Blue whales are found throughout the world's oceans. Blue whales generally spend winters in temperate and subtropical zones, migrating toward the polar regions in spring and summer. Dives last from 10 to 20 minutes. Usually they travel alone or in small groups of two to four, although off the coast of California some groups as large as 60 have been seen. Sexual maturity is reached between 5 to 10 years.
Females give birth every two to three years to one calf.
Gestation lasts 10 to 12 months and the average calf is 23 feet long and weighs 4,440 pounds at birth. A calf is fed by its mother for seven to eight months. Blue whales once were considered too difficult to hunt because of their speed and tremendous size. However, with the introduction of factory ships and the harpoon gun in the 1920s, blue whales were hunted intensively. By the 1960s they were nearly extinct.