Fish Disease Whirls Into Yellowstone Parasite That Attacks Trout At Work In Park Waters

Sun., Jan. 24, 1999

Staff

Whirling disease, an incurable parasitic ailment that can wipe out trout populations, has been detected in Yellowstone National Park and a tributary in the prime fishing section of the Missouri River.

The disease that has decimated young rainbow trout in Montana’s Little Prickly Pear Creek can be expected to begin impacting the Missouri River rainbow fishery next year, a Department of Fish, Wildlife and Parks official said recently.

In Yellowstone Park, officials said the disease was found in native cutthroat trout taken from the east side of Yellowstone Lake near the mouth of Clear Creek.

Native fish in the lake already are threatened by a growing population of voracious lake trout, which apparently are not affected.

Eleven of 41 cutthroat trout in three samplings taken during the fall tested positive for the disease. The microscopic parasite attacks the cartilage of some fish, such as cutthroat and rainbow trout. It may not kill the fish, but the resulting whirling behavior makes the ailing fish unable to feed normally and more prone to predators like lake trout.

Evidence of the crippling disease in Yellowstone Lake surprised park fisheries biologists. They had previously looked for but found no evidence of it in park rivers or streams linking the lake to known locations of the disease outside the park. The disease is more common to rivers and streams.

Montana’s first known outbreak of European-based disease struck the Madison River to the west of Yellowstone in 1994 and quickly decimated young rainbow trout.

With this background, biologists are pessimistic about some specific waters.

“You can kiss off (Little) Prickly Pear as a main contributor to the Missouri River for right now,” Dick Vincent told members of the Montana Whirling Disease Task Force recently.

Little Prickly Pear Creek, Sheep Creek and the Dearborn River are the three major rainbow trout spawning tributaries of the Missouri River between Helena and Cascade.

In four years, there has been a 90 percent drop in the number of young rainbow trout in Little Prickly Pear, Vincent said, and those young rainbows typically move into the Missouri.

So far, population declines due to the disease have not been found in the Dearborn River or Sheep Creek.

Vincent said it is too early to predict how large of an impact that decline will have on the Missouri River.

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