Blue whales occur in all oceans, primarily along the edge of the continental shelf or along ice fronts. Major populations are found in the North Pacific, North Atlantic and southern hemisphere. Based on acoustic data, size, and photo identification studies, there are two groups of blue whales in the North Pacific: eastern and western. The eastern group winters off Mexico and Central America, and feeds off California and British Columbia in summer/fall (from June to November). The western group feeds in summer southwest of Kamchatka and in the Gulf of Alaska, and in winter migrates to lower latitudes in the western and central Pacific, including Hawaii. Females give birth to a single calf every two to three years. Gestation is around twelve months. Calves are around 7 m when they are born The calf is weaned at about eight months when it is 15 m long, having gaining as much as 90 kg per day during the nursing period. Adults grow to 24 m and weigh up to 150 tons. They are the largest animal that has ever lived.