Sei Whale: Balaenoptera borealis

Taxonomic Classification

Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Cetacea
Family: Mysticeti
Genus/Species: Balaenoptera borealis
Common Name: Sei whale

Listing Status

Federal Status: Endangered
FL Status: Federally-designated Endangered
FNAI Ranks: Not ranked
IUCN Status: EN (Endangered)

Physical Description

Sei whales are large marine mammals that can reach a length up to 60 feet (18.3 meters) and a weight of 100,000 pounds (45,359 kilograms).  This species is primarily gray with oval shaped white scars due to bites from cigar sharks and lampreys (fish).  They have a pointed snout, short pectoral fins, and a dorsal fin that is erect and found at the end of the back.  Sei whales can blow water straight up out of their blow hole to a height up to 13 feet (four meters) (NMFS 2011, National Oceanic & Atmospheric Administration, n.d.). 

Life History

Sei whales are classified as baleen whales – they filter water for food through baleen plates.  Their diet primarily consists of zooplankton (copepods and krill); however, small fish and squid can also be part of their diet (National Oceanic & Atmospheric Administration, n.d.).

Breeding seasons differ in the geographical regions as the Northern Hemisphere sei whale population breeds between November and February, while the Southern Hemisphere population breeds between the months of May and July.  The total gestation period can last up to the 12 months, with the female giving birth to one calf every other year.  Calves are weaned around seven months.  Sei whales reach sexual maturity at ten years old and reach full-body size at around 25 years old (National Oceanic & Atmospheric Administration, n.d.). 

Habitat and Distribution

Sei Whale Distribution MapThe sei whale can be found around the outer continental shelf and slope in the Atlantic, Pacific, and Indian Ocean (National Oceanic & Atmospheric Administration, n.d.).


Historically, the sei whale population has been seriously depleted due to human capture and harvesting.  Between the 19th and 20th century, over 300,000 sei whales were killed and harvested by humans (National Oceanic & Atmospheric Administration, n.d.).  The whaling from that time period could still affect the population today if more females were killed than males.  In the 1980’s the International Whaling Commission External Website made whaling illegal; however, illegal poaching still continues today.   Sei whales also face a threat from ship and boat hits when they come to the surface for air.  Other threats include incidental capture in fish nets that can cause whales to drown.

Conservation and Management

The sei whale is protected as an Endangered species by the Federal Endangered Species Act and as a Federally-designated Endangered species by Florida’s Endangered and Threatened Species Rule External Website.  Sei whales are also Federally protected as a Depleted species by the Marine Mammal Protection Act.

Other Informative Links

International Union for Conservation of Nature External Website
National Oceanic & Atmospheric Administration External Website
Smithsonian National Museum of Natural History External Website
Texas A&M University Corpus Christi External Website



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National Marine Fisheries Service. 2011. Draft Recovery Plan for the Sei Whale (Balaenoptera borealis). National Marine Fisheries Service, Office of Protected Resources, Silver Spring, MD.105 pp.

National Oceanic and Atmospheric Association. (n.d.). Sei Whale. Retrieved June 7, 2011 from NOAA National Marine Fisheries Service: External Website

Image Credit NOAA

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