Tue Mar 17, 2015 3:31am GMT

Managers of Yellowstone National Park bison said Monday they are considering changes to policies that have seen thousands of purebred buffalo from the nation's last wild herd bison killed since 2000 to stem transmission of a disease to cattle.

Yellowstone bison have been at the center of a bitter debate over a management plan crafted 15 years ago that seeks to keep the population at 3,000 and permits killing of many buffalo that migrate from the park to bordering Montana for winter forage.

Montana cattlemen fear wandering bison exposed to brucellosis, a disease first brought to the park by domestic livestock, will infect their cows, causing them to abort their young and endangering the state's brucellosis-free status.

But buffalo advocates and those who promote Montana tourism argue the park's bison should be allowed to roam without being marked for death as the herd represents the nation's rich wildlife heritage and is a top draw for the roughly 3 million annual visitors to Yellowstone.

Government and tribal managers are considering six alternative management options submitted by agencies including the state of Montana and the National Park Service.

The alternatives are open for public comment until June 15. The options range from one in which Yellowstone would be home to thousands more bison without severe culling to a plan that would likely see many more of the massive, hump-shouldered creatures targeted for death.

Buffalo once thundered by the tens of millions west of the Mississippi, but extermination campaigns in the late 19th century pushed them to the edge of extinction.

The population at the park last year was estimated at 4,900, 1,900 more than allowed under the management plan agreed upon by Native American tribes and government agencies – including the park and Montana Department of Livestock – that oversee the herd.

The roughly 700 bison that have strayed into Montana this winter have been killed, mostly through transfer to tribes which have the animals slaughtered but also through hunting outside the boundaries of Yellowstone.

The culling has triggered protests by wildlife advocates but has been endorsed by livestock organizations like the Montana Stockgrowers Association and by leaders of some American Indian tribes.

The Greater Yellowstone Coalition, a conservation group, is arguing for expanded year-round habitat for bison outside the park where the animals would not be subjected to seasonal hazing or capture as has been practiced for more than a decade.

The Montana Stockgrowers Association did not immediately respond to a request for comment.