A majority of the popular Yellowstone River in Montana has been closed off after thousands of fish have been mysteriously killed in the area.

According to Slash Gear, Montana’s Fish, Wildlife and Parks department announced that last week, more than 2,000 mountain whitefish have died in the area because of a deadly parasite. However, the number of affected fish is still growing as experts estimate that other regions in the Yellowstone River are also affected.

OC Register reports that it's not just mountain whitefish that have been affected. The parasite has also affected populations of rainbow trout and Yellowstone cutthroat trout, which are both crucial players in the area’s fishing industry.

“This kill is unprecedented in magnitude. We haven’t seen something like this in Montana,” said Andrea Jones, spokeswoman from the Fish, Wildlife and Parks department.

In a news release from the Government of Montana, test results showed that the culprit of the mass death in Yellowstone River is the Proliferative Kidney Disease. It is caused by a powerful parasite that originally thrives in the U.S., Canada and Europe but have been isolated in Montana in the past two decades.

The disease could have been triggered by certain “stressors” such as high temperature, low water flow and human recreational activity. The parasite can affect and transfer to other waterways and parts of the river by transporting it via boats and other items that have been submerged or have touched the parasite-infected waters.

The said parasite is not dangerous to humans but poses a risk to the economy. As of today, officials have closed a 183-mile stretch of Yellowstone River to prevent further deaths and keep the parasite from spreading. Water activities such as fishing, boating, wading and floating have also been prohibited.

“We recognize that this decision will have a significant impact on many people. However, we must act to protect this public resource for present and future generations,” said Fish, Wildlife and Parks Director Jeff Hagener.