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Sei Whale Fact Sheet

Sei Whale
Balaenoptera borealis

New York Status: Endangered
Federal Status: Endangered


Drawing of Sei Whale

The sei whale (pronounced "say") measures 25-50 feet (8-15 m) in length and weighs 40 tons (36 metric tons), making it the third largest baleen whale, behind the blue and finback whales. It can be identified by its inverted "V" shaped water spout which reaches 6-8 feet into the air. The body is dark gray with variable white undersides, usually limited to the area of the throat grooves which do not reach as far as the navel. In Antarctic and Pacific populations, there are light-colored patches all over the upper body. However, Atlantic populations lack the patches and are more uniformly dark. The sei whale resembles a blue or finback whale, but has a smaller and more curved dorsal fin and is dark on the undersides of the flippers and tail flukes. This whale may be the fastest of the large whales, able to cruise at 16 mph (14 knots) with a maximum speed of 40 mph (35 knots) recorded. Sei whales usually travel alone or in small groups of five or less.

Life History

In autumn, these whales migrate several thousand miles to equatorial waters. The mating season occurs from December to April, during which time they eat very little or fast, living off their fat reserves. A single calf measuring 15 feet in length is born after a gestation period of 10-12 months. Calves are weaned on the summer grounds at the age of 6-9 months. Both sexes reach maturity at 5-15 years of age, and the female gives birth to one calf every 2-3 years. Members of this species may live for up to 70 years.

Distribution and Habitat

Sei whales are found in the North Atlantic Ocean ranging from Iceland south to the northeastern Venezuelan coast, and northwest to the Gulf of Mexico. There are also records from Cuba and the Virgin Islands. Sei whales are seen infrequently in U. S. waters. This whale breeds and feeds in open oceans, and is generally restricted to more temperate waters. Unlike most rorqual whales, the sei whale feeds mostly by filtering plankton while swimming (skim feeding), but is also known to gulp-feed krill, shrimp, and small fish.


This species became important to the whaling industry as populations of blue and finback whales declined. Its earliest exploitation began in the 17th century, off northern Japan. Hunting began in the North Atlantic in the 1800's. In 1972, stocks in the North Pacific were estimated to be only 21% of original numbers. According to the Cetacean and Turtle Assessment Program (1982), there may be as few as 2,200-2,300 individuals in U. S. Atlantic waters currently. Although sei whales are relatively free of ectoparasites, many suffer from endoparasitic helminths (flatworms) which may cause kidney and liver problems.

Management and Research Needs

Hunting is no longer a problem for sei whales as a result of protection received through the International Whaling Commission and the Endangered Species Act. Iceland is still allowed an annual quota in the North Atlantic Ocean. A stranding network on Long Island operated by the Riverhead Foundation for Marine Research and Preservation under contract to DEC serves to alert marine biologists to the possible rescue of live animals and the identification and autopsy of carcasses. The hotline number to report strandings is (631)369-9829.

Additional References

Katona, S. K., V. Rough and D. T. Richardson. 1983. A field guide to the whales, porpoises, and seals of the Gulf of Maine and eastern Canada. Charles Scribner's Sons, New York. pp. 69-74.

Leatherwood, S., D. K. Caldwell and H. E. Winn. 1976. Whales, dolphins, and porpoises of the western North Atlantic -- a guide to their identification.National Oceanic and Atmospheric Administration, National Marine Fisheries Service NOAA Tech. Report NMFS CIRC-396, Seattle, Washington. pp. 32-36.

Leatherwood, S. and R. R. Reeves. 1983. The Sierra Club handbook of whales and dolphins. Sierra Club Books, San Francisco. 320 pp

Drawing by Elane Eckert

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