Claim: Blue whales where hunted.

The blue whale -LRB- Balaenoptera musculus -RRB- is a marine mammal belonging to the baleen whales -LRB- Mysticeti -RRB- . At up to 29.9 m in length and with a maximum recorded weight of 173 tonne and probably reaching over 181 tonnes -LRB- 200 short tons -RRB- , it is the largest animal known to have ever existed .   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 -LRB- also known as the pygmy blue whale -RRB- found in the Indian Ocean and South Pacific Ocean . B. m. indica , found in the Indian Ocean , may be another subspecies . As with other baleen whales , its diet consists almost exclusively of small crustaceans known as krill .   Blue whales were abundant in nearly all the oceans on Earth until the beginning of the twentieth century . For over a century , they were hunted almost to extinction by whalers until protected by the international community in 1966 . A 2002 report estimated there were 5,000 to 12,000 blue whales worldwide , in at least five groups . The IUCN estimates that there are probably between 10,000 and 25,000 blue whales worldwide today . Before whaling , the largest population was in the Antarctic , numbering approximately 239,000 -LRB- range 202,000 to 311,000 -RRB- . There remain only much smaller -LRB- around 2,000 -RRB- 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 . As of 2014 , the Eastern North Pacific blue whale population had rebounded to nearly its pre-hunting population .