Yellowstone National Park officials have found whirling disease in native cutthroat trout taken from Yellowstone Lake near the mouth of Clear Creek (located on the east side of the lake) this fall. In three separate independent test procedures, 11 out of 41 of the fish sampled tested positive to the disease. Although whirling disease has been widely identified in streams in neighboring areas and is a major concern of regional fisheries managers and anglers, previous routine samplings for the disease in streams and rivers throughout the park were all confirmed as negative.

Whirling disease is caused by a microscopic parasite that attacks the cartilage of some fish, such as cutthroat or rainbow trout; the disease does not appear to affect lake trout. Although the parasite may not kill the fish, the fish is unable to feed normally, which can result in starvation and death. Also, the whirling behavior of an infected fish makes it more vulnerable to predators, such as lake trout. Once the disease is present in a fish, it converts to a spore form which is released into the water when an infected fish dies and decomposes. Whirling disease spores can survive in this form up to 30 years or more until another appropriate alternate host, such as tubifex worms, is infected. This European disease was first discovered in Pennsylvania in 1956 and has now been confirmed in 21 states. Insufficient data exists about the disease, such as transmission methods, why some fish are more susceptible than others, alternate host life cycles, or how the disease can be controlled and cured.

It is unclear what effect whirling disease will have on the native cutthroat trout population in the lake, which are already affected by the non-native lake trout first confirmed in the lake in 1994. Park biologists plan to test other cutthroat trout around the lake. Samples will be by-catch cutthroat trout taken during gillnetting procedures being done to control and gather data on the lake trout population.

Park biologists do not know how the disease reached the lake. The microscopic spore can be easily transported through water sources from another area; mud on boats, waders, or other fishing gear; fish entrails; birds; other live fish from another area; aquatic plants or weeds; or other unknown methods. As a precautionary measure, Yellowstone National Park Superintendent Michael Finley encourages park anglers to help park staff prevent further spread of the disease by checking and washing off any mud that might be clinging to any fishing equipment before departing from one fishing area to another and by not transporting water, aquatic plants, or other fish from one river basin to another.

For more information about whirling disease visit the Montana University System Water Center - Whirling Disease Initiative pages.

Back to Main Fishing Page