Sei Whale

Sei Whale: Balaenoptera borealis

IUCN status: Endangered

Status and distribution summary:

The Sei Whale population is around 55,000, with a world wide distribution except for the polar seas, but is more numerous in the southern hemisphere. The southern population is isolated and some scientists suggest that it is a different species. Most populations are migratory from sub-tropical waters to summer in cool temperate and sub-polar seas.


Heavily exploited by 19th and 20thC commercial whaling, which killed c250,000 animals, current populations are estimated at only 20% of pre-whaling numbers. Some limited whaling has resumed by the Iceland in the Atlantic and Japan in southern Pacific.

Other threats:

Not thought to be at risk other than from changes in prey abundance in response to climate change.

Where it is seen:

In deeper waters over the continental slope and the Abyssal Plain, but has very occasionally been sighted in shallower waters in the north of Biscay.

Frequency of sightings:

Sei Whales are encountered much less frequently in the Bay of Biscay than are Fin Whales, and are arguably less frequently seen that they used to be.


A large rorqual (up to 20 metres in length).

In regular surfacing they usually show the following features:

  • Appears blackish at sea, though in good light may appear dark bluish or browny grey, with a paler lower lip on both sides
  • Has a relatively tall, erect dorsal fin with a hooked back tip, which is set relatively far forward (less than 2/3 to the tail)
  • The blow is distinct, but not as tall or as columnar as Fin Whale


Usually seen alone or in pairs.

Often surfaces at a very shallow angle, so the blow is visible as the dorsal fin appears.

Even breathing pattern, blowing every 20 - 40 seconds for 2 to 4 minutes before deep diving.

Dorsal and back stay in view for longer than other species.

Tends to slide under the surface when diving, without arching the tail stock or showing the flukes.

Breaches only rarely, normally only once at a shallow angle.

Confusion species:

  • Fin Whale; is larger, with proportionally smaller dorsal fin set further back, simultaneous blow and dorsal fin are unusual
  • Minke Whale; much smaller, not normally visible blow and arches its back and tail stock on diving