And it seems to be no accident - the alien trout were likely slipped into Yellowstone Lake by anglers seeking to start a stock of catchable trophy fish. "An appalling act of environmental vandalism," says departing Yellowstone Park Superindent Bob Barbee.
The aliens are lake trout, also known as Mackinaw trout, native to Canada and the Great Lakes region.
Because of their popularity with anglers, over the years lake trout have been introduced into lakes around the West, either officially by wildlife agencies, or surreptitiously by anglers.
The practice fell into ill repute as the repercussions became known: Lake trout are so aggressive they often played havoc with local native fish. Lynn Kaeding, project leader for the U.S. Fish and Wildlife Service in Yellowstone, says lake trout wiped out a native species of cutthroat trout in some high-country lakes in Colorado.
Three other Yellowstone Park lakes - Heart, Lewis and Shoshone - already have been contaminated with lake trout.
But larger Yellowstone Lake, set amid surrounding peaks at an elevation of 7,733 feet, has long remained free of outside influences. Some 80 percent of the nation's remaining Yellowstone cutthroats live in the lake and the rivers and streams that drain into or out of it, making the lake "the heart of the whole population," " Kaeding said.
There have been scattered reports for 30 years of exotic lake trout in Yellowstone Lake, but never any proof until this summer, when anglers caught and turned in two of the fish. Biologists have learned of at least four other lake trout caught during the summer as well.
If the lake trout take hold in Yellowstone Lake, the national park could be "on the verge of an ecological disaster," " said Barbee.
Lake trout pose a danger because they are voracious, gobbling the smaller cutthroats, biologists say. The lake trout could also upset the food chain that sustains terrestrial wildlife from grizzly bears to birds of prey.
The two species have opposite habits. Cutthroat trout spawn in small streams each spring and live in shallow waters. Lake trout spawn in the fall only in deep water and live as deep as 100 feet below the surface.
It's feared that lake trout will be beyond the reach of predators who depend on cutthroat trout. Grizzly bears feed on spawning cutthroats after emerging from their winter hibernation. Birds, including ospreys, pelicans and eagles, catch cutthroats in the shallows.
Biologists are probing the lake's depths to determine how many lake trout might be present and how long they have been around.
Park managers have loosened regulations, allowing anglers to catch more fish, in hopes that lake trout will be taken. But managers concede there may already be too many of the lake trout to eliminate them.
If lake trout have been in the lake long enough to reproduce, Kaeding says, it may take "nothing short of a miracle'" to control them.
Rangers have posted a $10,000 reward for information leading to an arrest and conviction in the damaging transplant. Call 307/344-2281.
* Michael Milstein
Michael Milstein covers Wyoming for the Billings Gazette.