Sei Whale

Hunted until there were 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 whale. 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.

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 2025, the global population of sei whales is estimated to be between 50,000 and 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.

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 for 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. Japan previously took some sei whales for ‘scientific research’ until 2019, but as of 2025, sei whales are not being hunted for research or commercial purposes. Before this, sei whale flesh was considered a delicacy in Norway and was expensive because the fast-swimming whale was hard to catch.

Population

The sei whale population is believed to be between 50,000 and 80,000 as of 2025, 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.