They found an alien in Yellowstone National Park.

"We were doing a standard wildlife census around Yellowstone Lake when we saw it," said Ronald Jones, the Fish and Wildlife Service biologist called in to deal with the emergency. "And we got really excited really fast, because there's no way on Earth the thing could get here unless it got up and walked across the mountains."

That would have been a formidable task, because the alien intruder that sparked a full-bore governmental response here this month was a trout: the Eastern Brook Trout, Salvelinus fontinalis, a native of the cool, clear streams of Virginia and North Carolina.

Hundreds of these trout were found this summer in Arnica Creek near West Thumb Bay of Yellowstone Lake. The official reaction was swift and hard.

Biologists poisoned about eight miles of the creek to remove all traces of the alien species. Park Superintendent Bob Barbee offered a $1,000 reward for information on any person responsible for putting brook trout in the creek. He warned that the guilty party faced up to five years in prison and a $5,000 fine.

Why the flap over a fish? The answer lies partly in nature -- in the seamless web that links the inhabitants of the Yellowstone ecosystem -- and partly in the man-made laws dictating preservation of all native species here.

The intrusion of the eastern trout could mean extinction for Yellowstone's native trout species, the cutthroat, a valued sport fish that takes its name from its characteristic marking: two blood-red slashes beneath the mouth.

"Wherever that brook trout is introduced," Jones said, "it becomes dominant, and the native species die away."

The gradual replacement of one trout species by another would throw a monkey wrench into nature's intricate food chain, with dangerous consequences for nonaquatic Yellowstone animals.

Every year from May to July, the cutthroat trout leave Yellowstone Lake and swim upstream along the mountain creeks to spawn. Cutthroat in the creeks are the main summer food of many Yellowstone animals, including pelicans, osprey, black bear and two majestic but endangered species, the bald eagle and the grizzly bear.

The brook trout swim upstream to spawn, too -- but not until October or November. If the brook trout were to drive out the cutthroat, many land animals would be deprived of their chief summertime meal. Some of these animals would starve to death: The grizzlies, presumably, would look for other food, such as people.

Jones is also concerned that the coming of the alien species would mean "corruption of the genetic pool" of the native cutthroat trout.

At this point, the non-fishing city-dweller may wonder why anybody other than a few graduate icthyologists would care about corruption of the gene pool of a particular trout species.

The answer may lie in the fortuitous case history of another species, the Lake Trout, a native of the Great Lakes.

In 1890, the federal government brought some lake trout from Lake Michigan out to Yellowstone. The midwestern species was established in a pristine, pearl-blue mountain lake here.

Over the next half century, pollution essentially killed the Great Lakes and most of their fish, and the 1970s brought a major effort to bring the Great Lakes back to life. But this required reintroduction of native species. By then, the native lake trout had died off.

Then somebody remembered Yellowstone Park and the pure gene pool of lake trout thriving there.

Today, the original Lake Michigan trout species is home, playing a major role in the reclamation of the Great Lakes -- and only because the pure genetic pool had been maintained in a mountain lake 1,500 miles from the fish's native waters.

Yellowstone officials went to similar lengths last week when they set out to destroy the intruder in their creeks.

Using a precisely calibrated mechanism, they dropped four parts per billion of Antymicin A, a fish toxicant, into Arnica Creek for 24 hours.

This procedure -- the biologists call it "treating the stream" -- killed all 80,000 fish in the creek.

The victims included both native species and the alien trout.

But if the "treating" eliminates the eastern brook trout -- full results won't be known until next year -- the native trout will find its way back into the creek and repopulate it with fish that spawn at the right time of year for the eagles and grizzlies.

Meanwhile, officials here are still trying to figure out who put the alien species into Arnica Creek.

Nobody knows for sure, but Jones has a theory.

"It was probably somebody who likes fishing for brook trout," he said. "So he just brought a bucket of them and figured they'd establish themselves here where he likes to fish.

"It was probably innocent," Jones added. "But it broke the main rule of a national park: Leave everything the way nature made it."