A KILLER RUNS THROUGH IT

By RICHARD WOODBURY/BOZEMAN

Waist-deep in the fast-running Gunnison River, below the towering rock cliffs of Colorado's Black Canyon, angler John Duncan drifted a tiny brown imitation caddis fly on the filmy surface of the crystal-green water. Suddenly a form rose from below and took the hook. As Duncan played in his leaping, twisting catch, he could tell by its green back, silvery sides and blazing red stripe that he had hooked a rainbow trout. Then Duncan saw something else: a jet-black discoloration on the fish's tail and rear section. The trout was clearly diseased. "I was shocked," says the veteran sport fisherman. "You don't expect such a sight out in the remote wilds."

Duncan was lucky; he caught and released several more fish that afternoon. Other trout aficionados will make the pilgrimage to Colorado's and Montana's world-renowned wild-trout streams this fishing season and come away skunked. The cause: the tail-blackening "whirling disease," a mysterious and usually fatal ailment that is spreading rapidly through prized trout populations of the Rocky Mountain West. In Colorado, where the rainbow is the mainstay of a $1 billion-a-year game-fishing industry, the disease has infected hatcheries, devastated trout on a prime stretch of the Colorado River and spilled into 13 of 15 major river drainages. On one 55-mile stretch of Montana's famous Madison River, an estimated half-million fish have been killed since 1990--including 90% of the fingerling rainbows--and the catch rate of adult rainbows has plummeted 75%. Warns Dick Vincent, a Montana state-fisheries manager: "On some rivers, we're looking at a real threat to the entire species."

The urgency of that threat was underscored last week when 60 ichthyologists and wildlife managers gathered in Bozeman, Montana, to share what they had learned about the disease and map out a strategy for fighting it. Unfortunately, there was precious little hard information to share. No one is sure exactly how the disease gets started, how it spreads so easily, why it zeroes in on rainbows or how it can be stopped. "Our data base is almost zero," says Karl Johnson, a virologist who spearheaded the search for the Ebola virus, and is helping to lead this effort. "There are unanswered questions everywhere."

This much is known: the disease is carried by a microscopic protozoan called Myxobolus cerebralis, whose spores are released when infected fish die. These spores are not in themselves harmful to trout. It is only after they have been ingested by inch-long Tubifex worms in the mud that the parasites become dangerous. In the worm's gut, the protozoan takes a new form: grappling-hook-shaped spore cases that when released from the worm, can invade the gills and skin of tiny rainbow fry. The infection eats away at the cartilage of young trout, leaving them deformed, discolored and often spinning frantically in small circles until they die. Hence the name whirling disease.

The ailment was first detected in Pennsylvania in 1958--imported, scientists believe, in a shipment of trout fillets from Denmark--and has since turned up in 20 other states. Not all trout are susceptible. Brown trout show signs of infection but seem to be weathering it, and fishing for browns is still good in most streams. For some reason the rainbows of the Rockies, which breed in cold mountain water and create the wild-trout populations so valued by anglers, are especially vulnerable.

Fishermen themselves may have helped spread the disease. More than 80% of the trout caught in Colorado come from hatcheries, and although many of the state's hatcheries are known to be infected, the Division of Wildlife continues to pour its fish into Colorado streams to help maintain what it calls the angler-satisfaction rate. Montana, however, does not stock its streams; authorities suspect the Madison may have become infested when some angler unwittingly dumped infected rainbows into it. The spores can also be carried from stream to stream by boats, outboard motors and even mud on the soles of wading boots.

Scientists are desperately searching for solutions. They are perfecting DNA tests that will allow faster identification of the parasite and are searching for a trout species that is immune to the disease and could provide a substitute for the rainbow. Meanwhile, the Montana-based Whirling Disease Foundation, which is helping to coordinate the fight, has landed a big-name supporter in TV mogul (and part-time Montanan) Ted Turner. For streams like the Colorado and the Madison, where the wild-rainbow population is in free fall, the hope is that it may not be too late.