Balaenoptera borealissei whale

Geographic Range

These whales are found in all oceans and adjoining seas, except polar and tropical regions. These animals occupy temperate and subpolar regions in the summer, but migrate to sub-tropical waters during the winter.


These pelagic whales are found far from shore.

Physical Description

The largest known Sei whale measured 20 meters in length, although most whales are between 12.2 and 15.2 meters long. Of this length, the head and body make up about 13 meters. Males are slightly smaller than females. Sei whales have a relatively slender body with a compressed tail stock that abruptly joins the flukes. The snout is pointed, and the pectoral fins are short. The dorsal fin is sickle shaped and ranges in height from 25 to 61 centimeters.

The body is typically a dark steel gray with irregular white markings ventrally. The ventrum has 38-56 deeps grooves, which may have some feeding function. Each side of the upper part of the mouth contains 300 - 380 ashy-black baleen plates. The fine inner bristles of these plates are whitish.

  • Average mass
    2e+07 g
    704845.81 oz


During mating season, males and females may form a social unit, but strong data on this issue are lacking.

Mating occurs during the winter months. Sei whales in the Northern Hemishpere mate between November and February, whereas mating in the southern hemisphere occurs between May and July. Gestation lasts from 10 1/2 to 12 months. Females typically give birth to a single calf measuring 450 cm in length. There are reports of rare multiple fetuses. The calf nurses for six or seven months. Young reach sexual maturity at 10 years of age, but do not reach full adult size until they are about 25 years old. Sei whales may live as long as 74 years.

Females typically give birth every other year, but a recent increase in pregnancies has been noted. Researchers think this may be a response to the predation rate. Humans kill a great many whales each year, and this might have effects on their reproductive activity.

  • Breeding interval
    Females typically give birth every other year
  • Breeding season
    Mating occurs during the winter months
  • Average number of offspring
  • Average number of offspring
  • Range gestation period
    10.5 to 12 months
  • Range weaning age
    6 to 7 months
  • Average age at sexual or reproductive maturity (female)
    10 years
  • Average age at sexual or reproductive maturity (female)
    Sex: female
    3652 days
  • Average age at sexual or reproductive maturity (male)
    10 years
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    3652 days



Little is known about the actual social system of these animals. Groups of two to five individuals are typically observed, but sometimes thousands may gather if food is abundant. However, these large aggregations may not be dependent on food supply alone, as they often occur during times of migration. Norwegian workers call the times of great Sei whale abundance "invasion years."

Sei whales are among the fastest cetaceans, swimming at speeds of up to 50 kilometers per hour. Although distinguished by their speed, Sei whales are not remarkable divers. These whales dive only to shallow depths, and they remain submerged only five to ten minutes at a time.

Communication and Perception

Food Habits

The Sei whale obtains food by skimming through the water and catching prey in its baleen plates. These whales feed near the surface of the ocean, swimming on their sides through swarms of prey. An average Sei whale eats about 900 kilograms of copepods, amphipods, euphausiids and small fish every day.

Economic Importance for Humans: Positive

The current economic importance of this whale is questionable. However, in the past, these large whales provided a great deal of income to the whaling industry. It cannot be stressed enough, however, that the positive economic effects of hunting this animal have been acheived only by large scale decimation of Sei whale populations. By overharvesting the whales, the whaling industry experienced a short term economic gain at a long term cost-- the reduction in the number of whales available for harvest.

Conservation Status

Sei whales are listed as CITES appendix 1 from the equator to Antarctica. All other populations are listed as CITES appendix 2. The global population of these whales is estimated at only 57,000. Hunting of these whales by humans has been high since the 1950s. The take of these animals peaked in the 1964-65 season, when 25,454 of these whales were taken. The reported global catch of Sei whales in the 1978-79 season was only 150, showing the dramatic drop in whale populations. Some researchers have concluded that Sei whale populations are rising as a result of decreases in Blue and Fin whale poulations. However, this conclusion must be taken with caution, as actual data are scarce, and the dietary overlap between Sei whales and these other species is not complete.


Nancy Shefferly (author), Animal Diversity Web.


Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


uses smells or other chemicals to communicate


the nearshore aquatic habitats near a coast, or shoreline.


animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


makes seasonal movements between breeding and wintering grounds


having the capacity to move from one place to another.


specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.


an animal that mainly eats plankton

seasonal breeding

breeding is confined to a particular season


reproduction that includes combining the genetic contribution of two individuals, a male and a female


associates with others of its species; forms social groups.


uses touch to communicate


reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.


animal constituent of plankton; mainly small crustaceans and fish larvae. (Compare to phytoplankton.)


Gaskin, D.E. 1982. The ecology of whales and dolphins. Heinemann, London, Exeter and New Hampshire.

Nowak, R.M. 1991. Walker's Mammals of the world, Fifth edition. John Hopkins University Press, Boston.