After the park's herd gained new protections and gradually rebounded, Yellowstone administrators sought to keep bison numbers in check by slaughtering the animals or shipping them elsewhere, said Keith Aune a bison biologist with the Wildlife Conservation Society.
Those shipments continued into the 1960s, ending after the park adopted a policy of regulation in which bison numbers would be controlled by natural deaths.
But the park's herds soon began spilling over its border, and thousands of those migrating animals have been captured and shipped to slaughter over the past decade to guard against livestock being infected by brucellosis. The disease can cause pregnant animals to miscarry. Ranches that suffer infections are subject to lengthy quarantines.
Schweitzer has said he will allow the state to transfer 66 disease-free Yellowstone bison to eastern Montana's Fort Peck and Fort Belknap Indian reservations.
Another 143 Yellowstone bison are being held at a ranch near Bozeman. Those are the animals that Salazar is suggesting could be sent to federal lands elsewhere. Before being put on the Turner ranch for temporary holding, the bison spent several years in a government-run quarantine near the park to ensure they were brucellosis-free.
The quarantine compound is expected to be used beginning next year to study the effectiveness of chemical contraceptives on bison. Salazar said in his letter that he has asked the National Park Service to evaluate whether a new quarantine facility should be built.
Aune said the relocation of disease-free bison captured from Yellowstone has potential to help the species recover in other parts of the country.
Garrit Voggesser, director of tribal partnerships for the National Wildlife Federation, said that could take several years to arrange, and that his organization would not endorse specific proposals.
Associated Press writer Steven Paulson contributed to this story from Denver.