Flow Regime, Temperature, and Biotic Interactions Drive Differential Declines of Trout Species Under Climate Change
Broad-scale studies of climate change effects on freshwaterspecies have focused mainly on temperature, ignoring criticaldrivers such as flow regime and biotic interactions. We usedownscaled outputs from general circulation models coupled with a hydrologic model to forecast the effects of altered flows andincreased temperatures on four interacting species of trout across the interior western United States (1.01 million km2), based onempirical statistical models built from fish surveys at 9,890 sites. Projections under the 2080s A1B emissions scenario forecast amean 47% decline in total suitable habitat for all trout, a groupof fishes of major socioeconomic and ecological significance. We project that native cutthroat trout Oncorhynchus clarkii, already excluded from much of its potential range by nonnative species, will lose a further 58% of habitat due to an increase in temperatures beyond the species’ physiological optima and continuednegative biotic interactions. Habitat for nonnative brook troutSalvelinus fontinalis and brown trout Salmo trutta is predictedto decline by 77% and 48%, respectively, driven by increases in temperature and winter flood frequency caused by warmer, rainier winters. Habitat for rainbow trout, Oncorhynchus mykiss, isprojected to decline the least (35%) because negative temperature effects are partly offset by flow regime shifts that benefit the species. These results illustrate how drivers other than temperature influence species response to climate change. Despite some uncertainty, large declines in trout habitat are likely, but our findings point to opportunities for strategic targeting of mitigation efforts to appropriate stressors and locations.