The U.S. National Park Service plans to kill as many as 900 bison this winter in an attempt to limit the herd’s annual migration up the Yellowstone River valley and into traditional winter range in Montana.

Ranchers fear the bison could infect their cattle with brucellosis, a bacterial disease.

There were 4,900 bison in Yellowstone National Park as of summer 2015.

Yellowstone National Park’s annual culling of the last wild herd of bison, or buffalo, in the United States has been controversial since it began in 2000, and now it has prompted a First Amendment battle between National Park Service officials and reporters determined to document the grim spectacle.

The U.S. National Park Service originally proposed to kill as many as 1,000 bison this winter in an attempt to limit the herd’s annual migration up the Yellowstone River valley and into traditional winter range in Montana, where ranchers fear the bison could infect their cattle with brucellosis, a bacterial disease that can cause miscarriages.

Brucellosis is a livestock disease first introduced in the US by cattle brought over from Europe. Bison are not the only species that can carry the disease — and in fact, according to the Buffalo Field Campaign (BFC), recent studies of brucellosis transmission to cattle in the Yellowstone ecosystem found that elk were the more likely source.

There has never been a documented case of domestic livestock contracting the disease from bison in the wild, the BFC says.

After consulting with state and federal agencies as well as local Native American tribes, the Park Service plans to begin trapping and facilitating the killing of up to 900 bison starting February 15, EcoWatch reports. The parts of the park where the trapping operations take place will be off limits to the public, including media.

“It’s ironic that to benefit Montana ranchers grazing their cattle — an invasive species — Yellowstone Park has agreed to facilitate the capture and killing of 900 American bison, an iconic, native species,” said Justin Marceau, a law professor and attorney with the Animal Legal Defense Fund.

Marceau is part of a group of lawyers and law professors who have filed a federal suit on behalf of journalist Christopher Ketcham and activist Stephany Seay, arguing that by denying access to Yellowstone National Park’s trapping operations, park officials are violating the First Amendment.

“I want full access to the operations so I can effectively report on the issue,” Ketcham said, according to EcoWatch. “I want to be able to see the suffering of these animals up close and thus bring readers up close.”

Calves and females are being targeted in order to limit the herd’s reproductive capabilities. “Hunters, including from tribes with treaty rights in the Yellowstone area, are anticipated to kill more than 300 of the animals. Others would be captured for slaughter or research purposes,” per the Associated Press.

There were 4,900 bison in Yellowstone National Park as of summer 2015. More than 6,300 have been slaughtered and almost 1,900 killed by hunters since the 1980s. Officials trapped 737 animals last year, falling short of their goal of up to 900.

“Through the legal agreement the National Park Service has to do this,” Yellowstone spokesperson Sandy Snell-Dobert said, according to the AP. “If there was more tolerance north of the park in Montana for wildlife, particularly bison as well as other wildlife, to travel outside the park boundaries, it wouldn’t be an issue.”