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Sei Whale


Latin: Balaenoptera borealis
Gaelic: Muc-mhara-sei



Months: July – October 

Length: Up to 19.5 metres

Range: Patchy global distribution, absent in polar regions

Threats: Marine litter, acoustic disturbance, pollution

Diet: Plankton, small fish and squid



Sei whales have a large, slender body that can measure up to 19.5 metres in length. They are dark grey on the upper side with a light underside and often have circular scars on the skin which are thought to be caused by predators or parasites.The dorsal fin is relatively large and curved, and is positioned about two-thirds along the body. Sei whales do not raise their tail flukes above the water when diving. The blow is tall and columnar and is visible at the same time as the dorsal fin when the whale surfaces.


Sei whales commonly travel alone or in small groups, although they can occur in larger groups where food is abundant. They are among the fastest of the large whales, capable of reaching speeds of 30 mph in short bursts and can occasionally breach.


The global distribution of sei whales is unpredictable and poorly understood. They occur throughout all major oceans worldwide, except the polar regions, although they are known for their sudden influxes to an area and subsequent disappearances. Migration patterns and population divisions are also poorly understood. Sei whales are predominantly an offshore species. They are rarely seen in Hebridean waters and reports include stranded animals.



Sei whales feed by skimming the surface of the water to filter smaller prey such as zooplankton, as well as lunging at high speed into a concentration of small fish or squid. A sei whale will eat about 900 kg of prey each day. They regularly swim on their sides during feeding.


Following large-scale commercial hunts for sei whales, the worldwide population was estimated at about 54,000 animals in 2006. Threats to the species now include entanglement in, and ingestion of, marine litter, disturbance from boat traffic and acoustic interference.