Colorado State University study: Changing Climate Could Cut Western Trout Habitat in Half


Here’s the release from Colorado State University (Kimberly Sorensen):

A new study shows a changing climate could reduce trout habitat in the Western United States by about 50 percent over the next 70 years, with some trout species experiencing greater declines than others. The results were reported by a team of 11 scientists from Colorado State University, Trout Unlimited, the U.S. Forest Service Rocky Mountain Research Station, the U.S. Geological Survey and the University of Washington Climate Impacts Group.

The study, published this week in the peer-reviewed science journal, “Proceedings of the National Academy of Sciences,” predicts native cutthroat throughout the West could decline by as much as 58 percent and introduced brook trout could decline by as much as 77 percent. Rainbow and brown trout populations, according to the study, would also decline by an estimated 35 percent and 48 percent, respectively. These losses would have major impacts on trout fishing, which generates hundreds of millions of dollars in recreation annually in the United States and is a major factor drawing anglers to Colorado and the West.

The study notes that the decline of cutthroat trout is of particular significance because cutthroats are the only trout native to much of the West and a keystone species in the Rocky Mountain ecosystem.

“The study advances our understanding of climate change impacts by looking beyond temperature increases to the role of flooding and interactions between species,” said Seth Wenger, the paper’s lead author. “The study also is notable in scope, using data from nearly 10,000 sites throughout about 400,000 square miles of the Western United States.”

“This research also builds on 15 years of work with graduate students at CSU to find ways to prevent our native cutthroat trout from going extinct in the face of declining habitat and nonnative trout invasions,” said co-author Kurt Fausch, professor in CSU’s Department of Fish, Wildlife and Conservation Biology and an expert on trout ecology and management in the West. “It’s exciting to see these ideas being used, but the impending loss of trout habitat is both startling and depressing. The West is iconic for trout fishing, but much of this is projected to go away.”

Wenger was quick to point out that, while predictions are indeed dire, there is some hope. By restoring and reconnecting coldwater drainages and by protecting existing healthy habitat largely located on public lands in the West, some of the decline in trout populations might be avoided.

“Trout Unlimited is working to protect remaining strongholds and restore degraded habitat – exactly the kind of things that need to be done to reduce the impact of a changing climate on coldwater fisheries in the West,” Wenger said.

“This report is a wake-up call,” said Chris Wood, president and CEO of Trout Unlimited. “The good news is that we’re already working to protect high-quality trout habitat, such as backcountry roadless areas on national forests. We’re reconnecting tributaries to mainstem rivers, and we’re restoring degraded habitat. It is imperative that we accelerate the scope and the pace of that work if we are to have healthy trout populations and the irreplaceable fishing opportunities they provide through this century.

“However, this study also reinforces the danger in congressional proposals that would remove protection from backcountry roadless areas and cut funding for state and federal natural resource agencies,” Wood said.

Wenger and fellow researchers used an ensemble of climate models to arrive at the study’s findings. Some models predicted more warming than others, but under even the most optimistic model, cutthroat trout populations in the West could decline by 33 percent. Scientists note that most of the 14 unique forms (subspecies) of cutthroat trout are already in trouble—two are extinct, and most of the rest now occupy less than 15 percent of their historic native range with several of these listed under the Endangered Species Act. Declines from a changing climate would impact native cutthroat trout beyond the impacts they’ve already suffered.

The study can be read in its entirety online at the Proceedings of the National Academy of Sciences website:

The research was funded by the National Fish and Wildlife Foundation, the U.S. Geological Survey and the U.S. Forest Service Rocky Mountain Research Station.

Here’s the 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.

More coverage from Steve Bunk writing for From the article:

Today’s paper, in the peer-reviewed Proceedings of the National Academy of Sciences, also predicts that by 2080, rainbow trout, whose native habitat includes Idaho in the Rocky Mountain states, could be reduced by 35 percent. Two introduced trout species in the study will not do well, either: Brook trout habitat could decline by an estimated 77 percent, and brown trout by 48 percent…

They base their conclusions on statistical models they constructed that use data from almost 10,000 fish surveys, which were primarily done in the western parts of Colorado and Wyoming, eastern and northern Idaho, the western half of Montana, and in much of Utah. They write that the real value of their work, rather than predicting the futures of local populations of fish species, is to help identify how species and their habitats will react to various environmental effects of climate change. As an example, they point to a 1994 laboratory study that indicated brook trout in Wyoming fare better in warm-temperature waters than cutthroat. This apparently was contradicted by a 2011 study done in the Columbia River Basin showing cutthroat do better than brookies in warm water. The authors say their study resolves this conflict by showing that cutthroat generally have a wider thermal range than brook trout, although this varies in specific habitats, due to localized genetic adaptations…

The new report takes into account temperature shifts, seasonal flooding, inter-species competition, topography, and various land uses throughout the study area. It shows that brook and brown trout, which spawn in the fall, are hurt by high water flows in winter, which scour the eggs from stream bottoms. Such flows are expected to increase with climate change. The spring-spawning cutthroat suffer a little from such high flows, but rainbow, which also spawn in spring, actually benefit from them, possibly because of genetic adaptation.

More coverage from David O. Williams writing for the Colorado Independent. From the article:

Besides Colorado State University, the study was conducted by Trout Unlimited, the U.S. Forest Service Rocky Mountain Research Station, the U.S. Geological Survey and the University of Washington Climate Impacts Group. In addition to rising water temperatures, the study looks at other factors such as “flow regimes” and “biotic interactions.”[…]

Kurt Fausch, professor in CSU’s Department of Fish, Wildlife and Conservation Biology, said the study builds on 15 years of research by CSU graduate students trying to find ways to prevent the degradation of habitat for native cutthroat trout, considered a keystone species in the Rocky Mountain ecosystem. “It’s exciting to see these ideas being used,” Fausch said, “but the impending loss of trout habitat is both startling and depressing. The West is iconic for trout fishing, but much of this is projected to go away.”

More climate change coverage here.

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