Park authorities initiated an intensive bear management program in 1970. The objectives were restoring the grizzly bear and black bear populations to subsistence on natural forage, and reducing bear-caused injuries to humans. As part of the bear management program, regulations prohibiting the feeding of bears were strictly enforced. As the bears became more desperate for food and more aggressive, more of them were encountered in camping areas. Over a hundred grizzly bears had to be euthanized in the next several years, putting the park's bear population on the brink of extinction. On July 28, 1975, under the authority of the Endangered Species Act, the United States Fish and Wildlife Service listed the grizzly bear as a threatened species in the lower 48 states.