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Some Montana Waters Under Cloud

Thu., April 17, 1997

A microscopic parasite will be on the minds of thousands of anglers as they fish Montana’s Blue Ribbon trout streams this year.

The parasite, which has the tongue-twisting moniker of myxobolus cerebralis, has decimated trout populations in some of the world’s most famous streams and is threatening many other rivers.

Biologists and anglers first realized the parasite was threatening to destroy trout populations in 1994. A study showed that populations of young rainbows in the Madison River dropped from 3,300 to 300 per mile.

Fred Nelson, fisheries biologist, says in a new report on the whirling disease that the decline in adult rainbows has continued, reaching an all-time population low last fall.

On the bright side, however, the population of older brown trout “remains healthy and yearling browns (6 to 9 inches) exceed normal historic levels.” Also, he notes, researchers have recorded a “significant increase in the population of yearling (5 to 8 inches) rainbows from previous years.” He added that biologists don’t know yet whether the increase marks the beginning of a population recovery or is simply a one-year aberration.

Most Inland Northwest anglers, particularly fly fishers, fish the streams in the Missoula area and the Missouri and Big Hole rivers. Prospects are for good fishing this year in nearly all of them.

Because of the near-record snow pack, anglers won’t be able to fish the Missoula-area streams until well into June, perhaps later.

The rivers have been high and muddy this spring, without offering the usual window of low water flows before the major runoff.

The whirling disease parasite devastated the population of 10-inch and shorter rainbows in the upper Rock Creek, a study has revealed. More than two out of every three rainbows died. The lower river, however, hasn’t yet been affected.

Despite the big drop in Rock Creek’s rainbow population, there still are plenty of rainbows in the world-renowned stream for good fishing this year.

The whirling disease killed most of the brown trout in the upper Clark Fork, but it hasn’t had much of an affect yet on the lower river. Biologists will try to learn whether the parasite is killing rainbows in the Superior area this summer.

Neither the Bitterroot nor the Blackfoot has yet to show evidence of whirling disease penetrating trout. Biologists are optimistic the Bitterroot could escape effects that have crippled the Madison.

Parasites have been found in the Missouri River, but biologists won’t know for a year or so whether a high percentage of the rainbows and browns will be infected.

Besides the Bitterroot and Blackfoot, these streams presently test free of the disease: Yellowstone, Bighorn, Stillwater, Boulder, Gallatin, East Gallatin, Big Hole, Smith, Flathead, Kootenai, Marias and Big Spring Creek.

, DataTimes ILLUSTRATION: Graphic: Whirling disease affecting trout


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