Over Labor Day weekend, earlier this month, anglers reported seeing several deceased mountain whitefish on the Upper Yellowstone River. Over the eight mile stretch from Grey Owl to Mallard’s Rest fishing access sites, located right outside of Livingston, Montana, just over 70 dead whitefish were totaled.
Scott Opitz, fisheries biologist with Montana Fish, Wildlife and Parks, manages the river fishery from the north Yellowstone National Park boundary down to Springdale, Montana. Opitz was responsible for surveying the kill in order to determine the distribution of mortality, identify which, if any, other species were infected, collect fish to be analyzed and classify the disease that may be causing the killings.
“Based on initial results from some of the fish samples as well as past history with whitefish kills on the Yellowstone River, PKD appears to be the cause of the kill this year,” Opitz said. Proliferative kidney disease (PKD) is caused by a parasite called Tetracapsuloides bryosalmonae, which has affected the Yellowstone River in the past as well. The parasite is held in freshwater sponges that carry PKD and can spread it to the fish. In 2016, the first and largest outbreak of PKD was reported, with an estimate of over 10,000 dead whitefish. Every year since, the parasite has reappeared at lower numbers than this year’s occurrence. Numbers of this year's kill have been significantly lower than 2016 and will have little to no impact on surrounding wildlife, according to Opitz.
If a fish were to survive the disease, it would develop an immunity, since fish do not contract PKD from other fish. However, the immunity is not passed to its offspring.
“Currently there is no way to control this parasite in a large, free flowing system like the Yellowstone River,” Opitz said. Treatment has proven to be effective in hatchery facilities, but wild fish are more difficult to manage. There is an abundance of research taking place in both the U.S. and in Europe with the goal to find a way to predict when these kills may occur and to develop methods to control or eradicate the parasite.
As of last week, the mortality rates have diminished and fishing will not be restricted for anglers. “At this time we have not been able to confirm that any species other than whitefish have been affected by this kill,” Optiz said. “Only two dead trout have been observed and results on the one trout that we were able to collect have not been received yet.”
To help researchers gain more knowledge about the disease and to maintain a stable fish population, Yellowstone River recreators should report fish killings while anglers should practice proper catch and release techniques to relieve stress on the fish.