Flow Regime, Biotic Interactions, and Temperature Determine Differential Responses of Four Trout Species to Projected Climate Change

Broad-scale studies of climate change effects on freshwater species to date have focused mainly on temperature, ignoring critical drivers such as flow regime and biotic interactions. We used downscaled outputs from general circulation models coupled with a hydrologic model to forecast the effects of altered flows, biotic interactions, and increased temperatures on four species of trout across the interior western USA (1.01 million km²) using empirical statistical models built from fish surveys at 9890 sites. Projections under the 2080s A1B emissions scenario predict a mean 47% decline in total suitable habitat for trout, a globally-distributed taxonomic group of major socioeconomic and ecological significance. We project that native cutthroat trout Oncorhynchus clarkii, already excluded from 33% of its potential range by nonnative species, will lose a further 58% of suitable habitat, mainly due to an increase in temperatures to levels outside the species’ physiological optimum, as well as negative biotic interactions with other trout species. Habitat for nonnative brook trout Salvelinus fontinalis is predicted to decline 77%, driven by temperature increases and altered flow regimes—in particular, an increase in the frequency of winter high flows caused by warmer, rainier winters. Nonnative brown trout Salmo trutta habitat is predicted to decline by 48% due to increased winter high flows. Habitat for rainbow trout O. mykiss (native to 6% of the region but introduced broadly), is projected to undergo a smaller 35% net decline due to temperature increases, although flow regime shifts may benefit the species. Effective management of trout populations under climate change must consider drivers other than temperature and take into account differences in climate responses among species.

Additional coauthors: Michael Young, USFS Rocky Mountain Research Station;  Marketa Elsner, University of Washington Climate Impacts Group; Bruce Rieman, USFS Rocky Mountain Research Station (retired); Alan Hamlet, University of Washington Climate Impacts Group; Jack Williams, Trout Unlimited.