Estimates of Spawning Stream Contributions to an Imperilled Bull Trout Metapopulation Utilizing Otolith Geochemistry and Redd Counts

Migratory fish species have experienced substantial declines throughout aquatic habitats, but actions to manage and conserve migratory species require a better understanding of the relative contributions of local populations to the metapopulation. This demographic information is critical for prioritization of conservation and restoration in the most productive spawning tributaries. Our objective was to estimate the contributions from spawning tributaries to Bull Trout rearing in Swan Lake and relate this information to historic redd counts to explore local population productivity and demographics. We used otolith geochemistry and random forest classification to identify spawning stream origins of Bull Trout collected as bycatch in Swan Lake during Lake Trout suppression efforts. Stream assignments for Bull Trout were compared to historic redd count data obtained from surveys over several decades. The results provided evidence that some spawning tributaries contribute disproportionately to Bull Trout in Swan Lake, and streams within the drainage have differential relative production that was not evident by examining redd count data alone. Understanding the demographic dynamics of Bull Trout among spawning tributaries in the Swan River basin will help prioritize effective mitigation actions to aid recovery of this important metapopulation.