Sei Whale

Balaenoptera borealis
Rudolphi's rorqual, Pollack whale, Coalfish whale, Sardine whale, Japan finner
A Sei whale is a slender cetacean, although it is more robust than a fin whale. These rorquals are long and slender, and so are much more streamlined than other large whales. They have a compressed tail stock which abruptly joins the flukes. Their snout is pointed, and their pectoral fins are short. Their dorsal fin is shaped like a sickle. They typically have a dark steel gray body with irregular white markings on the underside. These plates have whitish fine inner bristles.

population size

50-75 yrs

Life span

48 km/h

Top Speed

20-25 t


14-16 m



Sei whales live in all oceans and seas, except tropical and polar regions. They occupy sub-polar and temperate regions during the summer, migrating to sub-tropical waters in the winter. These whales occur in the open ocean and generally avoid coastal waters.

Habits and lifestyle

Sei whales gather in groups that number two to five individuals, although larger groups can form where food is very plentiful. They prefer to eat at dawn and may demonstrate unpredictable behavior during foraging and eating. They are fast swimmers, and are believed to migrate to warmer waters in lower latitudes during winter. Sei whales are one of the fastest cetaceans. However, they are not remarkable divers, descending only to shallow depths, remaining submerged for just five to ten minutes. Sei whales typically do not rise high above the water when they dive, usually only sinking below the surface. Their dorsal fin and blowholes are often visible above the surface of the water almost simultaneously. A Sei whale almost never lifts its flukes up above the surface, it is generally less active at the surface of the water and it rarely breaches.

group name

pod, gam, herd, school, mod

Diet and nutrition

Sei whales eat whatever is abundant locally, if it is a shoal (school) species: fish, squid or plankton.


Mating habits

Sei whales are polygynous, one male having exclusive mating rights with many females. Mating takes place during the winter. Females mate every 2-3 years and gestation lasts 11-13 months. Females typically give birth to one calf. Mothers nurse their calves and look after them until they are weaned at the age of 6 - 9 months. Sei whales are sexually mature when they are about 10 years old.

Mating behavior

Reproduction season

in winter

Pregnancy duration

11-13 months

Independent age

6-9 months

female name


male name


baby name

1 calf

baby carrying


Population status


Population threats

Sei whales are under threat by climate change and also by pollution, entanglement in fishing gear and ship strike.

Population number

According to the U.S. National Marine Fisheries Service, the total size of the Sei whale population is around 80,000 individuals. This species is classified as Endangered (EN) on the IUCN Red List.

Fun facts for kids

  1. The name "sei" comes from the Norwegian "seje", used to describe a type of fish which gathers Norway's coast around the same time as sei whales.
  2. The scientific name of Balaenoptera borealis has the meaning “winged whale” and “northern.”
  3. After the Blue whale and Fin whale, the Sei is the 3rd largest animal living on the planet. Being a baleen whale, the Sei has two blowholes and no teeth, which are replaced with as many as 400 baleen plates full of bristles that filter out prey from the water.
  4. Seis can stay up to 20 minutes beneath the water while looking for food.
  5. These whales communicate with growl-like low-frequency calls.
  6. Sei whales eat a lot and can consume up to 2,000 pounds of food each day.
  7. Roy Chapman Andrews, the American naturalist, compares Sei whales to the cheetah because, although they are able to swim very quickly for a short distance, they tire fairly quickly.
  8. Sei whales prefer to travel on their own or in a small pod but they will sometimes gather in their thousands where there is abundant food.


  1. Sei Whale Wikipedia article
  2. Sei Whale on The IUCN Red List site