GARDINER, Mont. (AP) — Yellowstone National Park plans to start shipping many of its famous wild bison to slaughter Wednesday to drive down the size of the park's herds and respond to concerns by the livestock industry over a disease carried by the animals.

The park's actions are driven by a 2000 agreement with state officials that requires it to control its bison herds. The meat will be distributed to American Indian tribes.

About 150 bison have been captured this winter attempting to migrate out of the park in search of food at lower elevations in Montana. Ranchers in the area worry about bison infected with brucellosis, a disease that can cause cattle to abort their young.

There have been no recorded bison-to-cattle transmissions of brucellosis, and critics say the slaughters are unnecessary.

Tens of millions of bison, also known as buffalo, once roamed North America. Commercial hunting drove the species to near-extinction in the late 1800s before conservationists — including former President Theodore Roosevelt — intervened when only dozens were left.

Yellowstone is home to one of the few remaining wild populations. Millions of tourists visit the park each year to see the animals, a top attraction at the nation's first national park. The animals also are the symbol of the National Park Service.

Since the 1980s, worry over brucellosis has prompted the killing of about 8,200 park bison, most of them sent to slaughter. In recent years, officials have tried to emphasize public hunts that occur just outside the park boundary.

The park had 4,900 bison at last count, well above the 3,000 dictated under the agreement.

During the past decade, Montana Gov. Steve Bullock and his predecessor, former Gov. Brian Schweitzer, moved to allow bison into areas adjacent to the park. Yellowstone administrators have supported those efforts, but they say they are bound under the 2000 agreement to keep the bison herds in check.

Alternatives — such as transferring some Yellowstone bison to lands outside the park — are under consideration but unlikely to take effect soon. Park workers could hold back from slaughter at least some captured bison calves for potential future placement elsewhere if the opportunity arises, park spokeswoman Amy Bartlett said.

Captured bison are tested for exposure to brucellosis before they are consigned to slaughter. That's for research purposes only, and animals that test negative are not spared.