Whirling Disease Parasite Found in Upper Henrys Fork Trout

The parasite that causes whirling disease in trout has now been discovered in trout in Upper Henrys Fork. Since it was first found in Idaho waters in 1987, Idaho Department of Fish and Game biologists have inspected hundreds of streams, at thousands of sites, to determine whether fish populations have been exposed to Myxobolus cerebralis, the parasite that can cause whirling disease. Recently, Fish and Game detected this parasite in trout collected from the "Coffee Pot" section of the Henrys Fork Snake River, as well as four other sites: Medicine Lodge Creek, Birch Creek, the Portneuf River, and the upper Blackfoot River. While the parasite has previously been found in Henrys Lake, and from the Henrys Fork downstream of Mesa Falls, this is the first time it has been seen in the free-flowing portion of the Henrys Fork upstream from the falls. While not harmful to humans or other wildlife, infections of whirling disease have been responsible for population declines of wild rainbow trout, most notably in Montana and Colorado. The new findings bring to 20 the total number of populations in Idaho which have been documented as being exposed to Myxobolus cerebralis. Population-level declines of rainbow trout are suspected in only two of the Idaho systems where it has been found. Although the parasite causing whirling disease is known to have been present in the Henrys Fork below Mesa Falls since 1997, no adverse affects to the population or the fishery are apparent. A few of the fish collected from the upper Henrys Fork did show some slightly misshapen heads, which can be a sign of whirling disease. Dr. Keith Johnson, Fish and Game fish pathologist, notes that it is important for anglers and biologists to recognize that exposure to the parasite, or the presence of whirling disease in some fish, does not necessarily mean that fish populations will decline, but it does create cause for concern. Fish and Game biologists will continue to monitor the health of juvenile trout from the Henrys Fork and the other four sites. Idaho anglers are encouraged to learn more about the parasite through obtaining a brochure from Fish and Game regional offices and fish hatcheries entitled "Whirling Disease and Idaho Trout". This explains the life cycle of the parasite and its distribution (as of 2001) in Idaho waters. The brochure also details ways to reduce the spread by washing mud from equipment and by not placing inedible fish parts into public waters. Anglers are encouraged to do what they can to help prevent the spread of whirling disease.