Twenty-one more bison were captured Tuesday as the animals migrated out of the snow-packed park to find food at lower elevations in Montana. That brought to more than 300 the number of bison being held in corrals near Gardiner, and livestock agents were driving another 38 toward the site Wednesday.
Conservation groups appealed to Montana Gov. Brian Schweitzer for the animals to be spared, but the governor told The Associated Press that his hands were tied by an agreement with the federal government that sharply limits where Yellowstone bison can roam.
"We can't unilaterally change that" agreement, he said as he also defended the Montana Department of Livestock, which faces criticism for opposing increased leniency for bison.
"They are the agency that has to do something about them once they come to Montana," he said. "We don't want to put our livestock industry at risk."
The impending slaughter stands in stark contrast to recent initiatives to expand bison habitat in some areas outside the park. Those efforts have been frustrated by the animals' tendency to wander and political opposition from Montana's livestock industry and its supporters.
Roughly half of the park's bison, also known as buffalo, have been exposed to brucellosis, which causes cattle, bison and some other animals to prematurely abort their young.
With more bison expected to exit Yellowstone due to a harsh winter - and the corrals capable of holding only 400 animals - a park spokesman also indicated that even bison that test negative for the brucellosis might not be released as originally planned.
"There are a lot more bison in the park than we could conceivably hold," Yellowstone spokesman Al Nash said. "The question that no one can answer is what will the rest of the winter see in terms of bison movement."
Fears that bison leaving Yellowstone could infect Montana livestock have driven a government-sponsored capture and slaughter program that has killed about 3,800 bison since 2000.
However, no cattle-to-bison transmissions have been recorded, and the number of cattle on ranches surrounding Yellowstone has dropped sharply in recent years.
"It's more and more clear that it's the elk that transmit brucellosis, yet we're stomping all over these buffalo," said Julia Page, a retired rafting company owner from Gardiner and critic of the government slaughter program. "This is a heck of a way to treat these last remaining bison from the vast herds that used to roam the Great Plains."
But livestock producers want the state and federal agencies that oversee bison to hold the line. Forty-seven-year-old rancher Nelson Story in Montana's Paradise Valley said until brucellosis is eradicated from the animals they will remain a threat to his livelihood.
"It's a tough one, but I don't have any answers except to manage them. That's what we have to do with our cattle. That's what the park should do with (bison)," said Story, who lives about 30 miles downstream from the park, just west of the Yellowstone River.
Even with a major slaughter this winter, he added, "they'll still end up with 2,000 to 3,000 (bison), which is a lot."
Researchers estimate that tens of millions of bison once roamed North America, from Canada to northern Mexico. Early European settlers hunted the species to near-extinction in the late 1800s, encouraged by railroad companies that did not want the vast herds blocking new tracks that were being laid across the West.
Yellowstone contains two of the most genetically pure herds left in the world, numbering a combined 3,900 animals last year.
Two pending efforts to find new territory for the park's bison have yielded little success to date. Last month, 25 bison that tested negative for disease were herded onto a 2,500-acre patch of the Gallatin National Forest north of Yellowstone - an area where they had not been allowed in decades. Only 10 of the animals remain after one was shot, another went missing and 13 were hazed back to the park after repeatedly leaving the Forest Service land.
A second initiative to relocate dozens of disease-free bison to other parts of Montana has made little headway. Those bison are now in a government-run quarantine compound north of the park and Montana wildlife officials have proposed moving them onto state-owned land.
However, several bills pending before the Montana Legislature would impose new restrictions on the program that could make it impossible to move the animals.