ECOLOGY : Lake’s Cutthroat Competition Rises : A bigger, bully trout is threatening the native fish at Yellowstone. And the Mackinaw may win out.

When a young angler casting from fishing guide Tom Hansen’s boat reeled in a mottled blue-green fish in July, Hansen knew immediately it was not a native cutthroat trout for which Yellowstone Lake is famous.

While Yellowstone cutthroats carry a distinctive red slash below their mouths, this fish had none.

“I knew it was something that didn’t belong,” recalled Hansen, a teacher in Pittsburgh when he is not guiding anglers in Yellowstone National Park’s largest lake.

Biologists have since confirmed that the fish was a lake trout, or Mackinaw, an intruder indeed. In following months, anglers and federal netting crews nabbed more of the exotic fish that experts now fear may wreak havoc on the largest intact ecosystem left in the Lower 48 states.

Lake trout pose a danger not only because they gobble smaller cutthroats, but also because by doing so, they could send shudders through the food chain that sustains wildlife from grizzly bears to birds of prey.

“This may be the beginning of a major change in the traditional picture of the Yellowstone ecosystem,” U.S. Fish & Wildlife Service biologist Lynn Kaeding says as his motorboat skims the surface of the icy blue lake on a sunny but cold fall day.

Set amid snowy peaks at an elevation of 7,733 feet, Yellowstone Lake is the last stronghold of the Yellowstone cutthroat--squeezed from 90% of its original range by human development and alien predators. The national park’s cutthroat may be the largest undisturbed population of native trout left in the world.

Even the first military caretakers of Yellowstone safeguarded local cutthroat from big and voracious lake trout, native only to Canada and the Great Lakes.

Government agencies brought the Eastern species to many Western waters, including empty lakes in Yellowstone, in a kind of Johnny Appleseed approach to fish management at the turn of the century. But they always spared 85,000-acre Yellowstone Lake and its prized cutthroat.

Catches this past summer, however, proved lake trout have finally invaded the largest high-altitude lake on the continent. There are no natural links between Yellowstone Lake and nearby waters containing lake trout, so rangers suspect renegade anglers seeking a new challenge surreptitiously planted the big fish.

Calling the action “an appalling act of environmental vandalism,” Yellowstone managers are offering a $10,000 reward for information leading to the conviction of those responsible.

After discovery of the lake trout, rangers loosened rules so anglers can haul in all the out-of-place fish they want, provided they kill all they catch. Park biologists also are asking experts how to control the invading fish. Options include large-scale netting, poisoning of spawning grounds or introduction of sterile sea lampreys, which have attacked lake trout in the Great Lakes.

Scattered reports of lake trout in Yellowstone Lake had surfaced before, but authorities ignored them as erroneous. The varying sizes of fish caught this summer suggest the lake now holds lake trout of many different ages, evidence the species may have been multiplying here for years.

If that’s true, fishery managers concede, there may already be too many of the alien fish to get rid of.

Female lake trout may produce 18,000 eggs a year for as many as 20 years. Such a horde leaves little room for cutthroat that lay fewer than one-tenth as many eggs. Lake trout grow as big as 50 pounds and can gulp a three-pound adult cutthroat with ease.

“This is the worst of all possible creatures,” says John Varley, Yellowstone’s top scientist and an expert on cutthroat trout, pointing to a stuffed lake trout as big as a German shepherd on his office wall. “This is the epitome of a predator.”

Long the sole inhabitants of many mountain lakes, cutthroat never evolved tactics to outwit such large predators.

Once lake trout arrived in Heart Lake, a smaller lake in the Yellowstone backcountry, cutthroat nearly vanished. In some alpine lakes in Colorado, Kaeding says, the aggressive fish erased a native species of cutthroat trout altogether.

“We can only guess how completely lake trout will take over,” he says, watching a crew haul up nets cast into the lake to find out how many lake trout are present. “They could unseat bears and otters from the top of the food pyramid.”

Cutthroat trout spawn in small streams each spring, while lake trout spawn in the fall only in deep water.

The opposite habits put lake trout beyond reach of grizzlies that look to spawning cutthroat for food when they emerge from winter hibernation. Osprey, pelicans, eagles and otters that feed in the shallows may also go hungry if cutthroat falter, since they cannot reach the 100-foot-plus depths where lake trout hover.

This year, fewer cutthroat spawned in one closely watched lake tributary than in any year since the mid-1960s, when overfishing depressed populations. Biologists blame the decline on drought and 1988 forest fires but acknowledge that they cannot rule out lake trout as a culprit.