Fisheries scientists are warning that prolonged drought, reduced water flows and elevated temperatures in cold-water streams across the West are hallmarks of a "new normal" that could push already stressed fish populations closer to a permanent decline.

Those conclusions, based on an analysis of five river basins by the Forest Service, U.S. Geological Survey and Colorado State University, help solidify years of anecdotal evidence collected by scientists and sportsmen suggesting climate change is a more than a casual contributor to changing stream conditions and fisheries' health.

In fact, many bioclimate models predict that large reductions in native trout populations will occur across the Rocky Mountains during the 21st century, the study states. But the models lack details about how and where such changes will occur.

Yet even with data limitations, monitoring records from case histories of river basins in northwest Montana, central Idaho, the Greater Yellowstone Ecosystem, western Wyoming and southern Colorado "show trends in temperature and stream flow that suggest trout habitats have already been altered by climate change during the last 50 years."

"Unfortunately, similar long-term records for trout populations are lacking, so scientists are unable to confirm simultaneous changes in trout populations," Forest Service biologist Daniel Isaak of the agency's Rocky Mountain Research Station said in a statement.

For some fish species, scientists believe climate change has triggered wholesale migrations to more hospitable environments, often to colder water at higher elevations, while others have been able to adapt to generally warmer and shallower habitat conditions. The most sensitive species, however, will almost certainly experience population declines as climate stressors ultimately overcome their ability to adapt.

"Despite the best intentions, we will not be able to preserve all populations of native trout in the Rocky Mountains this century," the research paper published in the December issue of the journal Fisheries states. "However, it should soon be possible to have the tools and information to know when and where resource commitments are best made under a given set of assumptions about future climate change."

Salmon and char also face stress

Such information should help fisheries managers reach new conclusions about how and where to invest millions of dollars in public and private money toward preserving the best remaining habitat for species such as trout, salmon and char. But such decisions will not be easy and will require a broad shift in thinking for scientists and managers alike.

"A willingness to accept and manage in concert with many of these changes will require changing mind-sets from last century's paradigm of dynamic equilibrium to one of dynamic disequilibrium for the 21st century," the authors say. "Under the new paradigm, stream habitats will become more variable, undergo gradual shifts through time, and sometimes decline. Many populations and species will retain enough flexibility to adapt and track their habitats, but others are likely to be overwhelmed by future changes."

William Geer, manager of climate change initiatives for the nonprofit Theodore Roosevelt Conservation Partnership, said the Forest Service's findings are consistent with what he has observed in his evaluations of freshwater fish habitat across five Western states. He noted that while some adaptation efforts -- on the part of both fish and humans -- should be successful, some of the more dramatic effects of climate change will be difficult to ameliorate.

"Climate change doesn't hurt everything, and sometimes it can even do some good for certain species, but those are anomalies," Geer said from his Lolo, Mont., home. "What it comes down to most of the time is adapt, migrate or die."

Yet even with the mounting evidence of climate change's effects on Western streams and fish populations, some remain unsure about the degree to which a global phenomenon like climate change is responsible for shifting conditions on the ground.

Outfitters have their doubts

Robin Cunningham, executive director of the Fishing Outfitters Association of Montana, said his members, including more than 300 outfitters, have observed changing weather conditions such as higher air temperatures that may not favor trout and other native fish over the long run. But he said, "The distinction between localized conditions and a global situation is often hard to distinguish.

"What we find is that alterations from other man-made activities -- such as agriculture, expanding subdivisions, irrigation and even aquifer changes -- result in more immediate and obvious changes to our habitat and our fishery than what we see as global-warming-related change."

Cunningham said that he has extensively polled his members on their views about how climate change is affecting their businesses' bottom lines, and that most believe the general economic downturn of the last four years has posed a greater risk to their viability than climate change.

Geer said he often encounters such views when addressing sportsmen's groups in formal presentations around the West. But, he said, the reluctance to acknowledge the role of climate change in fisheries health has more to do with the lack of understanding about the long-term relationships between rising temperatures and habitat conditions than an outright denial of climate science.

"When you get up there and present the evidence about biological effects, about what actually happens to fish and wildlife [under shifting climate regimes], people say, 'Oh, crap.' And then you stress to them that fish and wildlife can't go to the public square and make their opinions known. They vote with their feet or their fins or their wings."