BILLINGS, Mont. — Montana wildlife officials closed a 183-mile stretch of the Yellowstone River on Friday to fishing, rafting and other activities to prevent the spread of a parasite that is believed to have killed tens of thousands of fish.

A fishing guide who runs a business along the river said the move could be catastrophic to the area’s sizable outdoor industry, which depends heavily on the business its gets during the busy summer season.

The closure is needed to stop the spread of the parasite that causes proliferative kidney disease and to protect the fishery and the outdoor economy it sustains, said Fish, Wildlife and Parks officials.

“This kill is unprecedented in magnitude. We haven’t seen something like this in Montana,” agency spokeswoman Andrea Jones said.

The closure extends from Yellowstone National Park’s northern boundary to the city of Laurel, along with tributaries in those areas.

Over the past week, wildlife officials have documented more than 2,000 dead mountain whitefish and believe the total number killed is in the tens of thousands. Reports are emerging that the die-off is beginning to affect some rainbow and Yellowstone cutthroat trout – species crucial to the area’s fishing industry.

Fishing, wading, floating, boating and all other activities are not allowed on the river until further notice. Numerous fly fishing outfitters and rafting companies operate in the closed stretch of river.

Fish, Wildlife and Parks Director Jeff Hagener acknowledged that the move would have a significant effect on those who use the Yellowstone. He said the agency had to balance that against the risk to the fishery, given that recreational activities disturb fish and exacerbate the effects of the disease.

Fishing guide Dan Gigone with the Sweetwater Fly Shop in Livingston said one of his guides reported seeing hundreds of dead trout Thursday. He called the closure catastrophic but said he would not fight the move.

“We have trips on the books through September,” Gigone said. “It’s definitely a big part of the Livingston and area economy. But we need to protect the resources as best we can for future years.”

Gov. Steve Bullock said a threat to Montana’s fish populations is a threat to the state’s outdoor economy and the jobs it sustains.

The wildlife agency will monitor the river and lift the closure when stream conditions such as flow and temperature improve and fish mortality ceases.

It has set up two decontamination stations near the affected area to try to reduce the chance of equipment spreading the parasite to other rivers. The agency is asking the public to properly clean all equipment prior to moving between bodies of water.

The closure comes just days ahead of a planned celebration at Yellowstone National Park to commemorate the 100th anniversary of the National Park Service. Interior Secretary Sally Jewell is slated to speak next Thursday as part of the event.

Yellowstone spokeswoman Charissa Reid said park fisheries scientists were reviewing the closure announcement and deciding how the park would react.