Northern Ireland's Priority Species

Balaenoptera borealis – sei whale


Balaenoptera borealis Lesson, 1828
Family: Balaenopteridae

The sei whale is a large, deep-water, baleen whale. It is found in warm and temperate waters in all oceans but not normally in the coastal waters around Northern Ireland. In 2006, a juvenile male, 10m long, swam into Larne Lough, County Antrim and eventually became stranded on the shores of Islandmagee. This is the first confirmed stranding of this species in Ireland since 1914.

In brief

  • Very rare in the seas around Northern Ireland
  • Found mainly in the open ocean
  • Large, fast swimming whale
  • Listed as a UK Priority species

Species description
The sei whale is a large, streamlined, fast-swimming whale that can grow up to 20m long. The body is dark bluish-grey, with lighter mottling on the sides and a paler underside; it has an almost metallic sheen in the water. The hooked dorsal fin can be up to 60cm tall and is located two-thirds of the way along the back. The head is narrow, with a single ridge running from the two blow holes to the end of the snout. There are 38-60 throat grooves that allow the throat to expand when the whale is feeding. The 300-400 baleen plates, on the upper jaws, are black with a metallic sheen. Each plate has a distinctive fringe of very fine, soft, white bristles.

Life cycle
Sei whales feed by skimming around 900kg of food a day (mainly plankton, small fish and squid) from the surface waters of the ocean.
They are usually found singly or in small family groups. Females give birth to a single 4-5m long calf, which is fed on milk for 6-9 months. The lifespan of a sei whale is thought to be about 70 years.

Similar species
The sei whale can be confused with the fin whale and the minke whale. Fin whales are larger (21-26m long). The minke whale is smaller (8-10m long) and has a distinctive diagonal white band on the upper surface of the front flippers.

How to see this species
Since the sei whale is usually found in deep water, far out in the open ocean, you would be unlikely to see this species in the seas around Northern Ireland. There have been no confirmed sightings off the coast of Northern Ireland and only one known stranding in Co. Antrim in 2006.

Current status
The European offshore population was estimated to be around 360 individuals in 2007.

  • Listed on Schedule 5 of the Wildlife (N. Ireland) Order 1985
  • Listed in Schedule 2 of the Conservation of Habitats and Species Regulations 2010
  • Listed in Annex II and III of the Convention on the Conservation of European Wildlife and Natural Habitats (Bern Convention)
  • Listed in Annex IV of the Conservation of Natural Habitats and of Wild Fauna and Flora (the Habitats Directive)
  • Listed in Appendix I CITES
  • Listed in Schedule 2 of the The Conservation (Natural Habitats, etc.) Regulations (Northern Ireland) 1995

Why is this species a priority in Northern Ireland?

  • Listed as a UK Priority species
  • Classified as Endangered (EN) on the IUCN Red List 2008

Threats/Causes of decline
Sei whales were targeted extensively by whalers in the 1960s and 70s when other large whale species became hard to find. Over 25,000 sei whales were taken in 1964-1965 but the catch dropped to just 150 animals in 1978-1979, by which time the species had been hunted almost to extinction. “Scientific whaling” by Japan still presents a threat to this endangered species.

  • Entanglement in fishing gear
  • Collisions with ships
  • Ocean pollution (chemical and noise)

Conservation of this species

Current action
Sei whales are included in the UK Grouped Species Action Plan for baleen whales, which was published in 1999.

  • Post mortem and tissue studies are carried out on stranded whales to establish the cause of death and condition of the animals at the time of death
  • In 2008 the Northern Ireland Environment Agency initiated a cetacean monitoring programme to provide information on the distribution and relative abundance of cetaceans in Northern Ireland waters. The information collected will enable the future selection of marine protected areas for cetaceans

Proposed objectives/actions

  • Avoid accidental harm to whales when present in Northern Ireland waters
  • Contribute to international measures for the conservation of whales

What you can do
To report sei whale sightings to CEDaR, telephone 028 90395264 or email

Further information

Irish Cetacean Review 2000-2009, The Irish Whale and Dolphin Group

Grouped species action plan for baleen whales

National action plan - Grouped plan for baleen whales

Joint Nature Conservation Committee (JNCC) priority species page

NBN Gateway: Sei Whale (Balaenoptera borealis) grid map

Irish Whale and Dolphin Group

ARKive - Sei whale


Text written by:
Angela Ross

iNaturalist: Species account : iNaturalist World Species Observations database