00:54 900 Yellowstone Bison to be Killed Al Roker explains the controversial bison cull in Yellowstone National Park.

As many as 900 wild bison at Yellowstone National Park will be killed or removed this winter as government agencies continue working to reduce the effects of their annual migration into Montana.

Park officials released their plans on Tuesday to cull the bison herd by 600 to 900 animals by either hunting or capturing the large creatures. It could potentially be the largest hunt in one winter since 2008, and could remove nearly 20 percent of the current population of about 4,900 bison that currently roam the park.

The bison migration occurs annually as the animals move to the lower elevations of Montana, where their historical winter grazing grounds are located. Since the 1980s, about 8,200 park bison have been killed due to fears over the spreading of the animal disease brucellosis.

Yellowstone Superintendent Dan Wenk said in a statement that the National Park Service was uncomfortable with the practice and interested in alternatives, such as sending disease-free animals to other public, private or tribal lands.

"The park would gladly reduce the frequency and magnitude of these operations if migrating bison had access to more habitat outside the park or there was a way to transfer live bison elsewhere," Wenk said.

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Park officials originally proposed killing or removing up to 1,000 of the animals, but they scaled back that goal in part because of opposition from American Indian tribes. Wildlife advocates said they were disappointed by but not surprised at the large number of bison being targeted.

"The brucellosis argument has been discredited so much that they're shifting their argument and saying there are surplus bison that need to be killed," said Dan Brister, executive director of the Buffalo Field Campaign. "You never hear anyone talking about surplus elk or other wildlife species. We really want to see bison treated like elk and other wildlife."

Brister's group has tallied more than 90 bison killed so far this winter by hunters.

Capture and slaughter operations will begin on or after Feb. 15, park spokeswoman Sandra Snell-Dobert said.

Two weeks ago, Montana Gov. Steve Bullock approved a plan that for the first time would allow hundreds of bison to roam year-round on about 400 square miles of primarily public lands just west of Yellowstone. The proposal must be agreed to by the park, other federal agencies and American Indian tribes before it can go into effect.

Whether that will happen this winter is uncertain. But even if it did, it's expected to have little impact on the hunts and slaughters planned for this winter. Most of the population growth has been within herds that migrate into the Gardiner Basin north of the park, not on the west side where Bullock wants more tolerance for the animals.

"The governor's decision wouldn't change the number of animals, the goal of what we're trying to take this year," said Sam Sheppard with Montana Fish, Wildlife and Parks. Reaching that goal is dependent on the severity of the winter. In mild years, fewer bison leave the park to graze in Montana, Sheppard added.

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Under a 2000 agreement between federal agencies and Montana officials, the capture and slaughter of bison has become a regular occurrence. The practice is the subject of harsh criticism from wildlife advocates and members of Congress. A record 1,726 bison were removed from Yellowstone in 2008. Most were slaughtered.

In the past few years, officials have put more emphasis on hunting just outside the park's boundaries, including by members of American Indian tribes that have treaty rights to harvest the animals.

Despite such efforts, the park's herds remain at near-record levels.

Unlike most bison in the U.S., Yellowstone's herds are considered genetically pure, making them prized among biologists and the throngs of tourists who come to the park hoping to see one of the animals up-close.

Some ranchers, landowners and elected officials in Montana have a more negative view. That's because of brucellosis and the potential for bison to compete with cattle for grazing space on public lands outside the park.

Roughly half of the park's bison test positive for exposure to the disease. However, there have been no recorded brucellosis transmissions from bison to cattle.

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