Sei whale (Balaenoptera borealis)

Distribution data supplied by the Ocean Biodiversity Information System (OBIS). To interrogate UK data visit the NBN Atlas.Map Help



Balaenoptera borealis is a baleen whale and can be recognised as such by the plates of baleen (rather than teeth) suspended from the upper jaw and the two blowholes on the upper body. The sei whale is slender bodied and can reach up to 16 m in length. It is a member of the rorqual family with the characteristic ventral pleats of skin under the eye and the relatively flat and broad jaw. The ventral pleats do not extend up to the navel but end near the pectoral fin. The flippers are a uniform dark colour and the upper body is a uniform blue-grey colour. The sei whale has a dorsal fin rising at a steep angle on the back. It has a single prominent ridge on the snout.

Recorded distribution in Britain and Ireland

Occasionally seen off the coasts of north and north-west Scotland and west Ireland but one sighting also puts it off the west coast of devon.

Global distribution



The sei whale is an open ocean whale, not often seen near the coast. It can be found at the surface or diving down to a few hundred metres.

Depth range


Identifying features

  • Up to 16 m in length.
  • Uniform dark blue-grey dorsal and lateral colouration; white underneath.
  • Thick tail base.
  • Tall, steep dorsal fin on posterior third of the back.
  • Between 32-62 ventral pleats ending before the navel.
  • Longest ventral pleat ends past flippers.

Additional information

At a glance, the sei whale can be easily be confused with the minke whale Balaenoptera acutorostrata but can be distinguished by having dark coloured flippers and a uniform blue-grey upper body. The sei whale can also be differentiated from Bryde's whale Balaenoptera edeni by having only a single prominent ridge on the rostrum.

Sei whales usually congregate in small groups of up to 5 individuals, although in feeding areas up to 30 have been seen together. It seldom breeches, and when diving, it does not show the tail flukes. It can remain submerged for up to 20 minutes (Kinze, 2002).


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  4. Kinze, C. C., 2002. Photographic Guide to the Marine Mammals of the North Atlantic. Oxford: Oxford University Press.

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  1. Bristol Regional Environmental Records Centre, 2017. BRERC species records recorded over 15 years ago. Occurrence dataset: accessed via on 2018-09-25.

  2. Merseyside BioBank., 2017. Merseyside BioBank (verified). Occurrence dataset: accessed via on 2018-10-01.

  3. NBN (National Biodiversity Network) Atlas. Available from:

  4. Norfolk Biodiversity Information Service, 2017. NBIS Records to December 2016. Occurrence dataset: accessed via on 2018-10-01.

  5. OBIS (Ocean Biodiversity Information System),  2024. Global map of species distribution using gridded data. Available from: Ocean Biogeographic Information System. Accessed: 2024-05-23

  6. South East Wales Biodiversity Records Centre, 2018. SEWBReC Mammals (South East Wales). Occurrence dataset: accessed via on 2018-10-02.

  7. South East Wales Biodiversity Records Centre, 2018. Dr Mary Gillham Archive Project. Occurance dataset: accessed via on 2018-10-02

  8. Suffolk Biodiversity Information Service., 2017. Suffolk Biodiversity Information Service (SBIS) Dataset. Occurrence dataset: accessed via on 2018-10-02.

  9. Whale and Dolphin Conservation, 2018. WDC Shorewatch Sightings. Occurrence dataset: accessed via on 2018-10-02.


This review can be cited as:

Barnes, M.K.S. 2008. Balaenoptera borealis Sei whale. In Tyler-Walters H. and Hiscock K. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 23-05-2024]. Available from:

Last Updated: 24/06/2008