Graph of the Day: Western U.S. Trout Habitat Loss Projected to 2080s
Projected stream length of suitable habitat for trout under current conditions and climate change scenarios. Whiskers show 90% confidence intervals for projections. Abstract: Broad-scale studies of climate change effects on freshwater species have focused mainly on temperature, ignoring critical drivers such as flow regime and biotic interactions. We use downscaled outputs from general circulation models coupled with a hydrologic model to forecast the effects of altered flows and increased temperatures on four interacting species of trout across the interior western United States (1.01 million km2), based on empirical statistical models built from fish surveys at 9,890 sites. Projections under the 2080s A1B emissions scenario forecast a mean 47% decline in total suitable habitat for all trout, a group of 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 continued negative biotic interactions. Habitat for nonnative brook trout Salvelinus fontinalis and brown trout Salmo trutta is predicted to 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, is projected 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. […] Under the climate projections, both native cutthroat trout and nonnative brook trout showed a strong decline in length of suitable habitat. Cutthroat trout was projected to decline by 28% in the 2040s composite scenario and 58% in the 2080s scenario; brook trout was projected to lose 44% and 77% of its range, respectively, for these scenarios. Rainbow trout was projected to decrease modestly (13%) in length of suitable habitat in 2040s, with a moderate decrease (35%) in the 2080s. Brown trout was projected to decline by 16% in the short term and 48% in the long term. There were very large differences between the MIROC3.2 and the PCM1 model projections of suitable habitat. For example, cutthroat trout was projected to decline by 70% under the 2080s MIROC3.2 scenario but only 33% under the 2080s PCM1 scenario. We projected that the total length of habitat suitable for one or more trout species would decline by ∼47% under the 2080s composite scenario. This was accompanied by a range shift from larger, low-elevation streams to smaller, high-elevation streams, so habitat volume and trout biomass could decline more than is indicated by the change in stream length.
