BILLINGS, Mont. - Yellowstone National Park officials are holding all pregnant bison captured at the Stephens Creek corral to lessen any chance that the bison will transmit brucellosis to nearby cattle.

"We started to hold bison knowing that we get green-up in the middle of the month," said Al Nash, the park's spokesman. "But there certainly are no signs that we're going to have an early spring."

Park workers are on site at the corral 24 hours a day to isolate a pregnant bison when it gives birth or aborts and to clean up any tissue and fluids, which carry the greatest risk of disease transmission.

Brucellosis can cause cattle to abort. Cattle graze on private land only a mile away from the Stephens Creek facility on the north end of the park. There has been no documented instance of brucellosis transmission from a bison to cattle in the wild.

Buffalo Field Campaign, a group that opposes the capture and slaughter of bison, complained that keeping pregnant bison in the corrals could lead to more abortions.

"It is more likely that pregnant females would abort as a consequence of stress, handling and confinement in pens at Stephens Creek, operated by Yellowstone National Park," Darrell Geist, of Buffalo Field Campaign, said in an e-mail. "The abortions you seek to minimize may very well be caused by your own actions."

Keeping all pregnant bison in the Stephens Creek corral has nearly maxed out the pen's holding capacity. There are 187 bison in the corral, which has a capacity of 200 when animals are held for a few weeks. Up to 400 bison can be held in the corral but for a much shorter time period.

The situation could force the Park Service to ship more bison to slaughter, including nonpregnant cows that haven't been tested for exposure to brucellosis.

The decision comes as the slaughter numbers continue to rise, making this winter the worst in terms of the number of bison sent to slaughter. As of Thursday, 1,341 bison have been shipped to slaughter; another three died in captivity. The previous record was 1,084 sent to slaughter in the winter of 1996-97. Meat from bison shipped to slaughter is distributed to food-assistance programs.

The total number of bison killed so far this winter is 1,510, or almost one-third of the park's total bison population. The total includes the 166 bison that were killed by tribal and Montana hunters. With winter kill sure to have taken some bison inside the park, Nash predicted a drop in the park's total bison population below 3,000 animals. Should the bison population drop to 2,300 animals, the Interagency Bison Management Plan, under which the park operates, would require the cooperating agencies to try nonlethal ways to manage the bison.

Nash said bison have steadily streamed from the park in groups of about 20 to 30.

"Through all of this there has been no one single push of a large number of animals," he said.

Because of the steady movement, the park has conducted 35 capture operations on the north side of the park and more than 90 hazing operations.