Resource use by threatened, native Bull Trout ( <i>Salvelinus confluentus</i> ) in the presence of two non‐native salmonids

Introduction of non-native species contributes to the global decline of freshwater fishes by disrupting the diet and resources of native species through competition. Despite the potential for non-native trout (Family: Salmonidae) to compete with native trout, non-native trout species have been widely introduced worldwide to support recreational fishing. In this study, we examined resource use by native Bull Trout (Salvelinus confluentus), a Threatened species, and non-native Brook Trout (Salvelinus fontinalis) and Brown Trout (Salmo trutta) in headwater streams in Alberta, Canada. Our objective was to investigate potential for competition and its implications for Bull Trout populations. To assess changes in diet, we used stable isotope analysis. Brook Trout and Brown Trout had higher trophic positions (3.49 ± 0.02 and 3.53 ± 0.02) and greater proportions of terrestrial resources in their diets (0.53 ± 0.02, 0.54 ± 0.03) than Bull Trout (TP = 3.38 ± 0.02, proportion terrestrial = 0.36 ± 0.02), suggesting the non-native species may outcompete Bull Trout for preferred resources and larger prey. Moreover, a relatively wide isotopic niche of Bull Trout led to a lack of niche partitioning and substantial overlap in niche utilization between sympatric Brook Trout and Bull Trout, supporting the potential for future niche displacement, particularly in scenarios where resources are scarce. These findings underscore the importance of monitoring and managing the invasion of non-native salmonids in streams where native salmonid populations exist, and the challenges in understanding multi-species interactions in dynamic habitats. It highlights the need for effective fisheries management and conservation efforts in regions where native freshwater fishes are facing population declines.