BILLINGS, Mont. (AP) — Yellowstone National Park has transferred 55 wild bison to a Montana Indian reservation under a program that aims to establish new disease-free herds of the animals, park and tribal officials said Friday.

The male bison, also known as buffalo, were transferred in trailers and released onto the Fort Peck Reservation, home of the Assiniboine and Sioux tribes. They had been captured in March 2018 and held in quarantine to ensure they don't carry the disease brucellosis.

The relocation program is part of an effort to conserve a species that once roamed North America by the millions. Officials also want to reduce the government-sponsored killing of Yellowstone bison over disease concerns.

More than 10,000 Yellowstone bison were captured and slaughtered or killed by hunters over the past several decades. The animals try to leave the park during winter in search of food at lower elevations.

Officials plan to capture more bison this winter to keep the park's population of about 4,500 bison from growing. They also want to expand the quarantine program, Yellowstone Superintendent Cam Sholly said. But officials say that will require more quarantine facilities because the corrals now in use have a limited capacity.

Fort Peck officials have been trying to increase the size of their bison herd, which now numbers almost 400 animals with the latest additions.

"The return of the bison is a return of our culture," Tribal Chairman Floyd Azure said in a statement.

The 55 newly arrived animals will be held separately for another year as part of disease prevention efforts. Most of the animals eventually will be sent to other tribes that want to grow their existing herds or establish new ones, said Robert Magnan with the Fort Peck Fish and Game Department.

In June, Fort Peck transferred five of its bison to the Wind River Reservation in central Wyoming. It has also sent animals to Montana's Fort Belknap Reservation.

Brucellosis can cause animals to abort their young. Its presence in Yellowstone-area elk and bison herds traces back to their exposure to the infected livestock of European settlers.

The disease has since been largely eradicated from domestic livestock. Ranchers in the past have opposed the transfer of Yellowstone bison, but state and federal officials have tried to quiet those concerns by setting up a rigorous testing program.