Black bears regularly inhabit Stockton, Sand and Oak islands, and, due to their mobility, may be found on just about any of the Apostle Islands.
Black bears (Ursus americanus) are the most numerous and widespread North American bear. Though the black color phase is most common, "black" bears can also be various shades of brown or even white. Adult males usually weigh from 250 to 400 pounds, while females range from 150 to 225 pounds. They are omnivorous and depend greatly on their sense of smell to locate food. Bears can run as fast as 30 miles per hour. Their curved claws enable them to climb trees. They are also powerful swimmers, enabling them to utilize habitat available on the islands.
Island populations can grow rapidly from a single pair of individuals to very high densities, only to eventually crash. The Stockton Island population has been studied on and off for decades, and may be going through this boom to bust cycle. Thought to have included only a single pair of bears in the 1970s, this population grew to a peak of 31 bears in 1994. The population appeared to be stable between the mid-1990s and 2002 with approximately 26 bears, but a 2010 study revealed the population has decreased by 50%, and all of the female bears may be gone.