Yellowstone National Park has begun shipping wild bison for slaughter as part of a plan to reduce the park's population by as many as 900 animals this winter.

On Wednesday and Thursday, more than 150 bison captured near the park's northern border with Montana were removed from holding, loaded onto trailers and shipped off, according to the Buffalo Field Campaign, a wildlife advocacy group.

There were 4,900 bison in the park last summer. For more than two decades, officials have tried to curb the animals' winter migration into Montana to guard against potential disease transmission to livestock.

Park spokesman Al Nash confirmed the shipments of recently-captured animals for slaughter. But he said the park no longer plans to offer timely updates on how many bison are captured and shipped.

The information instead will be posted on a bi-weekly basis to an interagency bison management website. Nash said information on how many animals are being held also would be released on a bi-weekly basis.

"There will be additional bison captured in coming weeks but it's impossible to predict when and how many," Nash said.

The bison taken Wednesday were turned over to American Indian tribes. Their meat will be distributed to tribal members.

Stephany Seay with the Buffalo Field Campaign said group members have been in the field counting bison being put into Yellowstone's Stephens Creek capture facility over the past two weeks. They also are tracking the number being shipped out, Seay said.

Seay said the park's unwillingness to share that information shows "they're trying to hide it."

"I think they're ashamed of what they're doing," she said.

Park spokesman Nash responded that "giving people numbers on a regular basis is not hiding."

"We have promised to provide regular reporting of our bison management activities," he said.

The first such report was released Thursday. It covers a two-week period ending Jan. 16 and did not have any information on this week's shipments.

Seay estimated that about 80 bison remained at Stephens Creek on Thursday.

At least 73 bison have been killed by state-licensed and tribal hunters after crossing the park's border, according to wildlife officials.

Montana has tried to increase hunting opportunities in recent years as an alternative to the often-criticized shipments to slaughter.

Nash said upcoming bison captures would be planned to minimize impacts on hunters and ensure opportunities for hunting outside the park.

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