Parasite Blamed in Rainbow Trout Decline : Montana: Fish population in fabled river has declined dramatically in recent years. Officials think they’ve discovered the cause, and it’s incurable.

To trout fishermen, Montana’s Madison River is an almost-holy place, a shrine to the delicate sport of fly-fishing. Every year, tens of thousands of anglers descend on the river, which tumbles out of Yellowstone National Park and runs for more than 100 miles in a picturesque valley between the Tobacco Root and Madison ranges to Three Forks, where it joins with the Gallatin and Jefferson rivers to form the mighty Missouri.

Cold and mineral rich, the Madison boasts one of the nation’s best wild trout fisheries--for every mile of river there are several thousand brown and rainbow trout, none of them hatchery-bred.

But in the last few years there has been a precipitous decline in the rainbow population on the upper Madison above Ennis, Mont. A few days before Christmas, officials from the Montana Department of Fish, Wildlife and Parks announced they had found a likely culprit: a deadly parasitic disease for which there is no known cure.

The discovery of what is known as whirling disease among the Madison’s rainbows has alarmed fishermen, conservationists and fly-fishing guides who revere the storied river and depend on it for their livelihood. They fear not just for the Madison, but for the rivers to which it is connected as a principal tributary of the Missouri.

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“Through carelessness of some kind or another, one of America’s premier trout fisheries has been badly trashed,” said Pete Rafle, a spokesman for the national conservation group Trout Unlimited. “There are outfitters and shop owners literally weeping over this.”

“Potentially, it’s a disaster,” agreed Dave Kumlein, a Bozeman, Mont., fishing guide. “If it moves throughout the Missouri River drainage, you’ve covered some of the best trout waters in the world. If it moves upstream, as it can, all the great trout rivers in Montana east of the Continental Divide could be affected--the Big Hole, the Gallatin, the Jefferson, the Beaverhead, the Ruby. It’s a who’s who list of world-class trout streams.”

Whirling disease--named for the characteristic tail-chasing of young fish whose cartilage is attacked by parasitic protozoa--is a relatively new phenomenon in the United States. Accidentally introduced into the country in 1956, whirling disease has been detected in 18 states and has been associated mostly with hatcheries.

The disease twists the spines of young fish and makes it very difficult for them to feed and more susceptible to predation. It is suspected of causing a sharp drop-off in rainbow trout in the upper Colorado River, said Peter Walker, a fish pathologist with the Colorado Division of Wildlife.

Because aquatic creatures called tubifex worms also serve as hosts for the parasite, the disease is unusually persistent. “No one’s figured a way to get rid of it,” said Walker.

Glenn Hoffman, a retired fish parasitology researcher with the U.S. Fish and Wildlife Service and an expert on whirling disease, is less alarmed by the outbreak in the Madison. “If I had my choices, I’d prefer not to have it in my river or anyone else’s river,” said Hoffman. “But I don’t know of any documentation showing how much damage it does in the wild.”

But Montana officials believe the disease may be responsible for a 90% decline in rainbow trout along a 50-mile stretch of the Madison. “We’re taking this disease very seriously,” said Dick Vincent, the regional fish manager for the state’s Fish, Wildlife and Parks department.

Because the Madison is not stocked with hatchery fish, the appearance of whirling disease is a puzzle. Montana officials think someone may have illegally put infected trout into the river.

Whatever the cause, conservationists and biologists say the implications for such a premier wild fishery could be enormous. Though the Madison’s brown trout are unaffected by the disease, cutthroat trout and landlocked sockeye salmon in other rivers connected to the Madison ultimately could be infected.

Guides like Kumlein fret that with the river’s rainbows all but gone, the multimillion-dollar sport-fishing industry will suffer. With the river still loaded with brown trout, said Kumlein, “there’s still a ton of good fishing,” but browns are harder to catch than rainbows.