Trout habitat likely will be slashed in half due to climate change by the year 2080, according to a study published Monday, with native cutthroat trout expected to see the most severe decline.
Researcher Seth Wenger, the paper’s lead author, said cutthroat could see a 58 percent decline in suitable habitat due to warming rivers, altered streamflows and competition from nonnative species.
The study, published in the science journal Proceedings of the National Academy of Sciences, also predicts a decline in introduced brook trout populations by as much as 77 percent, while rainbow and brown trout populations could also decline by an estimated 35 percent and 48 percent, respectively.
The decline of cutthroat trout is of particular significance, Wenger said, because it is the only trout native to much of the West, and is a keystone species in the Rocky Mountain ecosystem. The westslope cutthroat trout — a subspecies included in the study — is Montana’s state fish and the focus of numerous conservation efforts.
The 11 researchers who contributed to the study are from Trout Unlimited, the U.S. Forest Service, the U.S. Geological Survey, Colorado State University, and the University of Washington.
Wenger said the paper was based on data collected from nearly 10,000 fish surveys conducted in the western half of Montana, as well as the western parts of Colorado and Wyoming, eastern and northern Idaho, and Utah. The data was used to build statistical models that forecast the decline in total suitable habitat.
The range of cutthroat habitat has already shrunk by more than 85 percent due to competition from introduced species like rainbow trout and brook trout, and two subspecies have already gone extinct.
Without competition from nonnative species, the chances of cutthroat adapting to a warming environment would dramatically improve, and their territory could increase by a full third, the modeling shows.
While most broad-scale studies on climate change and its effects on freshwater species have focused on temperature change, Wenger said the recent report looks at critical drivers like streamflow and interactions between trout species.
“This gives us a better understanding of all the variables and what is projected to happen under climate change,” he said. “We tried to go beyond what past research has done, which is focus on temperature, and take into account other complex changes, and in particular the role of increased streamflows.”
Besides competition from other species, the altered streamflows will have the most dramatic effect on spawning cutthroat.
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As warming continues and winter precipitation increasingly falls as rain instead of snow, heavier streamflows will scour gravel streambeds, where species like bull trout spawn in the fall, reducing the survival and growth of trout eggs and redds. Conversely, the diminished snowpack will reduce spring flows and lead to increased water temperatures when species like cutthroat spawn.
“It’s not just that the air is warming and the water is warming, but the snowmelt hydrology is changing and leading to lower base flows in the summer months,” said Clint Muhlfeld, an aquatic ecologist at the U.S. Geological Society’s Glacier Park field office. “If you have lower flows for a sustained period of time, you are going to have increased warming.”
Muhlfeld has focused his research on the effects of climate change on aquatic species in the Rocky Mountain Front. He praised the recent study for its broad assessment on the effects of climate change, but also emphasized the ecological variability of ecosystems in the western United States, and especially the Rocky Mountain Front region.
“The Crown of the Continent is sort of the best of the last,” Muhlfeld said. “We are one of the last strongholds for these species. We believe that these populations and the species up here have a much better chance of adapting to climate change. And it’s my hope as a researcher that we can give managers the proper tools to predict what changes are going to occur, and what habitats and species are the most susceptible to warming, so that they can inform their policies.”
Bruce Rieman, a retired fisheries biologist with the U.S. Forest Service Rocky Mountain Research Station in Seeley Lake and a contributor to the recent study, echoed Muhlfeld, saying northern Montana’s regional differences may help buffer the impact.
“This part of the world is likely to fare a little better than other places, because we are further north at a higher elevation,” he said. “We may not suffer nearly the declines that are projected elsewhere.”
Still, even the most optimistic models show cutthroat trout populations in the West declining by 33 percent, while some subspecies are already functionally extinct.
“The overall picture is fairly negative for trout,” Wenger.
“The sort-of good news is that there is still some uncertainty of how much warming we will see. But even the best-case scenario isn’t great.”