Bison in Yellowstone
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At least 100 Yellowstone National Park bison have been trapped in corrals near the town of Gardiner since Saturday as the National Park Service begins its grim winter work of shipping some of the wild animals to slaughter.

One large cohort of bison annually migrates north from the park in winter in search of forage as deep snow covers the interior. Another band migrates along the Madison River on the west side of the park exiting near West Yellowstone.

"We've been seeing them migrate through the month of December," said Morgan Warthin, park spokeswoman. "We had anticipated this given the predictions of wintry weather. On the plateau there's been a lot of snow and cold."

Animals trapped by the Park Service are shipped to slaughter facilities for processing with the meat distributed to designated tribes.

Hunt zone

When the animals cross the park boundary into Montana near either entrance they become fair game for licensed state hunters as well as tribal members with treaty hunting rights. So far this winter, tribal hunters have reported taking 80 bison, 36 from the west side and 44 from the north, while state hunters have harvested 19 bison, only six of which came from the north side, according to the Montana Department of Fish, Wildlife and Parks.

Last year the Park Service delayed its bison trapping until Feb. 15 to see if tribal hunters would be more successful. As a result, hunters killed 384 bison, rangers killed 18 wounded animals and 101 were sent to slaughter, short of the park's original goal of removing 600 to 900 animals through hunting and trapping.

"There's a balance between hunting and population management," said Sam Sheppard, FWP's Region 3 supervisor in Bozeman. "We fully support operation of the trap. They've always done a good job of allowing enough bison to pass by ... for tribal and state hunters."

Warthin said park staff wait until bison have moved back from the state boundary — often after being pressured by hunters — before herding them into the Stephens Creek Bison Capture Facility's corrals.

Winter removal

The Park Service, in concert with cooperating agencies, is aiming to reduce the northern herd that migrates to the Gardiner Basin by 900 to 1,400 animals this winter. The northern herd is estimated at about 4,000 animals. The entire park's bison population is close to 5,500, an 11 percent increase from 2015.

That rate of increase is much higher though — 24 percent — after adjusting for bison that were removed from the population through culling and harvest, according to the park's own analysis.

"This increase was substantially higher than the 15-year average of 14 percent and suggests that the bison population is undergoing exponential growth — characteristic of a population that has not reached carrying capacity," according to an annual report by park biologists. "This increase also suggests that the Park Service underestimated the number of bison in the population last year."

Yellowstone National Park's growing bison population has become a point of contention between the National Park Service and the state of Montana, which borders the park to the north and west. Because the bison migrate out of the park and carry a disease that can cause pregnant cattle to abort, Montana has pushed the Park Service to reduce Yellowstone's total bison population to less than 3,500. Yellowstone officials have calculated that a herd of 3,000 to 5,000 is necessary for the animals to maintain genetic diversity.

Controversy

This year's bison reduction efforts come on the heels of the animal being designated a national mammal of the United States because it is a historical symbol of the U.S. and was once "integrally linked with the economic and spiritual lives of many Indian tribes through trade and sacred ceremonies," according to the text of the National Bison Legacy Act.