Climate-driven shifts in stream temperature and discharge favor nonnative bass in a salmon-bearing watershed

Objective The implications of climate change are unfolding in riverine systems, carrying consequences for fish populations and further challenging fisheries management. In the Pacific Northwest, United States, significant expansions of nonnative bass (family Centrarchidae) habitat are projected, fueling concern for the recovery of imperiled Pacific salmon Oncorhynchus spp. due to known predation impacts. Whether these projections are realized can be informed by understanding how contemporary nonnative bass populations are responding to environmental change. This study integrates multiple data sources to examine recent patterns in available habitat and juvenile growth of Smallmouth Bass Micropterus dolomieu and the relative abundance of dominant fish predators in the main-stem Chehalis River (Washington), a large free-flowing, salmon-bearing watershed. Methods To quantify changes in thermally suitable habitat for Smallmouth Bass, we reconstructed a 35-year summer stream temperature time series (1990–2025) and estimated the total amount of habitat exceeding a known occurrence threshold. We then compared juvenile Smallmouth Bass growth among recent brood years and evaluated its relationship with thermal and hydrological conditions. Finally, we compared contemporary (2023 and 2025) and recent past (2007) fish predator catch per unit effort (CPUE) to characterize recent patterns in fish predator assemblages. Results Over the past 35 years, thermally suitable Smallmouth Bass habitat and August stream temperature increased at a rate of 0.8% (∼28.4 km) per decade and 0.23°C per decade, respectively. Discharge was below average over the past decade, and juvenile Smallmouth Bass growth was generally higher during the extreme drought and heatwave years of 2015 and 2021, consistent with the observed negative relationship between growth and discharge. Compared with the 2007 baseline survey, Smallmouth Bass CPUE and Rock Bass Ambloplites rupestris CPUE were significantly higher in contemporary surveys, whereas the CPUE of native Northern Pikeminnow Ptychocheilus oregonensis was significantly lower. Conclusions Recent stream temperature warming and persistently low discharge in the Chehalis River have created favorable conditions for Smallmouth Bass. Combined with higher contemporary relative abundance of both Smallmouth Bass and Rock Bass, these findings suggest that recent environmental changes are broadly favoring nonnative warmwater predators, raising concerns about increased predation pressure on Pacific salmon. Because similar hydroclimatic trends and stream temperature warming are occurring across many western U.S. watersheds, these dynamics are likely more widespread, increasing the geographic scope and management complexity surrounding nonnative bass.