BILLINGS, Mont. — Yellowstone National Park's bison herd has bounced back to near-record levels following the slaughter of more than 1,000 animals two winters ago to guard against the spread of disease, park administrators said Monday.

An estimated 4,700 bison, also known as buffalo, now roam the park — up from 3,600 last year. They make up the largest bison herd in the world.

During the winter of 2005-2006, after the population hit a record 4,900 animals, more than 1,000 bison migrating outside the park in search of food were captured and killed to prevent the spread of brucellosis. That disease can cause female bison and cattle to abort their calves and is considered endemic in the Yellowstone herd.

Following a public outcry and congressional hearing over the slaughter, only two bison were killed last winter. Park administrators and the Montana Department of Livestock instead concentrated on hazing migrant animals back into the park, to keep them from interacting with cattle.

Whether the same dilemma will be faced with this year's larger population remains to be seen, said Glenn Plumb, Yellowstone's chief of natural resources.

"The implication every year is different, it seems, depending on the winter weather and the circumstances for the winter," Plumb said. "That could include hazing and removal of bison if necessary."

Christian Mackay with the Montana Department of Livestock said a lingering drought and higher population have set the stage for a large bison migration if the winter proves severe.

Any decision on what to do with animals that leave the park this winter will be driven by a bison management plan involving several state and federal agencies. Mackay said the agencies involved in the plan were in discussions over whether to repeat last year's hazing strategy or return to an emphasis on capture and slaughter of bison.

He said a decision could be made within the next few weeks.

Amy McNamara with the Greater Yellowstone Coalition said the brucellosis issue goes beyond population levels and is likely to hang over the Yellowstone herd until the disease can be eradicated.

"In bad winters, they're going to seek out forage (at lower elevations), and they're going to do this whether there's 400 or 4,000 bison," she said.

Bison once numbered in the millions and were found across much of the western United States. They were all but wiped out by European settlers in the 1800s. By the turn of the last century — when bison restoration began in Yellowstone — only about two dozen remained in the park.