General description: Blue whales from the southern hemisphere can reach 100 feet in length (30 m). Northern hemisphere animals are usually smaller, reaching 75-80 feet (23-27 m) and weighing 200,000 pounds (90,000 kg). Fully grown females are usually larger than males. Blue whales are blue-gray in color with lighter mottling on the sides and back. The darkness of the color and the amount of mottling can vary between animals. The undersides of the fins are white. Growth of diatoms on the belly area of blue whales may give a yellowish color that has caused them to be called sulphur-bottom whales. The head of a blue whale, when viewed from above, is wide and flat, somewhat like a gothic arch. When breathing, a blue whale can spout water up to 25 to 30 feet in the air. Spouts may last for up to 5 seconds. The dorsal fin on a blue whale is small, about one foot high.
Distribution and migration: Blue whales are found in all of the world's oceans from the arctic to the antarctic. In the north Pacific, however, they rarely enter the Bering Sea and are only seldom seen as far north as the Chukchi Sea. Blue whales may migrate long distances between equatorial wintering grounds and high latitude feeding areas. In the eastern north Pacific they winter off southern and Baja California. During spring and summer, blue whales may be found from central California northward through the Gulf of Alaska. Historical areas of concentration in Alaska include the eastern Gulf of Alaska, the eastern Aleutians, and the far western Aleutians. Animals may move between these areas. Whales tagged in Russia and Canada have been found near Kodiak Island. Blue whales are found most frequently along the edges of continental shelves and are seldom seen in nearshore Alaska waters.
Abundance: Prior to whaling, the worldwide population of blue whales is believed to have been about 200,000 animals. Only 8,000-12,000 are estimated to be alive today. Blue whales have always been more abundant in the Antarctic than in the northern hemisphere. An estimated 4,900 to 6,000 blue whales are believed to have inhabited the north Pacific prior to whaling. The north Pacific population is now estimated at 1,200 to 1,700 animals. There have been very few sightings of blue whales in Alaska waters in recent years.
Food habits: Of all the great whales, the blue whale has the most limited diet. It feeds almost exclusively on small, planktonic, shrimp-like krill. It has been estimated that an average blue whale needs to eat 40 million krill per day to meet its energy requirements. In the north Pacific, the krill species Euphausia pacifica and Thysanoessa spinifera are the main foods of blue whales. These two species occur in dense aggregations near the ocean's surface. A blue whale feeds by gulping huge quantities of water into its large mouth and expandable throat area. The animal then tightens its throat muscles, forcing the water across its baleen plates and out of the mouth. The krill are caught in the filter created by the overlapping fibrous edges of the baleen plates. The krill are wiped off the baleen by the tongue and swallowed. The baleen in a blue whale is black. The animals have 270-395 plates on each side of the upper jaw.
Reproduction: The age at which female blue whales become mature is still not determined, but it has been estimated at either 5 or 10 years of age. Upon reaching sexual maturity, females bear a single calf every two to three years. In the northern hemisphere, blue whales calve and mate in late fall and winter. The gestation period is 10-11 months. Calves are about 23 feet long (7.6 m) when born and weigh over 5,000 pounds (2,250 kg). The calves nurse for about seven months and grow more than 200 pounds per day. When weaned, calves weigh about 50,000 pounds (22,500 kg) and are approximately 50 feet long (15.5 m).
Sources of mortality: Humans have been the main source of non-natural mortality for blue whales. Because of their speed (up to 20 knots) and size, blue whales were not hunted in the early days of whaling. In the late 1800s, however, the advent of steam powered vessels and more powerful harpoon guns led to the taking of hundreds of thousands of these animals. Of the approximately 360,000 blue whales killed by whalers between 1910 and 1966, about 330,000 were killed in the antarctic. About 8,200 were taken in the north Pacific during the same period. Hunting of blue whales was banned by the International Whaling Commission in 1966, and there have been no intentional killings of blue whales in recent years. Killer whales also kill blue whales, but the extent of this source of mortality is not known.
Text: Steve Zimmerman
Illustration: Sandra Noel, Alaska Fisheries Science Center
Revised and reprinted 1994
