A parasite responsible for killing thousands of fishes prompted state wildlife officials to close for an indefinite period of time the popular Yellowstone River in Montana and hundreds of miles of other waterways on Friday, Aug. 19.

The state's Department of Fish, Wildlife and Parks barred fishing, rafting and other activities, a move that according to some fishing guides and rafting operators with business interests along the river could impact the areas' outdoor industry.

Officials, however, said that the action is important to prevent the fish-killing parasite from spreading and causing further damage. The closure is aimed at limiting the spread of the parasite to adjacent bodies of water through boats, waders, tubes and other human contacts as well as to minimize further damage to fish populations.

The closure may also last for months if there would be no improvements in the river's condition and the fish keep dying.

"A threat to the health of Montana's fish populations is a threat to Montana's entire outdoor economy and the tens of thousands of jobs it sustains," said Gov. Steve Bullock. "Our state cannot afford this infectious disease to spread to other streams and rivers and it's my responsibility to do everything we can to stop this threat in its tracks and protect Montana jobs and livelihoods."

About 4,000 dead fishes have been counted by Friday but officials estimate the number of dead fishes including those that sank to the bottom of the river could be in tens of thousands.

The mountain whitefish, a native game species, has been particularly affected but there are reports that the rainbow trout and Yellowstone cutthroat trout have also been affected by the die-off and these species are considered crucial in the fishing industry.

Officials said that low water levels and warm temperature make the problem worse by adding more to the stress already faced by affected fish populations.

The parasite responsible for the die-off is not native to the area. This means that it was likely introduced here by people though contaminated fishing waders, boat or other means. It may have also been transported by birds from another waterway.

The wildlife agency already set up two decontamination stations to thwart possibilities of equipment spreading the fish-killing parasite to other rivers. The agency also asked the public to properly clean their equipment before they move between bodies of water to prevent further spread of the disease.