The humpback whale is distributed worldwide in all ocean basins, though in the North Pacific it does not occur in Arctic waters. In winter, most humpback whales occur in the subtropical and tropical waters of the Northern and Southern Hemispheres. The North Pacific population was considerably reduced as a result of intensive commercial exploitation during the 20th century and recovery has been very slow. Photo ID studies indicate that humpback whales from the Western and Central North Pacific mix on summer feeding grounds in the central Gulf of Alaska and perhaps the Bering Sea. Females normally reproduce every two or three years, giving birth to a calf that is 4.5 to 5 m long; it has grown to about 8 or 9 m when it is weaned at 10 or 11 months. The gestation period is 12 months. Adult males are 15 m long and adult females slightly larger at 16 m. Humpback whales in the high latitudes of the North Pacific are seasonal migrants that feed on krill and small schooling fishes. They frequently employ an interesting feeding behavior called bubble net feeding in which they surround a school of schooling fish with a curtain of bubbles.