Threats to Rearing Juvenile Chinook Salmon from Nonnative Smallmouth Bass Inferred from Stable Isotope and Fatty Acid Biomarkers

Abstract

AbstractNonnative Smallmouth BassMicropterus dolomieuare increasingly sympatric with juvenile spring Chinook SalmonOncorhynchus tshawytschain the rivers of western North America. Understanding the potential effects of introduced Smallmouth Bass is essential to efficiently direct salmon management efforts, especially given the potential for upstream range expansion of Smallmouth Bass in response to the climate‐induced warming that many rivers may experience. However, the predatory and competitive threats of Smallmouth Bass to juvenile salmonids remain largely unexamined, particularly in salmon rearing areas. To address this knowledge gap, we collected stable isotope (δ13C and δ15N) and fatty acid data to assess the trophic role of Smallmouth Bass in the upstream reaches of the North Fork John Day River, a tributary of the Columbia River where the upstream invasion extent of Smallmouth Bass co‐occurs with the native predator Northern Pikeminnow and river‐rearing juvenile Chinook Salmon. We found that Smallmouth Bass and Northern Pikeminnow occupy divergent dietary niches, and there was little evidence of a strong predation or competition threat to juvenile Chinook Salmon from either predator. In addition, we found a disproportionate reliance on autochthonous resources across the entire community and low isotopic uniqueness, suggesting potential competition among the broader fish community given the scarcity of resources. Importantly, this study focused on the overlap between the upstream‐most habitats of encroaching Smallmouth Bass and downstream‐most habitats of juvenile Chinook Salmon. Therefore, the relative scarcity of rearing salmon in comparison with other available prey in this zone may limit current predation threats. The results from this study help guide resource‐limited managers that are tasked with conserving and restoring native salmonid populations.