Cutthroat trout, a native species in trouble around the West, are facing an increasing threat in a key sanctuary, Yellowstone National Park. Whirling disease, spread by a European parasite that showed up in the park five years ago, now infects 12 to 20 percent of the cutthroats in Yellowstone Lake, according to biologists’ studies. And in Pelican Creek, an important lake tributary and spawning area, the cutthroat population has crashed. Tens of thousands of cutthroats migrated up the creek to spawn each spring in the early 1980s, but only a few were spotted last spring, and the infection rate in some tests surpassed 90 percent.

Pelican Creek has suffered "a total loss," says Todd Koel, a park biologist. "Here we are, in this pristine environment, watching the proliferation of an exotic (parasite). It’s sad."

Whirling disease damages cartilage and nerves in trout and salmon, and can kill young fish directly or by causing them to spin in the water, making them easy prey for predators. Hundreds of streams around the West have been infected (HCN, 9/18/95: The West's fisheries spin out of control, by Ray Ring).

The disease adds to the pressure that Yellowstone cutthroats feel from another predator: illegally introduced lake trout. The fish’s decline could strain the park’s ecosystem, because birds, bears and other animals have evolved to feed on cutthroats. Biologists are carrying on a campaign to thin the lake trout, and they’re advising anglers and other people to be more careful: The whirling disease parasite can be spread through muddy clothing and gear.