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The blue whale is the largest animal to have ever lived, almost as big as a Boeing 737, and even larger than the biggest dinosaurs. The skin is greyish blue in colour with a mottled effect visible in some lights that can allow individuals to be identified. The underside, especially of whales living in polar waters, often has a yellowish tinge caused by microscopic algae (diatoms), and between 55 and 88 throat grooves run from under the chin to the navel. The blow (or spout) of this species is the biggest amongst all whales; the slender upright column of air can rise to nine meters.
The blue whale is a cosmopolitan species, found in all oceans except the Arctic, but absent from some regional seas such as the Mediterranean, Okhotsk and Bering seas.
The Antarctic form B. m. intermedia, which used to be by far the most abundant form of blue whale, occurs in the Antarctic in summer, from the Antarctic Polar Front up to and into the ice (Branch et al. 2006), including (in the past) the South Georgia area. Its winter distribution is poorly known, but the presumption has been that animals migrate in winter to lower latitudes, largely because blue whales were caught off Namibia, South Africa and Chile in winter (Best 1998, Mackintosh 1965).
Pygmy blue whales (B. m. brevicauda) are confined mainly to the area north of 55°S even in summer, but with one record at 56°15?S (Ichihara 1966). They are most abundant in the southern Indian Ocean on the Madagascar plateau, and off South Australia and Western Australia, where they form part of a more or less continuous distribution from Tasmania to Indonesia. Blue whales are found year round in the northern and equatorial Indian Ocean, especially around Sri Lanka, and the Maldives, and at least seasonally near the Seychelles and in the Gulf of Aden.
Blue whales occur in the eastern Pacific from around 44°S in southern Chile (Hucke-Gaete et al. 2005) as far as the Costa Rica Dome where they are present year-round (Reilly and Thayer 1990). There may be a gap from there to Baja California where they are quite common as also off the Californian coast (Calambokidis and Barlow 2004) but tracking of a tagged whale suggests that some of the Californian whales may migrate to the Costa Rica Dome in winter (Mate et al. 1999). North of 40°N, blue whales occur across the North Pacific from the coast of Oregon to the Kurile Islands (Russian Federation), and north to the Aleutian Islands (US -Alaska) but not far into the Bering Sea. In the past blue whales were caught off southern Japan and the Korean peninsula, but none have been seen there in recent years.
In the North Atlantic the summer distribution of blue whales extends in the west from the Scotian Shelf to the Davis Strait (Canada) (NMFS 1998). Blue whales occur in the Denmark Strait, around Iceland and north to the ice edge, and in the northeast to Svalbard (Norway). Historically, blue whales were commonly caught along the coasts of North and West Norway, the Faeroes and the NW British Isles. They also occur in low numbers off NW Spain (Bérubé and Aguilar 1998) and in the past near the Strait of Gibraltar, but not in the Mediterranean (Reeves and Notarbartolo di Sciara 2006). The winter distribution is poorly known but it appears that in the past blue whales were widely distributed in the southern half of the North Atlantic in winter (Reeves et al. 2004).
McDonald et al. (2006) use song to suggest nine different groupings of blue whales. They argue that because song is used in mating, that these different song types, five of which have data spanning over 30 years and showing stability, should form the basis for population structure hypotheses. Although some of the geographic locations correspond to IWC stocks, for example the northern Indian Ocean, others do not. Thus, the population structure in this account likely underestimates the true number of discreet groups of blue whales.
Blue whales inhabit sub-polar to sub-tropical latitudes. Poleward movements in spring allow the whales to take advantage of high zooplankton production in summer. Movement towards the subtropics in the fall allows blue whales to reduce their energy expenditure while fasting, avoid ice entrapment in some areas, and engage in reproductive activities in warmer waters of lower latitudes. Although the species is often found in coastal waters, blue whales are thought to occur generally more offshore than humpback whales, for example.
Blue whales were not initially among the most heavily hunted species due to their size, speed, and remote habitat. Technological advances from 1860-1920, however, allowed whalers to pursue the species. The estimated total kill of blue whales in the 20th century was 350,000 animals. By the 1960's, blue whales were on the edge of extinction. Despite the opposition of the whaling industry, blue whales gained protection after the 1965/66 whaling season. Estimates of the remaining population range from 2,000 to 6,000 individuals and it is not yet clear that the blue whale will escape extinction. Southern hemisphere populations have been surveyed extensively and are estimated at 400 to 1,400 animals. Northern hemisphere populations are estimated at about 5,000 individuals but the scientific rigor of these surveys has been criticized.
Blue whales are subject to some ship strikes and entanglements (NMFS 1998) but reported cases are few. The remote distribution of some blue whale populations probably makes them less vulnerable to human impacts than some other cetacean species, but local populations that inhabit waters with significant levels of human activity may be subject to some threat, such as disturbance from vessel traffic, including ship noise (e.g. Gulf of St Lawrence population, NMFS 1998). Globally, there appear to be no major threats to blue whales at present.
During this century, a profound reduction in the extent of sea ice in the Antarctic is expected, and possibly a complete disappearance in summer, as mean Antarctic temperatures rise faster than the global average (Turner et al. 2006). The implications of this for blue whales are unclear but warrant monitoring.
Small populations such as the surviving Antarctic population can have a number of interacting effects that accelerate overall risk (Gilpin and Soule, 1986). Among those effects are demographic stochasticity, inbreeding depression and density dispensation (Allee effects). Although the expectation is that these threats could be serious because cetaceans are social animals with low reproductive output, the fact that the Antarctic population is increasing is encouraging.
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