Balaenoptera borealis
Sei Whale [+]

Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Cetacea
Family: Balaenopteridae
Genus: Balaenoptera
species: Balaenoptera borealis
+ITIS +WoRMS +Conservation Status

Description & Behavior

The sei whale, Balaenoptera borealis (Lesson, 1828), is a slender cetacean, although more robust than the fin whale. It is a Rorqual whale, (Family Balaenopteridae, the family that also includes the blue whale, Bryde's whale, fin whale, humpback whale, and minke whale), and these long, slender whales are much more streamlined than other large whales.

Sei whale, Balaenoptera borealis

Local Names: Coalfish whale; Pollack whale; Rudolphi's rorqual; Sardine whale; Japan finner. The name 'sei' refers to the whales' habit of arriving off the northern Norwegian coast with the seje (coalfish).

There are two subspecies of sei whales:

1. sei (northern hemisphere), B. borealis borealis
2. sei (southern hemisphere), B. borealis schlegellii

The dorsal fin is well-defined and slightly hooked and is located about two-thirds back along the body. The head and jaws are rather narrow and slightly arched, unlike in other rorquals. It is a mottled dark gray in color, with paler undersides and between 38-60 throat grooves. There are 636-680 dark baleen plates per animal, the longest of which is 78 cm. The sei whale is between 13.6-16 m long, with the longest recorded animal at 18.3 m; females are generally larger than males. It weighs between 20,000-25,000 kg.

Sei whales tend to swim in pods of 3-5 animals, and rarely dive deeper than 300 m. Although little is known about how this species communicates, it has been found that low-frequency pulses are common.

The largest known sei whale measured 20 m in length, although most whales are between 12.2-15.2 m long. Of this length, the head and body make up about 13 m. 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 25-61 cm.

The body is typically a dark steel gray with irregular white markings ventrally. The ventrum has 38-56 deep 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.

Little is known about the actual social system of these animals. Groups of 2-5 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." During mating season, males and females may form a social unit, but strong data on this issue are lacking.

Sei whales are among the fastest cetaceans, swimming at speeds of up to 50 kph. Although distinguished by their speed, sei whales are not remarkable divers. These whales dive only to shallow depths, and they remain submerged only 5-10 minutes at a time.

World Range & Habitat

The sei whale is found in virtually every ocean and sea in the world. However, they do not like extreme environments and are less likely to be found in polar waters than other rorquals. The subarctic and subantarctic are favored for summer feeding, but the majority of sei whales are found in temperate and sub-tropical waters, and migrations between these areas are annual.

This species is essentially a dweller of the open ocean, not generally found inshore or in coastal waters. The sei whale tends to follow shelf contours and plankton gatherings.

» GBIF occurrence data in Google Earth [Requirements | Tips] | Ocean Biogeographic Information System (OBIS) [World Map] | [about]

Feeding Behavior (Ecology)

The sei whale will take whatever is in abundance locally, whether it be fish (up to 30 cm long), squid or plankton, as long as it is shoaling (schooling). It swims on one side through the shoaling fish and then filters out the water with its baleen plates and tongue.

The sei whale obtains food by skimming through the water and catching prey in its baleen plates. An average sei whale eats about 900 kgs of zooplankton (copepods, amphipods, euphausiids (includes krill) and small fish every day.

Life History

Mating occurs during the winter months. Sei whales in the Northern Hemisphere mate between November-February, whereas mating in the southern hemisphere occurs between May-July. Gestation lasts from 10 1/2-12 months. Females typically give birth to a single calf measuring 4.5 m in length. There are reports of rare multiple fetuses. The calf nurses for 6-7 months. Young reach sexual maturity at 10 years of age, but do not reach full adult size until they are about 25 years old.

Females typically give birth every other year, but a recent increase in pregnancies has been noted.

Longevity: Approximately 70 years. Estimated Current Population: 65,000 animals.

Comments

The sei whale was not traditionally a target for whalers because it lived in the same waters as blue, fin and humpback whales, which were the preferred quarry. However, when these stocks began to decline and then became protected, sei whales became the primary catch from the mid-1960s onwards. By the mid-1970s, the sei whale stocks were 'fished out' and the species earned protected status in 1979. The global population of these whales is estimated at only 57,000. 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 populations. 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.

References & Further Research

BioOne ~ CITES ~ Discover Life ~ GBIF ~ Google Scholar ~ ITIS ~ IUCN RedList ~ MarineBio Network ~ NCBI ~ SCIRIS ~ SIRIS ~ Tree of Life Web Project ~ Wikipedia

U.S. Fish & Wildlife Service Endangered Species
Whale and Dolphin Conservation Society (WDCS)
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.

Search the Web for Sei Whale » ARKive ~ Ask.com ~ Ask Jeeves ~ bing ~ deviantART ~ dmoz ~ Dogpile ~ Google Images ~ MySpace Images ~ OceanFootage ~ Picsearch ~ StumbleUpon ~ Yahoo! Images ~ YouTube

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Citation: Balaenoptera borealis, Sei Whale - MarineBio.org. Retrieved , from .

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Balaenoptera borealis Sei Whale [+]