Sei whale

This fast cetacean inhabits all oceans and adjoining seas except in tropical and polar regions. The sei whale became a major target for commercial whaling after the preferred stocks of blue and fin whales had been depleted.
Sei whale feeding. rel=
Sei whale feeding.
© Peter Duley / NEFSC / NOAA (images collected under research permit number 775-1875)

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Key Facts

  • Common Name

    Sei whale, coalfish whale, pollack whale, Rudolphi's rorqual; Baleinoptère de Rudolphi, rorqual boréal, rorqual de Rudolphi, rorqual sei (Fr); Ballena boba, ballena sei, rorcual boreal, rorcual de Rudolphi, rorcual norteno (Sp)

  • Scientific Name

    Balaenoptera borealis

  • Status

    IUCN: Endangered

  • Population

    Approximately 12,000 individuals

  • Length

    14 to 20 m

  • Weight

    About 20 tons

  • Skin colour

    Dark grey or bluish-grey

Background

This fast cetacean inhabits all oceans and adjoining seas except in tropical and polar regions. The sei whale became a major target for commercial whaling after the preferred stocks of blue and fin whales had been depleted.

Today, although commercial whaling has been officially halted, the species is subject to "scientific whaling" by Japan. It also remains vulnerable to pollution, shipping strikes and entanglement in fishing gear.

Recently a number of South Pacific countries, including five island nations, have declared their EEZs (Exclusive Economic Zone) to be whale sanctuaries in which commercial whaling is prohibited and where additional research on large whales is encouraged.

Physical Description

This species is identified by a dorsal fin, 38 to 56 ventral grooves, and two rows of 300-380 baleen plates. The sei whale is one of the fastest cetaceans, reaching speeds of up to 50 km per hour. A sei whale marked in the Antarctic and killed by whalers 10 days later had traveled more than 4,000 km (2,200 miles) during that period.

It is not usually a deep diver and periods of submergence generally last 5-10 minutes. To feed, the sei whale swims through swarms of prey, twisting on its side and also uses the skimming method of filtering food with its baleens. Although the species does not use echolocation to search for prey, it has been heard to emit a sonic burst of 7-10 pulses.

Like other great whales, the sei whale prefers to spend the summer feeding in the cooler northern waters before migrating south to warmer waters to breed and calve.

Size

This whale is 14 to 20 m long and weighs about 20 tons. Females are about 1-2 m longer than males.

Colour

The species is dark grey or bluish-grey on the back and sides, with a greyish white area on the ventral grooves of the lower jaw and underbelly. Its similarity to the Bryde's whale caused much confusion among whalers in the 19th Century, who often reported sei kills as Bryde's, or either as fin whales.
Sei whale and calf (Balaenoptera borealis) Azores, North Atlantic Ocean / ©: naturepl.com/Doug Perrine / WWF
Sei whale and calf (Balaenoptera borealis) Azores, North Atlantic Ocean
© naturepl.com/Doug Perrine / WWF

Priority species

Whales and dolphins are a priority species. WWF treats priority species as one of the most ecologically, economically and/or culturally important species on our planet. And so we are working to ensure such species can live and thrive in their natural habitats.

Ecology and Habitat

A global open water diver

Sei whales are pelagic, occurring far from shore in temperate oceanic waters worldwide.

Social Structure


Although sei whale groups mostly consist of two to five individuals, thousands may aggregate where plenty of food is available.

Life Cycle

Sei whales become sexually mature between 6 to 8 years old. Sei whales can live for 65 year.

Breeding

Mating season ranges from November to February in the Northern Hemisphere and from May to July in the Southern Hemisphere. Females generally give birth to a single calf every other year in winter, after a gestation period of 10.5-12 months.

Although we know very little about their breeding habits, some data indicate that sei whale migration is loosely organized around sex, age, or reproductive function. This presumably relates to mating strategies, but at this time nothing is known of their mating habits or calving grounds.

Diet

Like other baleen whales, sei whales feed by skimming and swallowing surface plankton, mainly copepods (tiny marine crustaceans) but also euphausiids (krill - shrimp-like crustaceans).
Baleen / ©: NOAA
Baleen makes up baleen plates, which are arranged in two parallel rows that look like combs of thick hair; they are attached to the upper jaws of baleen whales. Whales use these combs for filter feeding.
© NOAA

Population and Distribution

Protected... once it was already depleted

Previous population and distribution

The species was intensively exploited worldwide after blue and fin whale stocks had been reduced. Evidence shows that the stocks of sei whales were depleted before they were protected from commercial whaling in the 1970s and 1980s.

In 1980, it was estimated that the Southern Hemisphere population had been reduced to around 24,000 from an initial level of 100,000 or so. In the North Pacific, the most recent study, in 1977, indicated a decline from 42,000 in 1963 to 8,600 in 1974. Figures in the North Atlantic are the most uncertain, although some surveys have suggested a figure of around 10,000.

Current population and distribution

Sei whales inhabit all oceans and adjoining seas except in polar regions, feeding in cold water during the summer and migrating to warm tropical and subtropical waters during the winter.

In the western North Pacific, sei whales are common in the south-west Bering Sea to the Gulf of Alaska, and offshore in a broad arc between about 40 degrees North and 55 degrees North across the Pacific.

In the North Atlantic, sei whales can be found from the coast of Labrador, and along the coasts of Greenland and Iceland. In the East Atlantic, sei whales migrate north to northern waters off Norway, Shetland, Orkney and the Faeroe Islands and occasionally, Svalbard. Sei whales are also present in the Denmark Strait.

What little we know about sei whale population structure comes largely from whaling data. There are two separate populations in the Northern and Southern Hemispheres, but it is unclear whether they are separate populations within each ocean basin.

Estimates from the U.S. National Marine Fisheries Service puts the southern population at 24,000; 4,600 in the North Atlantic; and 22,000-37,000 in the Pacific. However, the extent to which stocks have recovered generally is uncertain, as little research has been carried out on this species over the last few decades.
Sei whale range / ©: Wikipedia
Sei whale range
© Wikipedia
Range States
Argentina, Australia, Canada, Chile, Cuba, Falkland Islands (Malvinas), Iceland, India, Indonesia, Japan, Kenya, Democratic People's Republic of Korea, Republic of Korea, Malaysia, Mexico, Mozambique, Norway, Poland, Russian Federation, Réunion, South Africa, Spain, Suriname, Taiwan, United Republic of Tanzania, Thailand, United Kingdom, United States, Uruguay

Ecological Region
Antarctic Peninsula and Weddell Sea , Bering-Beaufort-Chukchi Seas, Barents-Kara Seas, Mediterranean Sea, Northeast Atlantic Shelf Marine, Grand Banks, Chesapeake Bay, Yellow Sea, Okhotsk Sea, Patagonian Southwest Atlantic, Southern Australian Marine, New Zealand Marine, California Current, Benguela Current, Humboldt Current, Agulhas Current, Western Australia Marine, Panama Bight, Gulf of California, Galapagos Marine, Canary Current, Nansei Shoto, Sulu-Sulawesi Seas, Bismarck-Solomon Seas, Banda-Flores Sea, New Caledonia Barrier Reef, Great Barrier Reef, Lord Howe-Norfolk Islands Marine, Palau Marine, Andaman Sea, Tahitian Marine, Hawaiian Marine, Rapa Nui, Fiji Barrier Reef, Maldives, Chagos, Lakshadweep Atolls, Red Sea, Arabian Sea, East African Marine, West Madagascar Marine, Mesoamerican Caribbean Reef, Greater Antillean Marine, Southern Caribbean Sea, Northeast Brazil Shelf Marine.

What are the threats to sei whales?

Unwanted attention from whaling fleets

While the sei whale has been hunted by humans since the 1860s, it wasn't until the 1950s and 1960s and the declining availability of blue and fin whales that the killings seriously expanded.

Since 1985, the International Whaling Commission has officially halted all commercial whaling of this species. Today, 50 sei whales are killed annually by Japanese whalers in the North Pacific in Japan's "scientific whaling" programme.

They are threatened by global warming, but they can also be harmed by pollution, shipping strikes and entanglement in fishing gear.

What is WWF doing?

WWF efforts in this area over the coming years will be directed towards increasing awareness of the need for cetacean conservation at the national and regional levels, and to create opportunities for local communities to be involved in, and to profit from, cetacean conservation initiatives.

The majority of WWF's global conservation work to protect whales and dolphins takes place within the context of the International Whaling Commission (IWC).
Together we can make the world's oceans safe for whales. / ©: WWF / Ogilvy Indonesia
Together we can make the world's oceans safe for whales. Learn more about WWF's work to protect whales.
© WWF / Ogilvy Indonesia

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