Recent sightings of dead mountain whitefish have been reported from across the Upper Snake Region, including the Henrys Fork, South Fork Snake River, Teton River and main Snake River as far south as Firth. Exactly why these fish are dying has been a mystery until now. Preliminary results obtained by IDFG’s Fish Health Laboratory in Eagle show that the parasite Tetracapsuloides bryosalmonae (also called PKD or Proliferative Kidney Disease) may be behind the deaths. This rare parasite has not been documented in Idaho’s wild fish before, although it has been reported in both wild and hatchery trout and salmon in North America and in Europe. This may be the first time the parasite has been detected in any whitefish species. The life cycle of the parasite is not well understood, but involves a freshwater sponge as well as a fish. Transmission and disease signs are linked to elevated water temperatures, which have been common in area waters this summer. There are no known health risks to humans or other warm-blooded animals.

Although a likely cause of fish mortalities has been identified, new questions are arising as a result of this information. For instance, IDFG does not know the level of impact the current outbreak may have on mountain whitefish populations, or if the parasite will also affect other species, such as trout. Currently it appears only mountain whitefish have been affected. Also unknown is why only younger whitefish have been affected, if the kill occurs periodically, or if this is the first time an outbreak has occurred. Scientific literature indicates fish that survive an initial infection develop strong immunity to the parasite. Additional samples will be collected and analyzed to verify the preliminary test results, and to look for the presence of this parasite in trout. Structured sampling this fall will provide an insight into how extensive the kill has been on the South Fork Snake River.

For more information concerning this matter contact Upper Snake Regional Fisheries Manager Dan Garren at 208-525-7290 or

Logo courtesy Idaho Department of Fish and Game

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