Ranchers outside the park have a low tolerance for bison because of concerns the animals could spread disease and edge out cattle for grass. They've resisted efforts to allow bison to migrate into Montana and roam freely on adjacent public and private lands.
Wildlife advocates contend the capture and slaughter program is unnecessary, citing research that says Yellowstone could support far more bison than the 4,600 counted last summer.
Yellowstone's chief scientist, Dave Hallac, said the slaughter shipments and bison hunt will offset the population growth from bison calves born in the spring. As more die naturally this winter there should be at least a modest overall population decline, he said.
Tribes' participation in the slaughter marked a turnaround from prior years, when American Indians joined with opponents to protest the practice.
The change drew sharp criticism from some wildlife advocates. On Friday, the activist group Buffalo Field Campaign issued a statement accusing the tribes of "providing cover to the shameful actions of the livestock industry and the government agencies."
Jim Stone with the InterTribal Bison Council — which signed an agreement with the park in 2012 to take bison for slaughter — said the council is committed to the animals' conservation. But Stone said the council's leaders decided the only way to alter current practices was from the inside.
"You do it knowing it's a horrible thing to do," he said. "The only way you can change what's going on is if you know what's going on and can control what's going on."
By partnering with federal and state agencies that oversee bison management, Stone said the council is in a better position to push for the population goal to be revisited, for the creation of more bison habitat outside the park and to prevent the killing of young or pregnant animals.
Bison sent to slaughter this winter were captured and temporarily held in corrals along the park's northern border with Montana, near the town of Gardiner. The park refused multiple requests from The Associated Press and others to visit the site.
A protester was arrested along the road leading to the corrals on Thursday after he chained himself to a cement-filled barrel in an attempt to halt the trucks taking bison to slaughter.
Hallac said 500 to 1,000 bison remain near the park boundary, and Friday's announcement does not mean additional bison won't be captured this winter.
In past years, bison captured late in the season — when pregnant females are close to delivery — have been held and released in the spring.
Christian Mackay with the Montana Department of Livestock said bison are allowed to remain in the 70,000-acre Gardiner Basin north of Yellowstone until May 1. But if the animals come into conflict with area residents they will be hazed back toward the park, Mackay said.