Marine Mammal Stranding Database

Marine Mammal Species Description

Sei Whale

Balaenoptera borealis

Lesson, 1828


Order: Cetacea
Family: Balaenopteridae
Alternate Common Names:

Status: ESA Endangered/ MMPA Depleted


   Length: 46-62 ft (14-19 m)
   Weight: 18,700-34,400 lbs (8,500-15,600 kg); maximum 44,090 lbs (20,000 kg)

Sei whales have 32-60 ventral grooves or pleats that extend just beyond their flippers. They have a single ridge down the center of their head. These whales are in the suborder Mysticeti, or the baleen whales. Baleen whales do not have teeth, but have plates attached to their upper jaws. These baleen plates are hard, but flexible, which baleen whales use to filter their food from the water. They are dark gray over most of their body except for a white belly. Round scars may make them appear mottled.


Sei whales occur in deep waters, they are seldom seen in shallow, coastal waters. They typically occur in schools of less than five or six individuals while in the wintering grounds. They tend to migrate by themselves or as mother/calf pairs. On the northern feeding grounds schools can become large. If food is abundant, schools of up to 100 individuals can occur.


Sei whales eat copepods, squid, fish, and other small invertebrates. They skim feed as well as gulp food. They feed most often at dawn, and less often later in the day.


Sei whales become sexually mature when they reach 42-45 ft (12.7-13.7 m) length, or around 10 years old. In the past sei whales became mature at 12-13 years old. The reduction in age to sexual maturity is believed to be a response to overharvest by the whaling industry. The gestation period is about 12 months and females nurse their young for about 7 months.


Sei whales are one of the fastest swimming baleen whales; they can swim up to (30 knots).

Distribution / Range:

Sei whales occur in all oceans worldwide, except for the polar regions. Off the coast of eastern North America, they are most common in the North Atlantic, spending time in sub-polar areas in the summer for feeding and south to more tropical regions for calving in the winter.

Similar species:

Sei whales can be distinguished from the smaller minke whales by lacking a white band on their flippers and their larger size. Both fin and Bryde?s whales have a large, falcate dorsal fin. Sei whales also have a single ridge running the length of their head, where Bryde?s whales have three ridges that run the length of their head.



American Cetacean Society. 2004. American Cetacean Society Fact Sheet. Accessed January 2012.

Horwood, J. 2009. Sei Whale - Balaenoptera borealis. In: Encyclopedia of Marine Mammals 2nd Ed. Perrin W.F., B. Würsig, and J.G.M. Thewissen, eds. Academic Press, New York, pp. 1001-1003.

Jefferson, T.A., S. Leatherwood, and M.A. Webber. 1993. FAO species identification guide. Marine mammals of the world. Rome, FAO. 320 p. 587 figs.

Potter, C.W. and B. Birchler. 1999. Sei whale, Balaenoptera borealis. In The Smithsonian Book of North American Mammals. Wilson, D.E. and S. Ruff, eds., 248-249. Smithsonian Institution in association with the American Society of Mammologists, Washington DC.

Reilly, S.B., Bannister, J.L., Best, P.B., Brown, M., Brownell Jr., R.L., Butterworth, D.S., Clapham, P.J., Cooke, J., Donovan, G.P., Urbán, J. & Zerbini, A.N. 2008. Balaenoptera borealis. In: IUCN 2011. IUCN Red List of Threatened Species. Version 2011.2. Downloaded on 09 January 2012.