Sei Whale

The Sei Whale (Balaenoptera borealis) is a large baleen whale. Following large-scale hunting of Sei Whales in the Southern Ocean during middle part of the twentieth century when approximately 200,000 Sei Whales were killed, the Sei Whale is now an internationally protected species. The Sei Whale can grow to 20 m (66 ft) long.

Taxonomy and naming

This rorqual (The largest group of baleen whales) 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. Other names for the species include the Pollack Whale and Coalfish.

The word Sei comes from the Norwegian word for coalfish (also called pollack). Sei Whales feed on coalfish along with 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 subspecies have been identified by regions – the Northern Sei Whale (B.b.borealis) and Southern Sei Whale (B.b.schleglii)

Physical description

The Sei Whale is large, with Adults typically measuring 12-16 meters and weighing 20-30 tons, and with large individuals measuring up to 19.5 meters, and weighing 40-45 tons. The whale has a relatively slim body, With dark grey color 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. 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. They prefer to be in waters deep off the shores. 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 move alone or in small groups. The whale’s dive patterns are 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. 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.


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 moratorium 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 scientists. 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.