Lake trout aren't just found in low-elevation lakes with large recreational fisheries, like Montana's Flathead Lake. For more than two decades, they have thrived in the crystalline, icy waters of Yellowstone Lake, in the heart of Yellowstone National Park.
Biologists believe someone introduced lake trout to Yellowstone Lake back in the 1980s. Since then, the population has exploded, while the Yellowstone cutthroat trout, a beautiful, small-bodied native, has declined by more than 90 percent. Lake trout both eat and compete with cutthroat trout, as they do with bull trout in the Flathead drainage.
The loss of cutthroats has rippled through the ecosystem. Unlike lake trout, which spend their entire lives in lakes, cutthroat return to their natal streams to spawn. There, a host of animals – from spiders to bald eagles to grizzly bears – depend in some way on their carcasses for food. Scientists are finding that when those animals can't get trout, they'll turn to other things. Arthur Middleton, a biologist at Yale, recently published a paper in the journal Proceedings of the Royal Society B showing that, in the absence of cutthroat, grizzly bears have started eating more elk calves. As a result, some elk populations have declined. "That a change in the trout fishery can reach to elk migration is striking," Middleton says. "It shows how the broader effects can be felt far outside the park."
This helps explain Yellowstone's increasingly aggressive drive to eradicate lake trout. Unlike at Flathead Lake, where there is an entrenched lake trout recreational fishery and multiple managers, Yellowstone Lake is overseen solely by the National Park Service, which has a mission to preserve native species. Park biologists have had broad public support to deploy increasingly sophisticated methods, including using experienced gillnetting crews from the Great Lakes to sweep up lake trout.
In the first years, gillnetters caught 25,000 to 50,000 lake trout each summer; the catch rose to around 100,000 by the mid-2000s. As funding has increased – to over $1 million a year from both private and public sources – so, too, has fishing; gillnetters now remove between 200,000 and 300,000 lake trout per year. Although the agency has gotten some pushback from lake trout-loving fishermen, most angling groups support the effort, says Dave Hallac, division chief of the Yellowstone Center for Resources at the park.
The exact number of remaining lake trout is unknown, but Hallac guesses that it is in the hundreds of thousands, and that gillnetting will need to go on for years. But there are promising signs that the population is declining. In the last two years, gillnetters have caught fewer lake trout, and an independent monitoring program has found juvenile cutthroat trout in Yellowstone Lake for the first time in years. "It's too early to claim success just yet," Hallac says, "but we're definitely on the right track."