Study Projects Significant Decline of American Trout Habitat
Cutthroat trout, a species already eliminated from much of its historic range, faces new threats. (Image: CO Division of Wildlife/North Park Anglers)
The cold streams of the western United States are home to a species with an evocative name: cutthroat trout, the official fish of seven states, is so called for the often brilliant red streaks across its lower jaws and gills. Once widespread across western North America, pockets of the fish evolved into a dozen subspecies isolated by the rugged geography of the Rocky Mountain and Cascade ranges. Around the turn of the twentieth century, the introduction of nonnative species like the brook, brown, and rainbow trout—with whom cutthroat compete for resources and sometimes interbreed—wounded the native populations, even driving two subspecies to extinction.
Now experts believe that all western trout, not just the vulnerable cutthroat, face an unprecedented challenge. A recent study suggests climate change and the intrusion of nonnative species may reduce trout habitat in the western United States by 47% within seventy years. A team of biologists representing several public agencies and universities—including the Forest Service, the Geological Survey, the Universities of Colorado and Washington, and Trout Unlimited—used fish surveys from 10,000 sites in six states to model the viability of habitat under a changing climate. Cutthroat and brook trout are slated to suffer the greatest losses.
Rather than modeling the effects of temperature alone, the latest study also looked at the potential impact of altered stream flow regimes and continued interaction between competing species. The species’ distinct ecologies led to interesting outcomes—unique spawning schedules led the four trout to show differing responses to the predicted changes in seasonal stream flooding. Similarly, cutthroat trout proved more susceptible to biotic interactions than the nonnative fish. The models demonstrated that all four species, however, would be severely impacted by rising temperatures, with total habitat declining by an estimated 47%. Individual species showed projected losses from 35% (rainbow trout) to as high as 77% (brook trout).
The family of fish to which trout belong have long been the subject of extensive research because of their far-reaching environmental and economic significance. Trout populations help regulate nutrient cycling, maintain ecological balance, and bring in hundreds of millions of dollars from recreational fishing. Thankfully, management options in the face of climate change are as diverse as the fish itself. The authors suggest that by tailoring restoration projects to individual species and localities, managers will be better prepared to combat habitat and fish decline in the coming century.