Of the 1,142 BLT PIT tagged, 163 (14 %) were detected by a stationary PIT tag array during June through October. Mean length of all BLT detected on a stationary PIT tag array was 15% greater than the mean length of all BLT PIT tagged. Of the BLT that were detected moving, age varied from 1−4 years, but age-1 and age-2 BLT were more commonly detected moving downstream compared to other age classes. This study provides crucial information on the ecology of juvenile BLT in montane ecosystems that can be used to guide conservation and recovery efforts in USJRB. Furthermore, the observation that the densities and growth of juvenile BLT in the USJRB are similar to densities observed in natal streams used by more robust adfluvial BLT populations (e.g., Lake Pend Oreille) indicate that factors contributing to the low abundance of adults likely are not occurring in the headwaters of the St. Joe River. Thus, further conservation efforts may be more successful if they are focused on the migration corridor and(or) Coeur d’Alene Lake.