Sei whale  (Balaenoptera borealis)

Classified as Endangered (EN) on the IUCN Red List 2007 (1). Populations from the equator to Antarctica are listed on Appendix I of CITES, all other populations are listed on Appendix II (3). Also listed on Appendices I and II of the Convention on Migratory Species (CMS or the Bonn Convention) (4).

The sei whale is smaller in size than the fin whale (Balaenoptera physalus), and can be distinguished from this similar species because it has symmetrical colouring on the lower parts of its head (5). It is also similar to Bryde's whale (Balaenoptera edeni), but has only one ridge on the upper surface of the head, whereas Bryde's whale has three (5). The 'blow' or spout of the sei whale is a single thin cloud, which reaches about three metres in height (5). The skin is a mottled dark grey colour, with white grooves along the paler underparts (6) (2). The baleen is grey to black with paler fringes (2) and less than 80 centimetres in length (6). The dorsal fin is obvious, has a slightly hooked shape and is located two-thirds along the length of the body (7). The common name 'sei' arose from the arrival of this whale off the coast of Norway tending to coincide with that of coalfish 'seje' (6).

Found in almost every ocean of the world, but occurs less frequently in polar waters than the other members of this family (6).

An inhabitant of the open ocean, the sei whale tends to avoid coastal waters (6).

Few details of the natural history of this whale are known. They tend to occur in groups of between two and five individuals (5), but larger groups may form in areas where food is very abundant (5). Capable of travelling at great speed, this species is believed to migrate into warmer waters at lower latitudes during the winter months (8). Little is known of communication in this species, but individuals are known to make many low frequency sounds (7).

Although not a traditional target of the whaling industry, the sei whale began to be exploited after the blue, fin and humpback stocks became depleted and protected (6). This species was then relentlessly hunted in the 1960s and 70s (8), before the International Moratorium on Commercial Whaling came into effect in 1986 (9). At present, the species is vulnerable to chemical and noise pollution (8).

In 1976, this whale received protected status (8), and the moratorium on commercial whaling took effect from 1986 (9). There are ongoing problems with the moratorium however, and Iceland announced in 2001 that it might soon resume commercial whaling of sei, minke and fin whales (8). Other countries also oppose the ban and the future of endangered species such as the sei whale is not yet secure (5).

View information on this species at the UNEP World Conservation Monitoring Centre.

To help conserve this species by working in the field with Earthwatch, click here.

The UK Biodiversity Action Plan for this species is available at UK BAP.

For more on the sei whale and other whales and dolphins see: 

• The Whale and Dolphin Conservation Society:
  www.wdcs.org/
• Carwardine, M., Hoyt, E., Fordyce, R.E., and Gill, P. (1998) Whales and Dolphins. Harper Collins Publishers, London.
• The Smithsonian Institution's North American Mammals website:
  www.mnh.si.edu/mna/