The Sei whale (scientific name: Balaenoptera borealis), is a very large marine mammal, in the family of Rorquals (Balaenoptera), part of the order of cetaceans. The Sei is a baleen whale, meaning that instead of teeth, it has long plates which hang in a row (like the teeth of a comb) from its upper jaws. Baleen plates are strong and flexible; they are made of a protein similar to human fingernails. Baleen plates are broad at the base (gumline) and taper into a fringe which forms a curtain or mat inside the whale's mouth. Baleen whales strain huge volumes of ocean water through their baleen plates to capture food: tons of krill, other zooplankton, crustaceans, and small fish..
The Sei whale is relatively slender bodied and can reach up to 16 metres in length. It is a member of the rorqual family with the characteristic ventral pleats of skin under the eye and the relatively flat and broad jaw. The ventral pleats do not extend up to the navel but end near the pectoral fin. The flippers are a uniform dark colour and the upper body is a uniform blue-grey colour. The sei whale has a dorsal fin rising at a steep angle on the back. It has a single prominent ridge on the snout.
Unlike other rorquals, Sei whales have a dolphin-like dorsal fin. They are also unusual in using two different methods to fill their mouths with water during feeding: they both gulp and skim-feed. During feeding, these whales can be found in large numbers, typically centred around concentrations of copepods, a crustacean they favor. Otherwise, they occur in smaller groups of six or less.
The Sei whale is an endangered species, and it has been protected by the International Whaling Commission since the mid-1980s.
The common name, pronounced "sigh," comes from the Norwegian word for codfish, which Sei whales are known to eat. "Rorqual" is also a word of Scandinavian origin, meaning tubed, and refers to the grooved, expandable throats of the six species of whales in the family Balaenopteridae.
Superficially, the Sei whale can be easily be confused with the Minke whale, Balaenoptera acutorostrata, but can be distinguished by having dark coloured flippers and a uniform blue-grey upper body. The Sei whale can also be differentiated from Bryde's whale, Balaenoptera edeni, by having only a single prominent ridge on the rostrum.
Sei whales usually congregate in small groups of up to five individuals, although in feeding areas up to 30 have been seen together. It seldom breaches, and when diving, it does not show the tail flukes. It can remain submerged for up to 20 minutes (Kinze, 2002).
Few details of the natural history of this whale are known. They tend to occur in groups of between two and five individuals, but larger groups may form in areas where food is abundant. Capable of travelling at great speed, this species is thought to migrate into warmer waters at lower latitudes during the winter months. Little is known of communication in this species, but individuals are known to make many low frequency vocalisations.
The Sei whale is smaller in size than the Fin whale (Balaenoptera physalus), and can be distinguished from this similar species because it has symmetrical colouring on the lower parts of its head. It is also similar to Bryde's whale (Balaenoptera edeni), but has only one ridge on the upper surface of the head, whereas Bryde's whale has three. The blow or spout of the Sei whale is a single thin cloud, which reaches about three metres in height. The skin is a mottled dark grey colour, with white grooves along the paler underparts. The baleen is grey to black with paler fringes and less than 80 centimetres in length. The dorsal fin is conspicuous, has a slightly hooked shape and is located two-thirds along the length of the body. The common name Sei arose from the arrival of this whale off the coast of Norway tending to coincide with that of coalfish Seje.
There are two subspecies of the Sei whale:
- Balaenoptera borealis schlegelii, Southern Sei whale, Southern Hemisphere
- Balaenoptera borealis borealis, Northern Sei whale, Northern Hemisphere
The largest known Sei whale measured 20 metres in length, although most individuals are between 12.2 and 15.2 meres long. Of this length, the head and body make up about 13 metres, with males being 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 ventral markings. The ventrum has 38 to 56 deep grooves, which are thought to have some feeding function. Each side of the upper part of the mouth contains 300 to 380 ashy-black baleen plates. The fine inner bristles of these plates are whitish.
Sei whale body mass varies from 8500 to 11,300 kilograms (kg) in males; the sexual dimorphism leads to a body mass of 8600 to 15,000 kg in females. Diagnostic distinguishing characteristics of the Sei are:
- Blow readily visible as a six metre high vertical straight column
- Blow less dense than that of a Fin whale
- Curved dorsal fin mid-back
- Colour is slate grey
- Occasional round scars.
Sei whale key behaviors include natatorial, motile, migratory and social elements. Little is known about the compete social system of these mammals. Groups of two to five individuals are typically observed, but sometimes thousands may gather where prey are plentiful. 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 swiftest cetaceans, swimming at speeds of up to 50 kilometres 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.
Sei key reproductive features are:
- Seasonal breeding
- Gonochoric/gonochoristic/dioecious (sexes separate)
- Sexual reproduction
Females are sexually mature at about five to six years of age. During mating season, males and females may form a social unit, but detailed data on this subject are lacking.
Mating occurs during the winter months. Sei whales in the Northern Hemisphere mate between November and February, whereas mating in the Southern Hemisphere occurs between May and July. Gestation lasts from 10.5 to 12.0 months. Females typically give birth to a single calf measuring 450 centimetres in length. There are rare reports of multiple fetuses. The calf nurses for six or seven months. Young reach sexual maturity at ten years of age, but do not reach full adult size until they are about 25 years old.
Females typically give birth every other year, although a recent increase in pregnancies has been noted. Researchers think this may be a response to the predation rate. Humans kill a great many Sei whales each year, and this taking may have effects on their reproductive activity.
Maximum longevity of the species in the wild is 74 years of age.
Distribution and Movements
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.
The Sei whale is an open ocean species, seldomly observed near the coast. It can be found at the surface or diving downward to a few hundred metres.
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
The current economic importance of this whale is questionable. However, in the past, these large whales provided considerable 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.
Threats and 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 IUCN Red List classifies the Sei as Endangered. The global population of these whales is estimated at only 57,000. Hunting of these whales by humans has been intense since the 1950s. The take of these animals peaked in the 1964-65 season, when 25,454 of these whales were recorded as taken. The reported global catch of Sei whales in the 1978-79 season was only 150, underscoring the dramatic decline in whale populations. Some researchers have concluded that Sei whale populations are rising as a result of decreases in Fin and Blue whale poulations. However, this conclusion must be taken with caution, since actual data are scarce, and the dietary overlap between Sei whales and these other species is not complete.
Although not a traditional target of the whaling industry, the Sei whale began to be exploited after the Blue, Fin and Humpback whale stocks became depleted and protected. This species was then relentlessly hunted in the 1960s and 70s, before the International Moratorium on Commercial Whaling came into effect in 1986. At present, the species is vulnerable to chemical and noise pollution.
In 1976, this species received protected status, and the moratorium on commercial whaling took effect beginning in 1986. There are ongoing problems with the moratorium, however, and Iceland announced in 2001 that it may soon resume commercial whaling of Sei, Fin and Minke whales. Other countries also oppose the ban, and the future of endangered species such as the Sei whale is not yet secure. There are signs, however, that populations of this little known cetacean are starting to recover from past exploitation.
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