Sei Whale Facts

Sei Whale Profile

Sei whales (pronounced ‘say’ whales) are large, slim marine mammals, sometimes measuring up to 20m in length. They are incredibly fast swimmers and are often considered one of the fastest of all whale species.

These huge, steel-grey whales belong to the group of baleen whales, which all have bristly mouth plates which act as filters to separate krill and plankton from the ocean waters.

Sei whales are very adaptable and are found in all oceans and seas except for polar and tropical waters. There are two subspecies of the sei whale, the northerrn and southern. They are rarely found near shore and prefer swimming in more open areas, not far from the surface of the water.

sei whale facts
Photo credit: © ritaleitao (

Sei Whale Facts Overview

Habitat: Marine, preferring deeper waters
Location: Nearly all the world’s oceans and seas
Lifespan: 74 years in the wild
Size: Around 15m in length, but some have been known to measure up to 20m
Weight: Up to 17 tonnes
Color: Dark-grey with a white underbelly
Diet: Plankton, fish and crustaceans
Predators: Killer whales. Calves sometimes get killed by larger shark species
Top Speed: 50 kph (31 mph)
No. of Species:
Conservation Status:

Sei whales are usually found travelling in small pods of around six whales, but are often seen travelling alone. They feed near the ocean’s surface, opening their huge mouths to scoop up krill, plankton and fish in the water. Upon closing their mouth, sei whales push out the water through their baleen plates while krill, fish or plankton are swallowed. When prey is abundant, sometimes hundreds of individuals are seen feeding together.

Though they are fast and streamlined animals, sei whales and especially their calves, are predated upon by killer whales and large sharks. They tend to avoid areas where these predators are found in large numbers and they use their speed to try out-swim their pursuers. Sei whales are also able to breach; leaping out of the water, and this tactic is sometimes used to warn other whales that predators are nearby.

Sei whales generally appear to be a quiet and relaxed species but like most whales, they do enjoy a bit of play time. Leaping out the water, chasing fish and swimming close to one another are some of the activities sei whales partake in after feeding.

Like most whales, sei whales do not produce young very often. Females are capable of birthing a calf around every 3 years, after a pregnancy that can last up to a year. A single calf is usually born and can measure over 4m in length at birth. The calf is completely dependent on its mother until about seven months old. Calves are most vulnerable to predators when very young and usually swim within touching distance of their mother. Young sei whales will only reach adult size when they are 25 years old or more.

Sei whales are an important part of the ecosystem in which they inhabit. One of the main benefits they provide is maintaining a stable food chain. Sei whales can eat over 900kg of food in a day and this helps prevent overpopulation of certain species. When they dive, they also stir up nutrients in the water, which leads to more phytoplankton production and thus more food for other animals. Their faeces also adds nitrogen to the water, which in turn increases phytoplankton and krill numbers.

Sei whales once numbered in the hundreds of thousands but their population is currently at around 80,000 individuals worldwide. In the 1900s, sei whales were hunted in their thousands for their oil and meat. This caused dramatic population crashes and the sei whale was nearing extinction.

Fortunately the sei whale is now legally protected, and while they are classified as endangered, their numbers are slowly increasing.

Interesting Sei Whale Facts

1. Sei whales are the 3rd largest whale alive

They are the third-largest rorqual after the blue whale and the fin whale. Measuring up to 20m in length, they are some of the biggest whale species in the ocean.

Sei Whale
Photo credit: © Isaac Clarey (

2. Their Latin name means ‘northern winged whale’

Due to the shape and position of their pectoral fins, sei whales look as if they have wings.

3. The are often found near a species of fish called pollock

The name ‘Sei’ comes from the Norwegian word ‘Seje’, which means pollock. The sei whale is often found near shoals of these fish and hence they are named after them.

They are also sometimes called the ‘coalfish whale’ and ‘sardine whale’, for the same reason! 1

4. They are one of the fastest whales in the ocean

Swimming at speeds over 50 kph (31 mph), sei whales are one of the fastest whales in the ocean. Although able to swim very quickly through the water, sei whales do tire rather quickly.

When searching for food, they travel at a leisurely pace which, due to their streamlined body shape, is still fairly fast compared to many other whale species. 

5. Sei whales eat a huge amount in a day

A single sei whale can eat almost a tonne of food in a single day.

When feeding, they swim on their side near the surface with their mouth open through swarms of its prey, before closing it and filtering out the water.

Their diet consists primarily of copepods, krill, and small fish such as anchovies, sardines and mackerel.

6. They migrate

Other than the polar regions, sei whales are found in most oceans across the globe. They do, however, migrate annually from cool and subpolar waters in the summer to temperate and subtropical waters in the winter, where food is more plentiful. 2

7. They have two blowholes, rather than one

Like other baleen whales, sei whales have 2 blowholes rather than 1. Both blowholes sit next to each other and are allow the whale to take breaths without exposing their heads. When pushing air out of their blowhole, water shoots up into the air.

It is during these surface visits that sei whales are most commonly seen.

Another indicator that a sei whale is swimming by is their sickle-shaped dorsal fin, though short compared to other species it still can measure over 60cm in height. Sei whales are also referred to as winged whales due to their short pectoral fins which flap like wings when swimming.

8. Their blows can reach very high heights

The column of water and air produced from a sei whale blowhole can reach around 13ft high.

Sei Whale blowing water!

9. Sei whales do not dive deep

Unlike other whales, sei whales do not dive head-first but rather slowly sink into deeper waters.

10. They are relatively long-lived

Healthy sei whales may live to be well over 70 years old in the wild.

11. Females are larger than males

Although commonly weighing up to 17 tonnes, some females may weigh much more than this.

12. They can hold their breath for a long time

When feeding, sei whales can spend more than 20 minutes without coming to the surface to take a breath.

13. Sei whales bellyflop when they breach

Though they don’t breach often, sei whales are able to get most of their body out of the water before landing on their backs or bellies.

14. They sing

Like other whale species, sei whales make long, loud, low-frequency calls to each other and ‘sing’.

The research took place over two years in the Falkland Islands using acoustic devices. It’s believed to be linked to reproductive behaviour, with males singing to attract females.

Sadly, not all their vocalisations are as uplifting.

A former whaler was quoted that they used to make crying sounds when they were hunted and killed for oils in the early 1900’s.

“When we killed the sei whales, they used to make a noise, like a crying noise. They seemed so friendly, and they’d come round and they’d make a noise, and when you hit them, they cried really. I didn’t think it was really nice to do that. Everybody talked about it at the time I suppose, but it was money. At the end of the day that’s what counted at the time. That’s what we were there for.”

Roddy Morrison, from Lewis in the Outer Hebrides shared his painful memories from the time. 3

15. Sei whales are endagered

The current population is estimated to be 80,000, accounting for nearly one-third of the pre-whaling population.

But their true number is unknown the sei whale is listed as endangered under the endangered species act. The sei whale has various threats that include pollution, vessel strikes, entanglement in fishing gear and traps, increased ocean noise which can cause whales to strand, and climate change. 4

Beached Sei Whale
Photo credit: © Steve Dimock (

Sei Whale Fact-File Summary

Scientific Classification

Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Artiodactyla
Family: Balaenopteridae
Genus: Balaenoptera
Species Name:
Balaenoptera Borealis

Fact Sources & References

  1. WDC, “Sei whale, Balaenoptera borealis“,
  2. NOAA Fisheries (2023), “Sei whale“, NOAA Fisheries US Gov.
  3. Adam Nicholson (2014), “Britain’s Whale Hunters: The Untold Story“, BBC.
  4. COSEWIC Assessment and Status Report (2003), “Sei whale“,