IDAHO FALLS, Idaho — Biologists have confirmed thousands of dead whitefish on the South Fork of the Snake River in southeastern Idaho, the Idaho Department of Fish and Game said Friday.

Biologists suspect it’s an outbreak of proliferative kidney disease, the same pathogen believed responsible for killing thousands of whitefish in the Yellowstone River last month and led to the temporarily closure of that acclaimed river in Montana.

The disease was also responsible for whitefish die-offs on the South Fork of the Snake River in 2012, Idaho officials said.

The agency said the outbreak is unlikely to affect the river’s overall fish population.

“Because we monitor these fisheries so intensively, we can say with confidence that trout and Whitefish populations have continued to do very well even after we confirmed that PKD was present,” said regional fisheries manager Dan Garren.

Regional fisheries biologist Brett High recently boated a stretch of the river and saw thousands of dead whitefish in eddies and along the banks, mostly in the upper river.

High said they also “confirmed that healthy whitefish and trout are still very abundant, and only a small percentage of fish showed signs of disease.”

Biologists won’t know for one or two weeks whether the South Fork fish kill is due to the same disease until pathologists can test samples of sick fish but Garren suspects it’s the case.

On Aug. 19, Montana wildlife officials closed a 183-mile stretch of the Yellowstone River and its tributaries to prevent the spread of a deadly parasite believed to have killed tens of thousands of whitefish.

Montana has been since re-opened stretches of the river. On Friday, it opened a roughly 4-mile stretch of the river to fishing, boating and other activities after officials reported not finding any more dead mountain whitefish there in the last week. The section is at the south end of the Paradise Valley.

All recreational activity remains prohibited along a 47-mile stretch of the Yellowstone from Point of Rocks to the U.S. Highway 89 bridge east of Livingston.