Upwards of 900 bison are to be culled from Yellowstone's herds/NPS file

Work is underway to cull between 600 and 900 bison from Yellowstone National Park, where an interagency plan calls for the park's herds to be capped around 5,000 animals.

The operations began February 13, though the park didn't announce them until Tuesday.

Under the work, bison capture and shipping operations begin each winter when bison migrate from the interior of the park into the Gardiner (Montana) Basin and may continue through late March.

Bison operations at Stephens Creek are directed as part of the Interagency Bison Management Plan, whose partners include federal, state and tribal groups. 

The population will be reduced using three methods: 1) public and tribal hunting outside the park; 2) capturing bison near the park boundary and then transferring them to Native American tribes for processing and distribution of meat and hides to their members; and 3) the Bison Conservation Transfer Program that results in brucellosis-free bison being moved to tribal lands.

Culling of Yellowstone's bison herds has long been controversial, as some believe the park should be allowed to hold a greater number of the iconic animals. But their migratory habit of heading out of Yellowstone to lower elevations in Montana to calf is opposed by many Montana politicians, who fear bison will transmit brucellosis to cattle herds in the state. However, the National Academies of Science has weighed in on that fear with a report that held that elk, not bison, spread the disease to domestic livestock.

This year's culling operations come as the park is embarking on an environmental impact study to see if changes are needed in its bison management. Since the Interagency Bison Management Plan was adopted in 2000, "some of the premises regarding disease transmission in the initial plan were incorrect or changed over time," according to park staff. "In addition, fewer cattle are near the park and federal and state disease regulators have taken steps to lessen the economic impacts of brucellosis outbreaks in cattle."

At the same time, the U.S. Fish and Wildlife Service has been ordered by a federal judge to revisit a lawsuit that argues that Yellowstone bison deserve Endangered Species Act protections and that there are two genetically distinct herds in the park and so the park's bison herds should include at least 6,000 individuals.

Located in the northern section of the park near Gardiner, Montana, the Stephens Creek administrative area includes park corral operations, equipment storage, a native plant nursery, and the bison capture and quarantine facilities. The administrative area is closed to the public year-round.