BILLINGS, Mont. - The bison death toll continues to climb for Yellowstone National Park, as park officials say they plan to slaughter an estimated 180 animals captured Monday to prevent the spread of disease.

The bison were captured on the north end of the park near the town of Gardiner - not far from Yellowstone's famed Roosevelt Arch.

More capture operations were planned today near Gardiner and in the West Yellowstone area, where a herd of about 180 bison has lingered outside the park for days, state and federal officials said.

A 2000 agreement between Montana and the federal government says the animals must be captured if they leave the park and enter areas where they could encounter cattle.

Many bison carry the disease brucellosis. It can cause pregnant livestock to abort their calves and suffer other health problems. But there are no recorded cases of a bison-to-cattle transmission - prompting criticism from animal rights groups and members of Congress that the slaughter program is heavy-handed and unnecessary.

The killing of the animals captured Monday will bring to about 470 the number of bison slaughtered this winter. An estimated 130 more have been killed by hunters.

The captured bison had migrated from the deep snow of Yellowstone's high country in search of food at lower elevations.

With heavy snow falling Monday - and the end of winter weather possibly months away - the death toll this year is fast approaching the 1,016 bison killed during the winter of 2004-05.

"All indications are that more are going to come out" of the park, said Christian Mackay with the Montana Department of Livestock.

Mackay said a herd of 180 bison have been lingering outside the park's western boundary. He said several dozen would be captured for slaughter Tuesday, with more to follow in coming days.

A member of a bison advocacy group hoped to block those operations by putting up a tent Monday on a platform inside a group of pens erected to hold the animals.

Buffalo Field Campaign spokesman Mike Mease said the unidentified member of his group wanted to make a statement about the government's brutal measures against the nation's largest herd of wild bison.

"They've got their gunsights aimed at killing an awful lot of buffalo this year," Mease said. "It's time that the buffalo get as much rights as the holy cows do in this land."

Mackay said he was confident the protester would be removed and the pens would be ready to receive bison by this morning.

A representative of Montana's livestock industry said he "applauds" the Park Service's capture and slaughter operations this winter. Montana Stockgrowers Association Vice President Errol Rice said brucellosis remains a "real threat."

"We're still not discounting (the possible) transmission from bison to cattle," Rice said.

Since Montana officials and several federal agencies signed the 2000 bison management agreement, more than 2,300 bison have been killed - mostly through the slaughter program.

Yellowstone's bison herds totaled 4,700 animals at the start of the winter. The animals were exterminated from most of the rest of the country in the 1800s.