BILLINGS, Mont. — 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.