SALT LAKE CITY (AP) — Once a year, hundreds of horseback riders gather on an island in Utah's Great Salt Lake to nudge bison toward holding pens in a roundup of one of America's largest and oldest public herds.

Bison have occupied Antelope Island, about 25 miles (80 kilometers) northwest of Salt Lake City, for more than 120 years, after a dozen of them were brought there by a homesteader. Today, several hundred bison roam the island, which is now a state park.

The animals are rounded up each fall so they can receive health checkups and vaccinations and be affixed with a small external computer...