Photograph by Jay Fleming

Read Caption

Yellowstone cutthroat trout (pictured) are declining thanks to pressure from larger invasive lake trout.

Photograph by Jay Fleming

Lake Trout Are Bad News for Yellowstone Lake

Ospreys, bears, and especially cutthroat trout suffer because of non-native fish.

It's a case of trout versus trout, and in the face-off between native Yellowstone cutthroats (Oncorhynchus clarkii bouvieri) and the intruders on the scene, lake trout (Salvelinus namaycush), the judgment handed down is a no-brainer. The lake trout must go.

The problem, explained Pat Bigelow, a fisheries biologist at Yellowstone National Park, Wyoming, emerged in 1994 when lake trout were discovered in Yellowstone Lake. Why lake trout were introduced into the lake is a mystery. Perhaps, biologists speculate, some anonymous angler wanted to diversify the fish population in the lake but didn't think through the consequences. "It's an example of bucket biology," said Todd Koel, supervisory fisheries biologist for the park. (Related pictures: Trout vs. Trout in Yellowstone Lake.)

A Yellowstone cutthroat—coppery in color, back peppered with spots, a blush of pink by the gills—is a thing of beauty. But it is outmatched in size and outlived by the lake trout, which weighs in at over 50 pounds (23 kilograms) and can reach the ripe old age of 20-plus years, about twice the span of cutthroats. More to the point, lake trout eat cutthroats.

Why the fuss? Isn't variety the spice of trout fishing? Not at all, said Bigelow. The cutthroat fishery provides an estimated 34 million dollars in economic benefit to the area. Lake trout—not as easy to catch—are no substitute for their native cousins. More important, Yellowstone cutthroats, and their genetic well-being, need protecting. Two of fourteen cutthroat species—the Alvord and yellowfin—are already extinct. (Related interview: Invasive fish species in the Great Lakes.)

There are other factors that speak to the wisdom of weeding out non-natives. Lake trout are deepwater dwellers and inaccessible to the otters, ospreys, bears, and eagles that prey on easily caught, shallow-water-dwelling cutthroats. Ten years ago, Koel pointed out, there were 50 nesting pairs of ospreys in the lake system area. Today there are only three or four.

Once the alien species was discovered, the Park Service was quick to respond. It started netting the intruders and in the past ten years has hired commercial fishermen to accelerate the process with large deepwater entrapment nets. Last year, more than 300,000 lake trout were removed from the lake, pushing the ten-year total take to about a million.

But lake trout aren't going away any time soon. Netting has slowed down, but not completely solved, the problem. "The war isn't won yet," said Koel. "It's a matter of keeping ahead of the curve."