Associated Press/Garrett Cheen/The Livingston Enterprise

As difficult as it is to predict precisely how the planet will warm over the next century or so, it is even harder to refine predictions of how those changes will affect specific species. That’s because warmer temperatures alone are not the only driver of what happens to, say, four varieties of trout.

But a group of scientists set out to do just that. In a study published online Monday in the Proceedings of the National Academies of Science, they estimated not just what warmer rivers will mean for the life cycle of these western trout, but also how more winter rain or earlier snow melts — and the resulting higher flows — could affect eggs laid in the fall.

It also describes how competitors, newly empowered by temperatures and stream flows that favor them, could muscle traditional species out of the way.

The news is not good. As the authors, from the conservation group Trout Unlimited, the Forest Service, the University of Washington, Colorado State University and the United States Geological Survey write, “Our models forecast significant declines in trout habitat across the interior western United States in the 21st century.” They say that prediction applies to much of the rest of the temperate world because three of the species studied — rainbow, brown and brook trout — “are common on multiple continents.”



“The decline will have significant socioeconomic consequences as recreational trout fisheries are valued in the hundreds of millions of dollars in the United States alone,” the study said.

In a moderate plan for a warming climate drawn up by the researchers, brook trout would lose more than three-quarters of their range in the West in the next 75 years. Brown and cutthroat trout would lose about 50 percent of theirs. Rainbow trout would fare the best, losing a little more than a third of the miles of stream in which they can thrive.

What to do? The authors suggest that human cannot intervene to protect fall-spawning species from newly high winter flows. But because river temperatures are often affected by human activities, the restoration of cooler flows to warming rivers might be achievable for wildlife and forest managers, they add.

Finally, the authors caution that the reliability of their computer modeling is only moderate. For one thing, nonnative species that could edge out the trout have been introduced to rivers and streams in an inconsistent way over the years, they note. Then there is the difficulty of analyzing the impact of fires and debris on aquatic life.

The scientists suggest that their results are best suited “for understanding the different trajectories and relative climate sensitivities of different trout species, as well as the habitats that are most sensitive to change.”