BILLINGS, Mont. (AP) — Wildlife advocates have filed a legal petition to stop the capture and mass slaughter of wild bison leaving Yellowstone National Park during their winter migration.

Friends of Animals and the Buffalo Field Campaign filed Monday's petition with agencies including the National Park Service and U.S. Forest Service, which manages lands adjacent to the park. Thousands of Yellowstone bison have been shipped to slaughter in recent decades to keep them from coming into contact with livestock.

Many bison carry the disease brucellosis, which can cause pregnant animals to abort their young. Friends of Animals attorney Mike Harris says the regular removal of so many bison is destroying the animals' gene pool. Yellowstone administrators say 900 bison could be removed during the upcoming winter through hunting, slaughter and for research purposes.