Sei whale

Balaenoptera borealis

Derived from the Norwegian word for Pollack, there are two subspecies of the little-known sei whale: the northern and the southern.

The nicknames ‘pollack whale', ‘coalfish whale' and ‘sardine whale' come from the fact that the appearance of sei whales often signalled the presence of large numbers of these fish.

Other names: Rudolphi's rorqual; Japan finner' Sardine whale; Coalfish whale

Baleen whale illustration
Male Female Calf
Maximum length 17m 19.5m 4.5m
Maximum weight Unknown 45,000kg 1000kg

IUCN conservation status: Endangered

What do sei whales look like?

The bodies of sei whales range in colouring from murky blues to steely greys to deep browns. Along their heads, a single ridge runs from the tip of the nose to  two blowholes and set  aloft  their backs is a sickle-shaped dorsal fin. Growing up to 19.5 metres long, female sei whales are the larger of the two sexes, eclipsing their male counterparts by over 2 metres.

What is life like for sei whales?

When they are not feeding, sei whales can be quite playful. Sometimes breaching, their bodies leave the water at a low angle and finish with a graceful belly flop. Fast movers when they want to be, sei whales can really ramp up the speed.

Usually seen in small groups, numbers of sei whales will often increase when food sources are plentiful. Most common in the southern hemisphere, they prefer deep waters and will only approach shores around islands. Although northern and southern hemisphere populations don’t mingle, sei whales have been known to mate with fin whales, even producing young together.

The current population size is unknown.

What do sei whales eat?

Sei whales don’t gulp-feed like the other whales in their family. Skimming the water for food while swimming, their short ventral pleats and bristle-packed baleen help them to collect as much delicious food as possible. Tucking into a variety of treats, fish, squid, krill, copepods and zooplankton are all part of their diets.

Where do sei whales live?

Sei whales are pretty adaptable. They are found in nearly all of the world’s waters from the sub-Arctic and sub-Antarctic, apart from the Mediterranean and Baltic seas. Feeding in colder waters in summer, it is believed that they turn tail in winter and migrate to warmer, lower latitudes.

Severely depleted by commercial hunting, populations of sei whale populations are still vulnerable, facing danger from noise and chemical pollution, vessel strikes, global warming, sonar and entanglement in fishing gear.

How to identify a sei whale?

Although they rarely display their flukes above water, sei whales leave ‘fluke prints’ on the surface of the water during shorter, shallower dives which allows researchers to follow them.

Distribution map

orca-distribution-map

Sei whales need your help

The main threats...

  • Pollution – toxic chemicals from plastics, litter and oil spills build up in sei whales, seriously harming their health.
  • Fishing gear – sei whales get accidentally caught in fishing nets and lines, injuring or even killing them.

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