Some of these exotic fish are more than 20 years old, and Park Service biologist Dan Mahoney says the fish have probably been lurking in the lake's deep waters all the while, even though they are a relatively recent discovery. Until recently, the lake had remained the last major stronghold for Yellowstone cutthroats, a species the U.S. Fish and Wildlife Service is considering for protection under the Endangered Species Act. Yet now, in tributary streams feeding one particular area of the lake called the West Thumb, cutthroat trout numbers are less than one-tenth of what they were in the 1980s.
"Cutthroats are having a very hard time all over. Lake trout are just one more threat to their future," says Rob Ament of American Wildlands in Bozeman, Mont.
The native Yellowstone cutthroat spawn in streams and linger in shallows, where grizzly bears, raptors and other wildlife can catch and feast on them. Lake trout, by contrast, spend much of their lives in deeper waters, where they remain inaccessible to the native diners that have long depended on the cutthroat. They also are voracious predators: one lake trout could eat an estimated 6,000 cutthroat in its lifetime.