Patricia Flebbe, research biologist at the SRS Fisheries Team in Blacksburg, VA, is the lead author of a study which maps out the potential distributions of three trout species in the Southern Appalachians as temperatures increase. Susan Adams, research aquatic ecologist with the SRS Center for Bottomland Hardwoods Research hydrology team based in Oxford, MS, collaborated with Forest Service scientists at the Rocky Mountain Research Station to predict changes to the bull trout populations in the Western United States.

Although these projects were conducted independently of each other, their findings fit together well.

According to Flebbe’s model, between 53 and 97 percent of wild trout populations in the Southern Appalachians could die out as streams become warmer. Trout are coldwater species that depend on relatively low stream temperatures to survive.

“Trout are already at the southern limit of their range,” Flebbe says. “Suitable habitat area will shrink and become much more fragmented as the climate becomes increasingly warm.”

Flebbe, with fellow researchers Laura Roghair from the Virginia Tech Conservation Management Institute and former SRS employee Jennifer Bruggink, first examined the current distributions of the native brook trout and the introduced rainbow and brown trout in the Southern Appalachians. They developed a regional map of wild trout habitat based on stream samples, expert knowledge, and suitable land cover.

Their model substitutes elevation and latitude for temperature, creating a more spatially explicit way to predict habitat loss over the next century. The resulting paper was published in the Transactions of the American Fisheries Society in 2006.

Their model substitutes elevation and latitude for temperature, creating a more spatially explicit way to predict habitat loss over the next century. The resulting paper was published in the Transactions of the American Fisheries Society in 2006.

The problem of habitat fragmentation will continue to worsen as temperatures rise. Disturbances such as changes in land cover, road building, increased angling, and other factors already limit the distribution of trout in this region. Flebbe predicts that the fish will eventually exist only in “islands” at the highest elevations, leaving them especially vulnerable to dying out.

“Small populations in isolated patches can easily be lost,” Flebbe says. “In a warmer climate, local extinctions may become irreversible.”

The extent of these extinctions will depend on how much the stream temperatures actually rise in the Southeast. Flebbe, Roghair, and Bruggink used two different global circulation models to predict habitat loss.

Using the Hadley Centre model, the team determined that 53 percent of trout habitat could be lost by the year 2100. Using the more extreme Canadian Centre model, they projected a loss of 97 percent.

Flebbe’s model does not address what people could do to mitigate the effects of climate change, but it is likely that humans will have to manipulate fish species in order to provide the fishing experience. State agencies already raise millions of trout and release them into streams for people to catch.

“We have a long history of altering aquatic communities by stocking game fish and introducing other species,” Flebbe says, adding a word of caution, “However, stocked trout do not function in the same way within the ecosystem.”

Still, uncertainty remains about how trout will react to the warming climate.

“There’s a real argument about whether fish can adapt to climate change quickly,” Flebbe says. “There are potential issues related to changing physiology. The next step will be looking at the fish and how they respond to temperature.”

Predicting the Bull Trout’s Future

While Flebbe modeled projections for three species of trout, including two nonnative varieties, Adams’ research focused on the bull trout in the Interior Columbia River Basin. Bull trout are native to five States in that region and are considered a threatened species under the U.S. Endangered Species Act.

“Bull trout may be especially vulnerable to climate change, since they require cold water for spawning and early rearing,” Adams says. “Some populations appear to face higher risks than others.”

Adams and her fellow researchers explored the implications of climate change over the next half-century by modeling the species’ response to three levels of projected temperature increases.

“We concluded that warming over the range modeled could result in a loss of 18 to 92 percent of stream area cool enough to support bull trout reproduction,” Adams says. “Perhaps more importantly, the models predicted a 27- to 99-percent loss in the number of large habitat patches that are critical to population persistence.”

The model predicts how climate warming will raise the lower distribution limits of bull trout population, as well as how the increasing fragmentation of the habitat with rising temperatures will accelerate the loss of bull trout.

Adams is the third author of the paper, recently accepted by the Transactions of the American Fisheries Society. She collaborated with a number of scientists from the Rocky Mountain Research Station: Bruce Rieman, Daniel Issak, Dona Horan, David Nagel, Charles Luce, and Deborah Myers.