Officials warn lake trout are disrupting the food chain across the West

On a hot July day two years ago at Yellowstone National Park, Danielle McCain dipped her fishing line into the icy waves of Yellowstone Lake hoping to hook one of the park's famous cutthroat trout.

What young Danielle pulled from the water instead was a husky lake trout, which elicited a curious, sidelong glance from her guide. Far more profound, her surprise catch sent federal wildlife officials scrambling to combat what they say is one of the greatest environmental threats to Yellowstone.

"Let me put it to you this way," explains John Varley, director of the Yellowstone Center for Resources, which coordinates scientific research in the park. "To say we're on the verge of an ecological disaster here is stating it mildly."

The lake trout may look benign enough, but their large size and voracious appetite for other fish threaten the survival of Yellowstone's pure genetic strain of cutthroat trout. At the same time, cutthroat trout are a nutritional staple for 43 species of mammals and birds in the park. The disruption to the ecosystem could be extreme.

So this summer, park officials plan to take unprecedented action by laying down a web of industrial-strength gill nets in Yellowstone Lake to try to suppress the population. The biological dragnet will use ocean weight nets nearly half a mile long with the objective of capturing and killing mature lake trout before they reproduce. Holes in the webbing will be big enough to allow cutthroats to escape.

Further, rangers are encouraging anglers entering the park to ignore normal fishing regulations and catch as many lake trout as they can. In fact, it now is illegal to land a lake trout in Yellowstone Lake and return it to the water.

Yellowstone is not alone in its battle with unwelcome intruders. Across the West, Mr. Varley has witnessed the devastating effect that lake trout have had on native fish populations in hundreds of lakes. He says it is part of a growing problem where zealous fishermen illicitly transplant all kinds of nonindigenous sport fish into tarns and waterways where previously they did not exist.

"Yellowstone Lake is just another domino that has gone down as far as trout lakes are concerned, but it is a big domino," says Varley, adding that Yellowstone Lake and the Yellowstone River, which flows out of it to the north, represent the last major haven for cutthroat trout on the continent.

"This whole business of stocking exotic fish is an epidemic throughout the West," he contends. "It's gotten so bad and widespread that in 10, 20, maybe 30 years there won't be a decent-sized lake with wild trout left in it."

Although a $10,000 reward has been offered for any information leading to the arrest of those responsible, the monetary enticement has turned up no leads.

Initially, biologists assumed that dumping lake trout in Yellowstone Lake had been a recent phenomenon, and that they had 20 or 30 years to devise a strategy. But recent surveys have turned up larger-sized lake trout than expected, indicating that not only have lake trout been present for years, but they are also actively reproducing.

According to the latest fish surveys, about 4 million catchable cutthroat trout thrived in the lake. However, Lynn Kaeding, a fisheries biologist with the United States Fish and Wildlife Service, says a proliferation of lake trout could reduce the fish population by 90 percent, causing ripple effects across the ecosystem.

"The problem has to be dealt with immediately," Varley says. "There's no margin. It's clear, especially this year, that we are behind the eight ball even further than we thought."

Scientists say it isn't simply a matter of nature replacing one fish in the environment with another. Behavioral traits of the two species are distinctly different.

In late spring and early summer, cutthroat trout move from the lake to spawn in the shallow confines of the Yellowstone River and various tributaries where they are eaten by predators. Lake trout, meanwhile, are fish that dwell and spawn in deep water, thus making them inaccessible to the animals that depend on cutthroat trout.

Park officials concede that netting and exerting additional fishing pressure from anglers are merely stopgaps.

Scientists considered a broad range of radical options. One alternative called for spending $28 million to poison Yellowstone Lake, killing all the fish, and starting over with a new generation of cutthroat trout. But that was deemed too environmentally risky. Strategists now realize that total eradication of the lake trout is unachievable.

"The downside is there isn't a silver bullet. We see nothing in the data now, nor do we anticipate finding any in the future, that provides an easy way to eliminate them," Mr. Kaeding says.

"We can attempt to control the lake trout, but the kind of control we're talking about is a forever proposition if we want to save the cutthroat fishery," he says. "Long into the future, it will represent a maintenance responsibility ... just like patching potholes in the road."