Sei Whale (Balaenoptera borealis)


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Sei whales (pronounced “say” or “sigh”) are members of the baleen whale family and are considered one of the “great whales” or rorquals. Two subspecies of sei whales are recognized, B. b. borealis in the Northern Hemisphere and B. B. schlegellii in the Southern Hemisphere.

These large animals can reach lengths of about 40-60 feet (12-18 m) and weigh 100,000 pounds (45,000 kg). Females may be slightly longer than males. Sei whales have a long, sleek body that is dark bluish-gray to black in color and pale underneath. The body is often covered in oval-shaped scars (probably caused from cookie-cutter shark and lamprey bites) and sometimes has subtle “mottling”. This species has an erect “falcate”, “dorsal” fin located far down (about two-thirds) the animals back. They often look similar in appearance to Bryde’s whales, but can be distinguished by the presence of a single ridge located on the animal’s “rostrum”. Bryde’s whales, unlike other rorquals, have three distinct prominent longitudinal ridges on their rostrum. Sei whales have 219-410 baleen plates that are dark in color with gray/white fine inner fringes in their enormous mouths. They also have 30-65 relatively short ventral pleats that extend from below the mouth to the naval area. The number of throat grooves and baleen plates may differ depending on geographic population.

When at the water’s surface, sei whales can be sighted by a columnar or bushy blow that is about 10-13 feet (3-4 m) in height. The dorsal fin usually appears at the same time as the blowhole, when the animal surfaces to breathe. This species usually does not arch its back or raise its flukes when diving.

Sei whales are usually observed singly or in small groups of 2-5 animals, but are occasionally found in larger (30-50) loose aggregations. Sei whales are capable of diving 5-20 minutes to opportunistically feed on plankton (e.g., copepods and krill), small schooling fish, and cephalopods (e.g., squid) by both gulping and skimming. They prefer to feed at dawn and may exhibit unpredictable behavior while foraging and feeding on prey. Sometimes seabirds are associated with the feeding frenzies of these and other large whales.

Sei whales become sexually mature at 6-12 years of age when they reach about 45 feet (13 m) in length, and generally mate and give birth during the winter in lower latitudes. Females breed every 2-3 years, with a gestation period of 11-13 months. Females give birth to a single calf that is about 15 feet (4.6 m) long and weighs about 1,500 pounds (680 kg). Calves are usually nursed for 6-9 months before being weaned on the preferred feeding grounds. Sei whales have an estimated lifespan of

This species is classified as ENDANGERED by the IUCN's Red List.
This species is classified as ENDANGERED by the IUCN’s Red List.

50-70 years.


ESA Endangered – throughout its range
MMPA Depleted – throughout its range
CITES Appendix I – throughout its range

Species Description

up to 100,000 pounds (45,000 kg)
up to 40-60 feet (12-18 m)
long, sleek body that is dark bluish-gray to black in color and pale underneath; similar toBryde’s whales
50-70 years
plankton (like copepods and krill), small schooling fish, and cephalopods (like squid)
usually observed alone or in small groups of 2-5 animals

RELATIVE SPECIES: All baleen whales. Similar species include: Byrde’s Whale and Fin Whale.

NEIGHBORING SPECIES: Fin, Blue, and Humpback Whales. All other marine species except for those found in extreme northern and southern latitudes.


THREATS: Entanglement in nets, ship strikes and whaling. They used to be a targeted species for whaling, however they are now protected as an endangered species.

DIET: They have a more diverse diet than other baleen whales. Small fish, krill, squid, zooplankton and other copepods.

MANNER OF FEEDING: Gulping large mouthfuls of water and food or skimming through the water with their mouths open as they filter food.

BEHAVIOR: Travel alone or in groups of 2-5. They are almost as fast as the Fin whale.Unlike other baleen whales, the Sei whale will rarely arch its back when stating a deep dive. They are also never seen raising their flukes out of the water. Most baleen whales have the same migratory pattern or range that they stay in every year. The Sei Whale has very unpredictable distribution. It may not return to an area for many years. They are erratic swimmers, often changing directions. These whales stay mostly at the surface.

REPRODUCTION: Become sexually mature between 6-12 years. Females breed in the winter time every 2-3 years in tropical waters. Gestation last 11-13 months. The calf stays with the mother for 6-9 months.

LIFE SPAN: 50-70 years


Sei whales prefer subtropical to subpolar waters on the continental shelf edge and slope worldwide. They are usually observed in deeper waters of oceanic areas far from the coastline.


Sei whales have a cosmopolitan distribution and occur in subtropical, temperate, and subpolar waters around the world. They prefer temperate waters in the mid-latitudes, and can be found in the Atlantic, Indian, and Pacific Oceans. During the summer, they are commonly found in the Gulf of Maine, and on Georges Bank and Stellwagen Bank in the western North Atlantic. The entire distribution and movement patterns of this species is not well known. This species may unpredictably and randomly occur in a specific area, sometimes in large numbers. These events may occur suddenly and then not occur again for long periods of time. Populations of sei whales, like other rorquals, may seasonally migrate toward the lower latitudes during the winter and higher latitudes during the summer.

Population Trends

The most recent stock assessment reports with population estimates are available on our website.



  • ship strikes
  • interactions with fishing gear, such as traps/pots


  • whaling and hunting
    During the 19th and 20th centuries, sei whales were targeted and greatly depleted by commercial hunting and whaling, with an estimated 300,000 animals killed for their meat and oil.

Conservation Efforts

In December 2011, we published a final recovery plan [pdf] for the sei whale. We published the draft recovery plan [pdf] in July 2011.

Regulatory Overview

This species is protected under the Marine Mammal Protection Act of 1972 and the Endangered Species Act of 1973 as amended.


Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Cetacea
Family: Balaenopteridae
Genus: Balaenoptera
Species: borealis


  • Reeves, R. R., P. A. Folkens, et al. (2002). Guide to Marine Mammals of the World. New York, Alfred A. Knopf. p. 226-229.
  • Shirihai, H. and B. Jarrett (2006). Whales, Dolphins and Other Marine Mammals of the World. Princeton, Princeton University Press. p. 56-58.
  • NOAA


  • Often found with pollack in Norway, the name “sei” comes from the Norwegian word for pollack, “seje.” (Reeves et al.2002)
  • Sei whales scientific name is derived from the Latin word Balaenoptera for “winged whale” and borealis for “northern” (Reeves et al. 2002)
  • Their blow can reach 10 feet (3 m) high.
  • Considered one of the fastest swimming cetaceans, they can reach speeds of over 34 mph (55 km/hr).
  • An average sei whale eats about 2,000 pounds of food per day.