Sei Whale – Balaenoptera borealis
The Sei Whale belongs to the family of the rorquals and is therefore part of the infraorder of whales (Cetaceans). Balaenoptera borealis was first described in 1828 by Lesson.
“Sei” comes from the Norwegian word for saithe, as Sei Whales feed on them and can therefore be found in their vicinity.
Description, Anatomy & Characteristics
The Sei Whale reaches a maximum length of 20m, whereby it can weigh up to 45t. Females become somewhat larger than males.
Its body is slender and elongated. While the back is dark grey, the underside has a white colouring.
Since it belongs to the baleen whales, it has no teeth. Instead, there are between 300 and 380 baleen in each half of the mouth.
The fluke is very small compared to the body.
Habitat, Distribution & Occurrence
The Sei Whale occurs world-wide in all oceans, with what they prefer subpolar, temperate and subtropical zones. In polar and tropical areas they are to be seen rather rarely. Its range is between 60° north and south latitude. Oceans with narrow access are excluded.
In addition, Balaenoptera borealis is divided into two subspecies, the Northern Sei Whale (Balaenoptera borealis borealis) and the Southern Sei whale (Balaenoptera borealis schleglii). Both species never meet, since summer and winter are opposite on both hemispheres. During summer they are closer to the poles, during winter they are closer to the equator.
Their habitat is usually above deep water, the so-called continental shelf.
Biology & Feeding
Balaenoptera borealis lives in pairs or small groups. Larger schools can be found on rich feeding grounds. Their food consists mostly of zooplankton (krill) and up to 30cm large schooling fish. To hunt, they turn on the water-surface on the side and comb through these with open mouth.
Reproduction & Development
The reproduction of Balaenoptera borealis takes place in each case in the winter-months. In the northern hemisphere this occurs from November to February, while in the southern hemisphere it occurs from May to July.
The gestation period is between 10.5 and 12 months. At birth a calf weighs up to 750kg and is almost 5m long. They are weaned at the age of 6 to 7 months. Females are pregnant about every 2 to 3 years.
Although the Sei Whale becomes sexually mature with approximately 10 years, lasts up to 25 years until they are completely adult. Their life expectancy is estimated on that occasion at 75 years.