BILLINGS, Mont. — Large numbers of migrating Yellowstone National Park bison are likely to face slaughter for at least the next couple of winters as officials weigh changes to a 15-year-old agreement that drives the practice, the park's superintendent said.

Most of the bison removed from America's first national park are captured and sent to slaughter over concerns they may transmit the disease brucellosis to Montana livestock.

Yellowstone Superintendent Dan Wenk said he's hopeful the slaughters eventually will be phased out and replaced by hunting. However, that's not feasible in the short term with a near-record population of about 4,900 park bison, Wenk told The Associated Press.

The park has agreed to reduce bison numbers as a way to minimize conflicts with landowners in Montana. It plans to cull 600 to 900 of the animals this winter using a combination of hunting and shipments to slaughter.

"Under current population numbers, we will have to capture and ship bison to slaughter," Wenk said. "That's just the world we're living in today. It might not be the world we're living in three years from now."

No bison-to-cattle transmissions of brucellosis have been recorded in the wild, which Wenk said was one gauge of success for a 2000 bison management agreement between federal agencies and Montana officials. Brucellosis was introduced into Yellowstone's bison herds by domestic livestock brought into the region by early settlers.

An overhaul of the 2000 agreement that began last year is expected to be completed by late 2017.

Yellowstone's bison herds numbered just a few dozen animals in the early 1900s. They recovered dramatically over the past century, and today the park has some of the only genetically pure bison left in the world.

Yet that conservation success in recent years has become overshadowed by the slaughters used to control bison numbers, Montana State University wildlife researcher Robert Garrott said. Sporting groups, wildlife advocates, members of Congress and the Government Accountability Office have criticized state and park officials for not coming up with alternatives, but Garrott said they have few viable options.

"It's a numbers game: How many can we accommodate?" Garrott said. "The source population every year will produce 6 to 10 percent (more bison) that will need a new home. ... Despite the fact that bison are an iconic symbol of the United States and North America once had 30 to 60 million of them, our society has said there is no place we're willing to accept them."

Bison generally are protected from being killed inside the park. They run into problems when they cross Yellowstone's border with Montana in winter to seek out food at lower elevations.

Hunters, many of them from American Indian tribes with treaty rights in the Yellowstone region, will wait just over that border for the chance to harvest one of the large animals. Also on the border are holding pens that the park uses to capture and hold bison until they can be shipped to slaughter.

Montana officials have softened their attitude against bison in recent years as public opposition to the capture and slaughter program has grown. Former Gov. Brian Schweitzer took the first steps in 2011, allowing large numbers of bison to roam free during winter in the Gardiner area just north of Yellowstone.

His successor, fellow Democrat Gov. Steve Bullock, last month took another step to increase tolerance for bison. Bullock proposed allowing for the first time bison to remain in parts of Montana year-round.

"Progress is being made on behalf of the bison," Wenk said. "It's not a short-term process."