The westslope cutthroat trout looks like a tough fish, but looks can be deceiving.
Named for the red slashes that mark its lower jaw, the fish—the only cutthroat trout native to Alberta—is listed as threatened under both Alberta’s Wildlife Act and the federal Species at Risk Act. And in the United States, it’s actually considered a “sensitive” species by the U.S. Forest Service, meaning its population has been seriously reduced by habitat loss and hybridization with other trout species, especially non-native rainbow trout.
While the westslope cutthroat has put up a good fight, scientists now worry climate change will turn the tides. A new study suggests warming temperatures have accelerated cross-species mating in Montana—particularly in the Flathead Watershed, one of the last strongholds of the species. This interbreeding will produce a generation of trout hybrids with less genetic capability to adapt to a variety of conditions.
Dr. Clint Muhlfeld is an assistant professor at the Flathead Lake Biological Station and Research Ecologist with the USGS Northern Rocky Mountain Science Centre in Glacier National Park. He says that historically, hybridization between the cutthroat and rainbow trout was largely confined to one downstream Flathead River population.
However, during the past 30 years, hybrid trout have spread upstream.
“Climatic changes are threatening highly prized native trout as introduced rainbows continue to expand their range and hybridize with native populations through climate-induced ‘windows of opportunity,’ putting many populations and species at greater risk than previously thought,” stated Muhlfeld in a press release.
From 1978 to 2008, the rate of warming nearly tripled in the Flathead basin, resulting in earlier spring runoff, lower spring flooding and flows, and warming summer stream temperatures. Now, genetically pure populations of westslope cutthroat occupy less than 10 per cent of their historical range.
In Alberta, genetically pure populations now only exist in a disconnected pattern at the fringes of their range in the Rocky Mountains and foothills.
While westslope cutthroat typically spawn and rear their young in streams that are clear and cold, rainbow trout are more adaptable, and tend to favour the new conditions climate warming has created. Muhlfeld says the locations with the greatest changes in stream flow and temperature experienced the greatest increases in hybridization.
“The study illustrates that protecting genetic integrity and diversity of native species will be incredibly challenging when species are threatened with climate-induced invasive hybridization.”
Cutthroat are not the only trout mating with the enemy. In Jasper National Park, the interbreeding of native bull trout and invasive brook trout concerns local scientists, but the extent to which it is happening is currently unknown.
“The best defence against hybridization is a healthy bull trout population,” says Parks Canada Aquatic Biologist Ward Hughson. He adds that when their numbers are strong, bull trout are such tenacious predators that they will control the number of smaller brook trout by eating them.