Blue whale

The Blue Whale (Balaenoptera musculus) is a marine mammal belonging to the suborder of baleen whales (called Mysticeti).[3] At up to 33.59 metres (110 ft 2.5 in) in length and 181 metric tonnes (200 short tons) or more in weight, it is believed to be the largest animal ever to have existed.[4][5]
Long and slender, the Blue Whale's body can be various shades of bluish-grey dorsally and somewhat lighter underneath.[6] 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. As with other baleen whales, its diet consists mainly of small crustaceans known as krill, as well as small fish and sometimes squid.
Blue Whales were abundant in nearly all oceans until the beginning of the twentieth century. For over 40 years 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[7] located in at least five groups. More recent research into the Pygmy subspecies suggests this may be an underestimate.[8] Before whaling the largest population was in the Antarctic, numbering approximately 239,000 (range 202,000 to 311,000).[9] There remain only much smaller (around 2,000) concentrations in each of the North-East Pacific, Antarctic, and Indian Ocean groups. There are two more groups in the North Atlantic and at least two in the Southern Hemisphere.

Threats other than hunting
Due to their enormous size, power and speed, adult Blue Whales have virtually no natural predators. There is however one documented case in National Geographic Magazine of a Blue Whale being attacked by Orcas, although the Orcas were unable to kill the animal outright during their attack, the Blue Whale sustained massive wounds and probably died as a cause of them shortly after the attack.[51]
Blue Whales may be wounded, sometimes fatally, after colliding with ocean vessels as well as becoming trapped or entangled in fishing gear.[52] The ever-increasing amount of ocean noise drowns out the vocalizations produced by whales, which may make it harder for whales to communicate.[52] Human threats to the potential recovery of Blue Whale populations also include accumulation of polychlorinated biphenyl (PCB) chemicals within the whale's body.
With global warming causing glaciers and permafrost to rapidly melt and allowing a large amount of fresh water to flow into the oceans, there are concerns that if the amount of fresh water in the oceans reaches a critical point there will be a disruption in the thermohaline circulation. Considering the Blue Whale's migratory patterns are based on ocean temperature, a disruption in this circulation which moves warm and cold water around the world would be likely to have an effect on their migration.[53] The whales summer in the cool, high latitudes, where they feed in krill-abundant waters; they winter in warmer, low latitudes, where they mate and give birth.[54]
The change in ocean temperature would also affect the Blue Whale’s food supply. The warming trend and decreased salinity levels would cause a significant shift in krill location and abundance.[55]