Fish biologists say a parasite caused the recent deaths of mountain whitefish in the Snake River watershed just across the Idaho border.

Unusual numbers of dead fish had been reported by fishermen in the Henry’s Fork, South Fork Snake River, Teton River and main stem of the Snake River as long as two weeks ago. The deaths, said to be dozens of fish, had gone unexplained until Wednesday.

Idaho Fish and Game biologists took samples and found that the parasite Tetracapsuloides bryosalmonae may be behind the deaths.

Linked to warm weather, the rare parasite has never been found in Idaho’s wild waters, but has been documented in both wild and hatchery trout and salmon in North America and in Europe. The life cycle of the parasite is not well understood, but involves a freshwater sponge as well as a fish, a Fish and Game statement said.

The parasite so far has not affected cutthroat or other trout in the watershed, Fish and Game spokesman Gregg Losinski said in a telephone interview.

“We haven’t seen it in any species other than whitefish, but it’s not all whitefish,” Losinski said.

At this point, only younger whitefish have been affected, Losinski said. The kills have occurred periodically, and appear to be traced to warm water, he said.

“We’re still trying to get a handle on this,” Losinski said. “We’re at the point now where nights are cooling down and water temps are dropping and it may just cease to be an issue.”

In coming weeks, fisheries biologists will work to assess the extent of the kills.

“Additional samples will be collected and analyzed to verify the preliminary test results, and to look for the presence of this parasite in trout,” a statement said. “Structured sampling this fall will provide an insight into how extensive the kill has been on the South Fork Snake River.”

Winter will not necessarily wipe out the parasite, Losinski said.

“There is a stage of the parasite where it goes into a form like a pod or an egg,” he said. “It can be isolated for a time and still be viable.”

Idaho Game and Fish is asking fishermen to continue reporting sightings of dead whitefish to 208-525-7290.