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Balaenoptera musculus:   (click for more)

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FAO Names
En - Blue whale, Fr - Rorqual bleu, Sp - Ballena azul.
3Alpha Code: BLW     Taxonomic Code: 4230200104
Diagnostic Features
The blue whale is the largest animal ever known; however, its size substantially overlaps with that of adult fin and sei whales. Like all rorquals, the blue whale is slender and streamlined. The head is broad and U-shaped (like a gothic arch) when viewed from above and relatively flat when viewed from the side. Along the centre of the rostrum, there is a single prominent ridge, which ends in an impressive "splash guard" around the blowholes. The flippers are long and pointed, and the dorsal fin is relatively small, variably shaped, and placed about three-quarters of the way back from the snout tip. The broad flukes have a relatively straight trailing edge and a prominent notch. In the Southern Hemisphere and northern Indian Ocean, a subspecies called the pygmy blue whale (B. m. brevicauda), which is shorter and has a relatively larger head, has been described. It is generally not possible to distinguish pygmy blue whales from other blue whales at sea.

  Blue whales are bluish grey dorsally and somewhat lighter underneath. The head is uniformly blue, but the back and sides are mottled. When viewed through the water surface they may appear dappled or uniformly light blue. There is light to extensive mottling on the sides, back, and belly, generally in the form of dark spots on a lighter surface, but sometimes the reverse. A chevron, with the vertex behind the blowholes, sometimes marks the transition of coloration between the head and the body. Diatom films on the surface may be seen as an orangish brown or yellow tinge, which gave rise to the alternative name "sulphur-bottom" whale.

  On the throat, there are 55 to 88 long pleats extending to or near the navel. The mouth contains 270 to 395 pairs of black, broad-based baleen plates, each less than 1 m long. The blow is tall and slender, reaching 9 m or more in height. 
Can be confused with: Blue whales can be confused with the other large rorquals, fin whale and sei whale. Although the great size of blue whale adults may aid in identification, the best clues for differentiating blue whales from fin or sei whales are colour pattern, head shape, and dorsal-fin shape and position.

Geographical Distribution
Blue whales tend to be open-ocean animals, but come close to shore to feed, and possibly to breed, in some areas. Blue whales can be seen from the equator to the pack ice edges in both hemispheres, with most poleward intrusions in both hemispheres in summer. Some blue whales are resident others are migratory.
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Habitat and Biology
Blue whales are usually seen alone or in pairs. However, scattered aggregations of a dozen or more may develop on prime feeding grounds. Although shorter dives are most common, dives of up to 30 min, generally interspersed with series of shorter surfacings (at 15 to 20 sec intervals), have been recorded. Fluking-up is not uncommon, although not all blue whales are "flukers". Remarkably, some blue whales have been observed breaching.
Calves are born in winter on tropical or subtropical breeding grounds.  Krill form the major part of the blue whale's diet, and on their feeding grounds, blue whales can be observed lunging, often on their sides or upside-down, through great clouds of these invertebrates.
Size
Most adults are 23 to 27 m long (with females growing larger than males), but an Antarctic blue whale measuring over 33 m was once taken by whalers. Newborns are about 7 m long. Adults can weigh up to 160 t.
Interest to Fisheries

Conservation Status : From the time faster catcher boats and explosive harpoons made them catchable, blue whales were hunted relentlessly from the late 19th through the mid 20th centuries. As the largest whales, they were the most sought after of the rorquals. Although most populations remain well below pre-exploitation levels, some stocks (such as those that feed off California) have shown encouraging signs of recovery since protection by the International Whaling Commission (IWC) in 1965. At least in the eastern North Atlantic and the eastern North Pacific, numbers appear to be on the rise.
IUCN:  Endagered.
Source of Information
Marine mammals of the world. Jefferson, T.A., S. Leatherwood & M.A. Webber - 1993. FAO species identification guide. Rome, FAO. 320 p. 587 figs. . 
 
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