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To anglers, the words "trout stream" or "salmon stream" mean one thing: free-flowing water that's clear and -- most of all -- cold. But in the decades ahead, the chilly streams that sustain these prized sport fish will grow increasingly tepid as a result of rising global temperatures.

According to a new study by NRDC and Defenders of Wildlife, global warming is likely to spur the disappearance of trout and salmon from as much as 18 to 38 percent of their current habitat by the year 2090. The study also found that habitat loss for individual species could be as high as 17 percent by 2030, 34 percent by 2060 and 42 percent by 2090 -- if emissions of heat-trapping pollution such as carbon dioxide are not reduced.

photo: Enrico Ferorelli Is fly fishing a vanishing pastime?

Why are salmon and trout so vulnerable to global warming? It's simple: cold-water fish such as trout and salmon thrive in streams with temperatures of 50 to 65 degrees Fahrenheit. In many areas, the fish are already living at the upper end of their thermal range, meaning even modest warming could render streams uninhabitable.

Projected increases in water temperature differ by location, but average 0.7 to 1.4 degrees Fahrenheit by 2030, 1.3 to 3.2 degrees by 2060, and 2.2 to 4.9 degrees by 2090, depending on future emissions of heat-trapping gases and the climate model on which projections are based.

The analysis covers four species of trout -- brook, cutthroat, rainbow and brown -- and four species of salmon -- pink, coho, chinook and chum. Researchers looked at air and water temperature data from more than 2,000 sites across the United States (not including Alaska or Hawaii). Then, using three internationally recognized climate models, they estimated changes in stream temperatures under a variety of pollution scenarios.

In reality, habitat loss could be even more extensive than predicted. NRDC's study examined only the direct effects of higher air temperatures on water, and did not cover indirect impacts of global warming, such as shifts in precipitation and evaporation. Nor did it take into account changes in the ocean, where salmon and some trout species spend much of their lives.

As with other consequences of global warming, the disappearance of trout and salmon is expected to vary by region. For trout, the greatest losses are likely to occur in the South, Southwest and Northeast, largely because stream temperatures in those areas already are warmer than in other regions. For salmon, significant losses are expected throughout the current range of the four species, with the most dramatic losses occurring in California.

Regardless of location, the disappearance of cold-water fish will come at a significant cost -- to jobs, recreation and regional culture. Roughly 10 million Americans spend an average of 10 days a year angling for salmon and trout, and the estimated value of the combined fisheries ranges from $1.5 billion to $14 billion a year. Trout are also central to the culture of the Rocky and Appalachian mountains, while salmon are an integral part of the Northwest's Native American heritage.

Of course, global warming isn't the only threat to salmon and trout. Logging, development, pollution, dams and water diversions have all degraded, if not destroyed, huge swaths of their habitat -- while inspiring high-stakes efforts to save various species from extinction. During the 1980s, for example, more than $1.3 billion was poured into the Columbia River basin in attempts to improve salmon runs.

It's now clear that such efforts -- if they're to succeed in the future -- must address the problem of global warming as well. To do this, the United States must reduce emissions of carbon dioxide and other heat-trapping gases. The good news is that many of the tools needed to clean up motor vehicles and power plants already exist. What's needed is the political will to put this technology to use.

Based on "EFFECTS OF GLOBAL WARMING ON TROUT AND SALMON IN U.S. STREAMS," a May 2002 report by Defenders of Wildlife and the Natural Resources Defense Council. The report is available on the Defenders of Wildlife website.

last revised 5/21/2002