Main Page | See live article | Alphabetical index

Sei Whale

Sei Whale
Scientific classification
Binomial name
Balaenoptera borealis

The Sei Whale (Balaenoptera borealis) is a large baleen whale, and as such is one of the largest animals in the world. Following very scale hunting of Sei Whales in the Southern Ocean during middle part of the twentieth century, when 200,000 individuals were killed, the Sei Whale is now an internationally protected species.

Taxonomy and naming

This rorqual is in the order Cetacea. Like all the biggest whales it has baleen plates rather than teeth. This places it in the suborder Mysticeti and family Balaenopteridae. The species was first described by Rene Lesson in 1828. A further description was given by A. K. Rudolphi and the species is often referred to as Rudolphi's Whale in older texts (see e.g. [1]). This usage has since died out. Other names for the species include the Pollack Whale and Coalfish Whale (more substantial list of other common names). The word Sei comes from the Norwegian word for coalfish (also called pollack). Sei Whales feed on coalfish, amongst other small fish and squid in Norwegian waters and so the fish and whales were and are often sighted together, giving the whale its name. Two geographically separated subspecies have been identified - the Northern Sei Whale (B.b.borealis) and Southern Sei Whale (B.b.schleglii). Genetic analyses may yet cause these two to be reclassed as separate species.

Physical description

The Sei Whale is large, weighing 600-750 kilograms and measuring 4-5 metres in length at birth. Adults typically measure 12-16 metres with large individuals upto 19.5 metres being recorded. Adults weigh 20-30 tonnes typically and can grow up to 45 tonnes. The Sei Whale looks similar to Bryde's Whale. At sea the most reliable distinguishing feature is the dive sequence. The whale has a relatively slim body, coloured dark grey on the upper side and light grey to white on the belly. The upper side commonly has white scar marks, believed to be caused by sharks. The dorsal fin is a little further up the body than most rorquals but still more than half way down the back. Carwardine (1995) describes the fin as erect. Contradictarily Reeves et al (2002) describe it as "very falcate". The tail is thick and the fluke is small in relation to body size.

Population and distribution

Sei Whales are found worldwide in a band stretching from about 60 degrees south to 60 degrees north. Deep off-shore waters are preferred. Sei Whales differ from other rorquals in that it is not easy to predict where groups will appear from one year to the next. A particular location may one year see an influx of many whales only for them not to return for several years afterwards. Sei Whales migrate annually from cool and subpolar waters in summer to temperate and tropical waters for winter.

The total population of Sei Whales is now believed to be between 50 and 60 thousand of which around 10,000 are in or close to Icelandic waters.


Sei Whales usually more alone or in small groups. Larger groups have been seen at particularly abudant feeding grounds. The whale's dive sequence is more regular than its close relative. Blows occur at intervals of about 40-60 seconds for a few minutes followed a "deep dive" lasting five to fifteen minutes. Between shallow dives the whale stays close to the surface and remains visible in clear, calm waters. The Sei Whale is a fast swimmer, the fastest of all the baleens, and can reach speeds of up to 25 knots over short distances. In 1916, biologist R. C. Andrews likened them to a cheetah .


On the advent of large fast whaling ships, Sei Whales, like other rorquals were hunted in earnest, particularly in Antarctic waters. Around 200,000 (the WWF quotes an exact figure of 203,588) were killed during the twentieth century, representing about 80% of the total worldwide population. The species was protected by the International Whaling Commission in 1976, with the morartorium being enforced since 1986. Since then some Sei Whales have been caught and killed by Icelandic and Japanese whalers under the auspices of scientific research. Currently around 50 Sei Whales are killed for this purpose each year by Japanese scienties. The main focus of the research is to examine what Sei Whales eat and whether they are reducing the amount of fish available for fisheries. Environmental campaigners dispute the necessity of the research, saying that it is known that Sei Whale feed on small fish, squid and plankton not hunted by humans.

The species remained listed on the IUCN Red List of Threatened Species in 2000, categorized as 'endangered'.