They're not spectacular like marlins, but trout are among the most legendary—think A River Runs Through It—and lucrative fish of mountain rivers and lakes. Across the western United States alone, trout fishing generates hundreds of millions of dollars annually. But trout habitat will likely be cut in half by 2080 due to warming rivers and altered patterns of flooding, according to a large study published today. "It's fairly shocking to us, as biologists," says co-author Kurt Fausch of Colorado State University, Fort Collins.
The study, published online today in the Proceedings of the National Academy of Sciences, is the first to tease apart the probable impacts of climate change on common trout species. This is not a case of winners and losers; the four species examined by the team are all likely to decline to varying degrees. The good news for fans of fly-fishing is that the rainbow trout, beloved for its large size, will suffer the smallest impact. On the other hand, the results will further alarm conservationists concerned about the plight of native cutthroat trout.
By running several climate models and plugging in data on habitat characteristics and fish present at 9890 locations in the Rocky Mountains and Great Basin, the team of scientists created predictions for trout habitat across more than 1 million square kilometers of the western United States. They factored physical aspects of habitat—water temperature, patterns of flooding—as well as competition among species. Here's what the study concludes about the future of trout:
Rainbow trout (Oncorhynchus mykiss): Introduced beginning in the 1800s for fishing, these trout live in larger rivers than do the other species. They typically grow up to 110 centimeters long and are relatively easy to catch.Increases in water temperature will restrict their range, but rainbow trout dodge a bullet. Climate change will mean more frequent and intense winter floods, which can scour away eggs laid in the fall, but rainbow trout spawn in the spring. The size of their habitat is predicted to decline by 35%.
Brown trout (Salmo trutta): Smaller than rainbow trout, they are the most temperature tolerant among the four species and like warmer temperatures. They have the ill fortune to spawn in the fall, so the increased flooding will make many streams inhospitable. Their habitat is predicted to decline by 48%.
Cutthroat trout (Oncorhynchus clarkii): The range of these native fish has already shrunk by more than 85% due to competition from introduced species. Two subspecies have already gone extinct. Warming temperatures and continued competition primarily from rainbow trout are predicted to reduce suitable habitat by a further 58%.
Brook trout (Salvelinus fontinalis): Runts compared with the other species, brook trout are the biggest losers in 2080. Because they spawn in the fall, the increase in winter flooding is expected to reduce their suitable habitat by 77%—a surprising amount given their success at invading new habitat. "I did not expect brown trout to decline so much," says co-author Seth Wenger, a biologist with Trout Unlimited, a conservation organization in Boise.
Frank Rahel, a fish ecologist at the University of Wyoming in Laramie, who was not involved with the study, says the findings help paint the big picture for trout this century. "They're very sobering results that will catch people's attention," he says. Not much can be done to reduce the impact of winter floods, but managers already try to keep streams cooler by planting trees and shrubs.