Sei Whale

Balaenoptera borealis

Last updated: August 30, 2021
Verified by: AZ Animals Staff
© Christin Khan, NOAA / NEFSC, Public domain, via Wikimedia Commons – License / Original

This whale is one of the fastest of the cetaceans


Sei Whale Scientific Classification

Scientific Name
Balaenoptera borealis

Read our Complete Guide to Classification of Animals.

Sei Whale Conservation Status

Sei Whale Locations

Sei Whale Locations

Sei Whale Facts

Krill and other euphausiids, copepods, small fish and squid
Group Behavior
  • Solitary
Fun Fact
This whale is one of the fastest of the cetaceans
Estimated Population Size
Biggest Threat
Humans, climate change, pollution
Most Distinctive Feature
Its tall, sickle-shaped dorsal fin
Other Name(s)
Japan finner, lesser fin whale, pollock whale, sardine whale, coalfish, Rudolph’s rorqual
Gestation Period
10.5 to 12 months
Common Name
Sei Whale
Number Of Species

Sei Whale Physical Characteristics

  • Grey
  • White
Skin Type
Top Speed
32 mph
70 years plus
Up to 31 tons
64 feet, females bigger

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“One of the fastest whales in the ocean.”

Hunted until there was only a handful left in the world, this sleek baleen whale has made an impressive comeback, even though it is still classified as endangered under the IUCN Red List.

4 Incredible Sei Whale Facts!

Read on for some fascinating facts about the sei whale.

  • Sei whales can be hard to tell from the fin whales. The descriptions of the two baleen whales are similar, but the fin whale is larger.
  • Though it’s one of the largest animals on the planet, its diet is made up of some of the smallest, including krill and copepods.
  • This cetacean is usually solitary, but thousands of whales can come together in locations where the food is especially abundant.
  • Like other cetaceans, the sei whale is probably descended from an animal called Pakicetus. The description of this terrestrial animal is of a wolf-like, wolf-sized creature with an unusually long snout and jaws full of vicious teeth.

Sei Whale Classification and Scientific Name

The Sei Whale is a baleen whale, which means it has baleen plates in its mouth to strain its food from the water. Its scientific name is Balaenoptera borealis, which translates into something like “finned whale from the north.” “Balaena” is Latin for “whale” and “pteron” is ancient Greek for “fin.” Borealis is Latin for “northern.” “Sei” is the Norwegian word for “pollock,” a fish that arrives in the northern seas at the same time as the whale. Like all whales and dolphins, the Sei Whale is a mammal, which means it is warm-blooded and nurses its young with milk.

Sei Whale Species

There are two subspecies:

  • Balaenoptera borealis borealis
  • Balaenoptera borealis schlegelii

Sei Whale Appearance

This animal can get up to 64 feet long with a body that’s a bit more slender than other baleen whales such as the Right Whale or the Humpback. The body tapers toward the tail and the skin is a dark gray that resembles galvanized steel. The ventral part of the whale is white or a light gray around the 40 to 50 grooves that expand when the whale eats. They run halfway between the flippers and the navel. The right lower lip is also gray, and the leading edges of this baleen whale’s flukes are white. The flukes are small in proportion to the rest of the animal. The baleen plates are grayish-black, though the inner bristles are white. Baleen is made out of keratin, the same material that makes up fingernails.

The description of the whale’s head includes a snout that’s a little arched and a rostrum that’s somewhat blunt but has a ridge down the middle. The location of the two blowholes is on top of the head. The dorsal fin is sickle-shaped and found near the tail.

Sei Whale Mother and Her Calf


Sei Whale Mother and Her Calf

©Christin Khan, NOAA / NEFSC, Public domain, via Wikimedia Commons – License

Distribution, Population, and Habitat

This species is found in most of the seas and oceans of the world, but it tends to avoid the tropics, the polar regions, and bodies of water that are partially closed off such as the Mediterranean Sea or the Gulf of Mexico.

As of 2021, the population of this species was about 80,000 individuals, but there used to be three times as many whales. Their numbers were decimated due to overhunting, and though the whale is protected, it is still considered endangered according to the International Union for Conservation of Nature, or IUCN.

Predators and Prey

The only predators of sei whales that scientists have solid information on are humans, who started to hunt the fast and elusive whale when other whale species were depleted. Over a quarter-million sei whales were killed during the 19th and 20th centuries. Threats to sei whales also include red algae blooms, which are toxic to the animal and seem to be responsible for mass deaths. Sei whales are rarely troubled by ectoparasites, but they often bear scars from lampreys and cookiecutter sharks, and their digestive tracts are full of endoparasites such as protozoans and parasitic worms.

As for the whale’s diet, it consists of small marine animals such as krill and anchovies. The whale feeds on its side with its mouth open. When it closes its mouth it uses its tongue to push seawater out through its baleen plates while leaving the food, which it then gulps down. Sei whales eat close to 2000 pounds of food every day.

From toxic red algae blooms to humans, the Sei Whale population has been severely depleted.

©Brurou, CC BY-SA 4.0, via Wikimedia Commons – License

Reproduction and Lifespan

Female sei whales normally give birth every other year. Sei whales mate during the winter months in whatever region they occupy. This means they mate from November to February in the northern hemisphere and from May to July in the southern. After this, the female is pregnant from 10.5 months to a year before she gives birth to one calf that’s about 15 feet long. She’ll nurse the calf for half a year or a little longer.

Both male and female sei whales are sexually mature when they’re about 10 years old but are not fully grown until they’re 25. The lifespan of a sei whale can be up to 74 years.

Fishing and Cooking

Sei whales have not been extensively hunted since commercial whaling was banned in 1986, though some numbers are still taken for “research.” Before this, sei whale flesh was considered a delicacy in Norway and was expensive because the fast-swimming whale was hard to catch.


The sei whale population is believed to be around 80,000 as of 2021, but this is down drastically from their pre-whaling numbers. Though females are thought to give birth every other year, biologists believe that some may give birth yearly due to population pressure.

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Sei Whale FAQs (Frequently Asked Questions) 

Where are sei whales found? 

Sei whales’ habitat is in the deeper parts of the world’s temperate oceans. They avoid locations such as gulfs, narrow seas, and tropical or polar waters.

What is a sei whale?

A sei whale is a baleen whale that grows to over 60 feet long. It uses baleen plates made out of keratin to strain its diet of copepods and other tiny animals from the ocean water. Another of the facts about the sei whale is that it’s also a rorqual whale, which means it has grooves along its belly that allow its mouth to expand. Not every baleen whale is a rorqual whale.

How many sei whales are left?

There are about 80,000 sei whales left, but they are still endangered and haven’t recovered from nearly a century of intensive whaling.

How long does a sei whale live?

A sei whale can live as long as 74 years.

Why are sei whales important?

Sei whales are important because they provide needed balance to the ecosystem of the earth’s waters. For example, they eat krill and other small marine life that might imbalance the ecosystem if their populations exploded.

What is the weight of a sei whale?

It’s not unheard of for a sei whale to have a weight of about 31 tons or 62,000 pounds.

Thank you for reading! Have some feedback for us? Contact the AZ Animals editorial team.


  1. MarineBio, Available here:
  2. Animal Diversity Web, Available here:
  3. International Whaling Commission, Available here:
  4. Wikipedia, Available here:

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