CASPER, Wyo. (AP) – A man who chained himself to a concrete-filled barrel in an effort to prevent trucks from hauling Yellowstone National Park bison to slaughter has pleaded guilty.

The Casper Star-Tribune reports Joshua Rivera, of Clifton, Colo., pleaded guilty Monday to trespassing and interfering with a government employee. He and another man were arrested Friday after they chained themselves to 55-gallon (208-liter) barrels in an attempt to block a road in Yellowstone.

The Bozeman Daily Chronicle reported the men belong to the group Wild Buffalo Defense. The group's spokesman, Monty Slate, said the protest delayed a shipment of bison for a few hours before another path was cleared.

Bison are shipped to slaughter each year as part of a population management plan for the park.

Rivera's co-defendant has pleaded not guilty.