Climate change could reduce the habitat of cutthroat trout, a keystone species already under stress in the West, by as much as 58 percent over coming decades, according to a study published today. Meanwhile, long-term efforts have begun in Colorado to restore selected cutthroat habitats by eliminating other trout.
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.
The researchers are from Trout Unlimited, the U.S. Forest Service, the U.S. Geological Survey, Colorado State University, and the University of Washington.
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 study advances our understanding of climate change impacts by looking beyond temperature increases to the role of flooding and interactions between species,” the paper’s lead author, Dr. Seth Wenger of Trout Unlimited, notes in a press release.
To date, temperature has been the focus of most broad-scale analyses of climate effects on fish. 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.
“We found that the predictive accuracy of our models was only moderate for most species,” the scientists warn.
They got marked differences in results, depending on whether the general circulation model they incorporated into their statistical work was set for high warming by 2100, low warming, or was a composite of 10 such models.
The report indicates that the biggest single factor in cutthroat populations, next to water temperature, is the presence or absence of other trout species. The other three species in the study showed no ill effects from sharing habitat.
Rather than examining the nature of these interactions between species, the study models the effects of them on cutthroat.
Analyzing the question of how much territory cutthroat would occupy today if no other trout species existed in their waters, the team’s statistical model indicates that the total suitable stream length would rise by one-third, from the current 159,000 km to 239,000 km.
Projecting into 2080, the modeling shows that total stream length suitable for cutthroat is expected to be 68,000 km, but if non-native trout species were absent, that figure would rise 26 percent, to 92,000 km.
“Little can be done to influence the predicted increase in winter high flows,” the authors observe, but they add that often this is not the case for rising stream temperatures. “Future increases can be offset by restoration measures such as maintenance of stream flows and reforestation.”
Another restoration measure, now being pursued by the U.S. Forest Service and Colorado Parks and Wildlife, involves wiping out the cutthroat’s competition.
The two agencies have embarked on a multi-year plan to restore historical cutthroat habitat in more than 16 miles of high creek on the Uncompahgre Plateau by poisoning fish and restocking with cutthroats.
A Forest Service Fisheries description of the project says two or three rounds of summer poisoning are scheduled to begin in 2013, using “non-selective” chemical toxicants, such as rotenone and antimycin.
Stocking, which will begin in 2016, is expected to continue for several years.
A fish barrier will be built on Big Dominguez Creek to prevent non-native fish from spreading upstream into the chemically treated waters of LaFair Creek.
Matt Dare, an aquatic biologist for the Grand Mesa, Uncompahgre and Gunnison National Forests, told the Daily Sentinel that although these forests contain “thousands of miles” of potential cutthroat habitat, only about 150 miles are currently occupied by the native fish.
The article indicates that similar Colorado cutthroat restoration projects are underway at Woods Lake near Telluride and Hermosa Creek near Durango.