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Sei Whales

Balaenoptera borealis

The Sei whale closely resembles the Bryde's whale in both size and appearance. From a distance it is almost impossible to tell them apart. This whale grows to a length of 12-17m (36-51ft) and weighs 20-30 tonnes. The Sei whale is called a "rorqual" a Norwegian word for "furrow" and refers to the pleated grooves running along its under-belly. These throat grooves, which extend from the lower jaw to the navel, in addition to streamlining the shape of the whale, allow the throat area (cavum vent-rale) to expand tremendously during feeding.

The head of the Sei whale forms up to a quarter of the total body length and compared with other rorquals is slender with a slightly arched forehead. It has twin blowholes with a low splashguard to the front. A single prominent longitudinal ridge along the top of the rostrum distinguishes the Sei whale from the Bryde's whale. The baleen plates in the mouth can be 75cm-80cm (30-32in) in length and number between 318-340 (Northern Hemisphere) and 300-410 (Southern Hemisphere).The baleen looks grey-black all over and fringed white near the tip of the snout.

A Sei whale will feed mainly on copepods, though it also takes krill and other crustaceans. It will often skim the surface of the water with its mouth half open taking in food from patches of water concentrated with plankton. Sei whales also feed in the way most rorquals do, by openning its mouth widely to gulp in large quantities of water out of which it seives the tiny organisms.

Most Sei whales live in the Southern Hemisphere while smaller populations inhabit the North Atlantic and North Pacific. There appears to be little or no mixing between the northern and southern populations. They are not normally found in extreme polar areas, although the subarctic and the subantarctic are favoured feeding grounds. They migrate from these polar and cold temperate feeding grounds to tropical to warm temperate breeding grounds. Sporadic annual invasions at specific locations are known as "Sei whale Years" but are not easy to predict. May be seen around islands but is rarely found close to shore elsewhere.

Sei whales were heavily exploited by the whaling industry, especially during the 1960's and early 1970's and the population has been severely depleted. Estimates place the original number of Sei whales at 256,000 with current figures standing at 54,000 that's just 20% of natures intention. Now on the endangered list, the Sei whale is protected (since 1986) worldwide by international law. It is not to be hunted by anyone for any reason.