The Antelope Island bison herd and the island remained in private hands until 1969 when the northern of the island were purchased by the state of Utah. The southern end of the island was acquired in 1981, granting the entire herd protected status on Antelope Island State Park. In 1986, park rangers saw the need to begin controlling the bison population to prevent overgrazing and disease. The first roundup was held in 1987 and it has since become an annual event that brings in revenue by way of the sale of excess bison and tourist dollars brought in by spectators. Every year, in late October, all the bison on the island are herded to a central area in a "Great Buffalo Roundup" and sent briefly into pens where they are examined, weighed and vaccinated, whereupon decisions are made on culling and selecting breeding stock. The majority of the bison are then released within a few days, and are again allowed to roam free on the island. The Antelope Island bison herd fluctuates in size between 550 and 700 head, and is one of the largest publicly owned bison herds in the nation. The reason for the variability of the size of the herd is that the bison produce approximately 150 to 200 calves annually, and since this is prime prairie habitat for bison, with no significant predators, the herd can increase by up to half every year. It has been determined that 700 head is near the maximum preferred carrying capacity for bison on the island, so the excess bison need to be culled. Bison from this island are often sent to other herd locations around North America due to their genetic isolation, some unique genetic markers contained in the population, and because of their disease-free condition. Some bison are also purchased at the pen site in a yearly public auction, and are taken as meat or breeding stock for commercial farms in other parts of the world.
