HELENA - Rainbow trout are rebounding in the Madison River, the world-class fishing stream where Montana's first known outbreak of whirling disease occurred about 15 years ago, devastating the rainbow fishery.
In the 1990s, whirling disease slashed the rainbow trout population by 90 percent compared to levels measured in the 1960s and '70s.
Now, after a succession of rebound years, rainbows under 10 inches have "pretty well recovered to pre-whirling levels" and the population of those larger is about 60 percent of what it was before the disease, said Dick Vincent, whirling-disease coordinator for the Montana Department of Fish, Wildlife and Parks until his retirement last winter.
Vincent has now embarked on a new career as a breeder of dachshunds, but the agency says Vincent remains the authority on whirling disease in Montana.
Rainbows in the blue-ribbon Madison River, where guides this summer charge about $425 per boat for a day of fishing, have developed considerable resistance to the disease, a parasitic condition often characterized by a tail-chasing whirl. How that resistance came about is unknown, Vincent said.
He said he knows of no other Western river with the depth of recovery seen in the Madison. Montana rivers not hit significantly by whirling disease include another renowned fishing destination, the Missouri.
Madison River fishing guide Sean Blaine said that although rainbow-trout numbers "aren't what they were back in the heyday, it's certainly better than it was in the 1990s." Blaine said many of his clients are unaware of whirling disease and often are simply interested in catching a fish, not necessarily a rainbow.
Another Madison guide, Mike Lum, questions the severity of whirling disease's impact. A serious drought cycle also affected trout's ability to thrive, Lum said.
"Trout populations in the Madison and everywhere else are never static," he said. "It's always peaks and valleys, no matter what."
Whirling disease, which does not infect humans and is known to exist in about 20 states, is attributed to a microscopic parasite that attaches to the skin of young fish. Institutions researching the disease include Montana State University, which estimates that in Montana, whirling disease affects 150 streams.
Rainbow trout dying release into the water spores that may remain viable for years and are eaten by tiny worms. The worms excrete an infectious parasite, which attaches to fish. Young ones are particularly vulnerable to parasitic attacks, which can cause nerve damage, skeletal deformities and sometimes death.
In the United States, whirling disease first was observed in about 1958, according to the state Division of Wildlife in Colorado, where the disorder has been confirmed in 13 of the state's 15 major river drainages. It was 1991 when Vincent began detecting declines in Madison River rainbows. Laboratory tests confirmed the disease.
Vincent said he suspects, but cannot prove, that whirling-disease resistance in Madison River fish is tied to disease-resistant trout that were in southwestern Montana's Willow Creek Reservoir, somehow ended up in the Madison years ago and shared favorable genes with fish there.
Dave Kumlien of the Bozeman-based Whirling Disease Foundation, part of the conservation group Trout Unlimited, said developments on the Madison are encouraging but "we're not entirely sure the population is recovered. The infection level in the river is still quite high."
Among the states with whirling disease problems, efforts at prevention and control include use of ultraviolet systems intended to kill spores at hatcheries, and restrictions on the stocking of trout from hatcheries that test positive.
While the improvement on the Madison is encouraging, Vincent said, "the parasite hasn't gone away. It's still there."