Small (around 2,000) concentrations in each of the eastern North Pacific, Antarctic, and Indian Ocean groups. There are two more groups in the North Atlantic, and at least two in the Southern Hemisphere.
The blue whale (Balaenoptera musculus) is a marine mammal belonging to the baleen whales (Mysticeti).
Long and slender, the blue whale's body can be various shades of bluish-grey dorsally and somewhat lighter underneath. There are at least three distinct subspecies: B. m. musculus of the North Atlantic and North Pacific, B. m. intermedia of the Southern Ocean and B. m. brevicauda (also known as the pygmy blue whale) found in the Indian Ocean and South Pacific Ocean. B. m. indica, found in the Indian Ocean, may be another subspecies. The blue whale has a long tapering body that appears stretched in comparison with the stockier build of other whales.The head is flat, U-shaped and has a prominent ridge running from the blowhole to the top of the upper lip. The front part of the mouth is thick with baleen plates; around 300 plates (each around one metre (3.2 ft) long) hang from the upper jaw, running 0.5 m (1.6 ft) back into the mouth. Between 70 and 118 grooves (called ventral pleats) run along the throat parallel to the body length. These pleats assist with evacuating water from the mouth after lunge feeding (see feeding below).
At 30 metres (98 ft) in length and 190 tonnes (210 short tons) or more in weight, it is the largest existing animal and the heaviest that has ever existed.
The calf weighs about 2.5 tonnes (2.8 short tons) and is around 7 metres (23 ft) in length.
Blue whales feed almost exclusively on krill, though they also take small numbers of copepods. The species of this zooplanktoneaten by blue whales varies from ocean to ocean. An adult blue whale can eat up to 40 million krill in a day. The whales always feed in the areas with the highest concentration of krill, sometimes eating up to 3,600 kilograms (7,900 lb) of krill in a single day.
Mating starts in late autumn and continues to the end of winter. Little is known about mating behaviour or breeding grounds. Females typically give birth once every two to three years at the start of the winter after a gestation period of 10 to 12 months. The calf is weaned after six months, by which time it has doubled in length. Sexual maturity is typically reached at five to ten years of age. Scientists estimate that blue whales can live for at least 80 years.
Blue whales most commonly live alone or with one other individual. It is not known how long traveling pairs stay together. In locations where there is a high concentration of food, as many as 50 blue whales have been seen scattered over a small area.
They do not form the large, close-knit groups seen in other baleen species.