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.
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.
- Peter Saundry; Encyclopedia of Life. 2011. Sei whale. Topic ed. C.Michael Hogan. Ed-in-chief Cutler J.Cleveland. Encyclopedia of Earth. National Council for Science and the Environment, Washington DC http://www.eoearth.org/article/Sei_Whale
- Sei whale, Encyclopedia of Life (accessed February 17, 2011)
- Horwood, J. (2002) Sei whale. In: Perrin, W.F., Würsig, B. and Thewissen, J.G.M. Eds. Encyclopedia of Marine Mammals. Academic Press, London
- Kinze, C. C., (2002). Photographic Guide to the Marine Mammals of the North Atlantic. Oxford: Oxford University Press.
Mammal Species of the World
More images, video and sound
- Original description: "Lesson, René Primevère, 1828. Histoire naturelle générale et particulière des Mammifères et des Oiseaux découverts depuis 1788 jusqu'à nos jours, Baudoin Frères, Paris, 1:342,"
The Sei whale according to MammalMAP
Sei whales (Balaenoptera borealis) are long, slender whales that are more streamlined than the other large whales belonging to the Balaenopteridae family. They are usually between 12 – 16 meters long and weigh in between the ranges of 20 000 – 25 000 kgs.
Little is known about social dynamics and communication structure of sei whales but groups of 2 – 5 animals are usually seen. Groups of thousands can occur when food is abundant and during times of migration. Sei whales typically live in sub-polar and temperate regions in the summer then migrate to sub- tropical waters in winter to breed (May – July).
Sei whales are among the fastest cetaceans, swimming up to 50 km/hr. However, they are not good divers – they rarely dive deeper than 300 m. Sei whales also tend to be surface feeders. Like other baleen whales, they skim the water for surface plankton, copepods and krill. They also ingest small schooling fish and squid.
The IUCN lists sei whales as an endangered species. The global population has been reduced by 80% due to commercial whaling in the past. Sei whales are now internationally protected although hunting occurs under a controversial research program conducted by Japan.
At a glance, 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 5 individuals, although in feeding areas up to 30 have been seen together. It seldom breeches, and when diving, it does not show the tail flukes. It can remain submerged for up to 20 minutes (Kinze, 2002).
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.
Biogeographic Regions: indian ocean (Native ); atlantic ocean (Native ); pacific ocean (Native )
In the North Pacific, Sei Whales in summer are distributed mainly north of 40°N, including the Gulf of Alaska and Aleutian Islands (US), and to some extent into the Bering Sea, but not into the Okhotsk Sea. The wintering grounds are poorly known, but Sei Whales were formerly caught in winter off the Bonin Islands (Japan) (IWC 2006); animals tagged there have been recaptured throughout the summer range (Masaki 1977).
In the North Atlantic the summer distribution seems to be quite variable from year to year; however, Sei Whales typically occur north of an arc running from south of Nova Scotia in the west, to the northwestern British Isles and Trondheim (Norway) in the east. They occur as far north as the Davis Strait and the northern end of the Denmark Strait. Occasional incursions into other areas have been noted (e.g. the Gulf of Maine, Schilling et al. 1993). Peaks of catch rates in early and late summer suggested a northward and southward migration through the former Nova Scotia whaling ground (Mitchell 1975). Sei Whales have been scarce in Norwegian waters in recent years despite significant catches there in the past. Sei Whales were caught in limited numbers, mainly in late summer, off northwestern Spain (Aguilar and Sanpera 1982), and have been recorded in winter as far south as the Caribbean Sea and Cap Blanc, Mauritania (Rice 1998).
The summer (Jan–Feb) distribution in the southern hemisphere is mainly in the zone 40–50°S in the South Atlantic and southern Indian oceans, and 45–60°S in the South Pacific (Miyashita et al. 1995). Known wintering grounds include a number of former low–latitude whaling grounds, including northeastern Brazil at 7°S (da Rocha 1983), Peru at 6°S (Valdivia et al. 1982), and in earlier years off Angola and the Congo (IWC 2006). Catches off western South Africa (Donkergat) and eastern South Africa (Durban) showed peaks in spring and autumn, suggestive of populations on migration routes (Horwood 1987).
Chilean whaling records (32–38°S) show catches and sightings of “Sei” Whales throughout the year including summer until the early 1980s, but some of these may have been Bryde’s Whales. However, some Sei Whales have been observed feeding in summer at around 42°S off Chile in recent years (Galletti et al. 2005).
occurs (regularly, as a native taxon) in multiple nations
Regularity: Regularly occurring
Type of Residency: Year-round
Regularity: Regularly occurring
Type of Residency: Year-round
Global Range: (>2,500,000 square km (greater than 1,000,000 square miles)) Worldwide, but distribution and movements during much of year are poorly known. Coast of Mexico to Gulf of Alaska in the eastern North Pacific. Bering Sea to Japan and Korea in the western North Pacific. Gulf of Mexico to Davis Strait (especially off eastern Canada) in the western North Atlantic. Norway to Spain and northwestern Africa in the eastern North Atlantic. In Southern Hemisphere, Antarctic Ocean to coasts of Brazil, Chile, South Africa, and Australia. See IUCN (1991) for further details.
- North-West Atlantic Ocean species (NWARMS)
- UNESCO-IOC Register of Marine Organisms
Distribution in Egypt
- IUCN Red Book, NMFS/NOAA Technical Memo
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.
Other Physical Features: endothermic ; bilateral symmetry
Average mass: 2e+07 g.
Length: 1860 cm
Weight: 2.0E7 grams
- IUCN Red Book, NMFS/NOAA Technical Memo
Size in North America
Range: 14-18.6 m
Range: 8,500-11,300 kg males; 8,600-15,000 kg females
Maximum size is less than that of fin and blue whales. Dorsal fin is decidedly taller, more falcate, and located farther forward (a little more than one-third the body length forward from the fluke notch) than in other large baleen whales. Differs from the Bryde's whale in having a taller, less sharply pointed dorsal fin and a single dorsal rostral ridge rather than three ridges. Differs from the fin whale in the more upward-angled dorsal fin (located farther forward on the body), the lack of asymmetrical lower lip coloration, lack of whitish dorsal chevrons, lack of mixed color baleen, and fewer throat grooves (32-60 vs. 56-100); dorsal fin of sei whale tends to surface simultaneously with the head, rather than later as in the fin whale. Differs from the blue whale in the less U-shaped snout and much larger and more anterior dorsal fin. (Leatherwood and Reeves 1983).
- North-West Atlantic Ocean species (NWARMS)
These pelagic whales are found far from shore.
Aquatic Biomes: coastal
Habitat and Ecology
Habitat Type: Marine
Comments: Generally in deep water; along edge of continental shelf and in open ocean.
- North-West Atlantic Ocean species (NWARMS)
- UNESCO-IOC Register of Marine Organisms
Water temperature and chemistry ranges based on 166 samples.
Depth range (m): 0 - 0
Temperature range (°C): -1.353 - 26.222
Nitrate (umol/L): 0.154 - 28.455
Salinity (PPS): 31.523 - 36.516
Oxygen (ml/l): 4.657 - 8.106
Phosphate (umol/l): 0.054 - 1.949
Silicate (umol/l): 0.775 - 70.561
Temperature range (°C): -1.353 - 26.222
Nitrate (umol/L): 0.154 - 28.455
Salinity (PPS): 31.523 - 36.516
Oxygen (ml/l): 4.657 - 8.106
Phosphate (umol/l): 0.054 - 1.949
Silicate (umol/l): 0.775 - 70.561
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.
- IUCN Red Book, NMFS/NOAA Technical Memo
Non-Migrant: No. All populations of this species make significant seasonal migrations.
Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).
Locally Migrant: Yes. At least some populations of this species make annual migrations of over 200 km.
Migrates between lower-latitude wintering grounds and higher-latitude feeding grounds. Movements in specific areas may be unpredictable (Leatherwood and Reeves 1983).
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.
Animal Foods: fish; zooplankton
Primary Diet: planktivore
Comments: Eats copepods, euphausiids, squid, and various small schooling fishes. May skim feed on copepods at surface or gulp feed on krill and small fishes (Leatherwood and Reeves 1983, Katona et al. 1983).
- IUCN Red Book, NMFS/NOAA Technical Memo
10,000 to >1,000,000 individuals
Comments: Total population is estimated at less than 51,000: about 14,000 in the Northern Hemisphere (mainly in the North Pacific), 37,000 or less in the Southern Hemisphere; a survey of Antarctic waters in the summer of 1989 found only 1500 in an area where perhaps 10,000 were expected (Matthews and Moseley 1990). North Atlantic population numbers a few thousand. See IUCN (1991) for further information on population sizes.
Usually travels in groups of 2-5, may concentrate in larger numbers on feeding grounds.
Life History and Behavior
Perception Channels: tactile ; chemical
- North-West Atlantic Ocean species (NWARMS)
Status: wild: 70.0 years.
Status: wild: 74.0 years.
Lifespan, longevity, and ageing
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: 1.
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 (male): 10 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous
Average birth mass: 680000 g.
Average number of offspring: 1.
Average age at sexual or reproductive maturity (male)
Sex: male: 3652 days.
Average age at sexual or reproductive maturity (female)
Sex: female: 3652 days.
Single calf is born usually in winter after a gestation period of about 11-12 months. Young nurse for about 5-9 months. Calving interval for individual adult females is 2-3 years. Sexually mature at an average age of 6-10 years.
- IUCN Red Book, NMFS/NOAA Technical Memo
Molecular Biology and Genetics
Barcode data: Balaenoptera borealis
Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species.
See the BOLD taxonomy browser for more complete information about this specimen and other sequences.
-- end --
Download FASTA File
Statistics of barcoding coverage: Balaenoptera borealis
Public Records: 2
Specimens with Barcodes: 2
Species With Barcodes: 1
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.
CITES: appendix i
IUCN Red List of Threatened Species: endangered
IUCN Red List Assessment
Red List Category
Red List Criteria
- 1994Vulnerable(Groombridge 1994)
National NatureServe Conservation Status
Rounded National Status Rank: N3 - Vulnerable
Rounded National Status Rank: N2 - Imperiled
NatureServe Conservation Status
Rounded Global Status Rank: G3 - Vulnerable
Reasons: Widespread but relatively rare throughout the world's oceans; difficult to protect due to migratory existence.
Date Listed: 06/02/1970
Lead Region: National Marine Fisheries Service (Region 11)
Where Listed: Entire
Population location: Entire
Listing status: E
For most current information and documents related to the conservation status and management of Balaenoptera borealis , see its USFWS Species Profile
Status in Egypt
- IUCN Red Book, NMFS/NOAA Technical Memo
About 14,000 Sei Whales are recorded caught by modern whaling in the North Atlantic. In addition, an unknown proportion of the approximately 30,000 unspecified large whales caught in the North Atlantic in the late 19th and early 20th centuries were Sei Whales.
Sei Whales appear to have been depleted in the eastern North Atlantic, with over 7,500 recorded taken in Norwegian and British waters prior to World War II (not counting the unspecified whales), but fewer than 200 since then. The species seems to be virtually absent there now: Norwegian surveys of northeast Atlantic waters in 1987, 1989, and annually from 1995–2005 have yielded only one Sei Whale sighting in the Eastern stock area. Small numbers of sei whales were caught during 1957–80 off northwestern Spain, mainly in late summer/autumn (Aguilar and Sanpera 1982); the stock identity of these animals is unclear.
By contrast, Sei Whales are still abundant in the central North Atlantic (Iceland-Denmark Strait IWC stock area), especially southwest of Iceland south to 50°N in summer. The only survey with reasonably complete coverage, designed specifically for the purpose of estimating sei whale abundance, was in 1989, and yielded a population estimate of 10,300 whales (CV 0.27) (Cattanach et al. 1993). Areas of Sei Whale abundance seem to shift markedly between years relative to the northern extent of the distribution, more so than for other baleen whale species (Gunnlaugsson et al. 2004).
No recent abundance estimates are available for the western North Atlantic. The population size during 1966–69 was estimated at 2,078 (Mitchell and Chapman 1977) from sightings surveys, using strip transect methodology. About 1,200 Sei Whales were taken off Nova Scotia during 1962–72.
North Pacific. The last assessment of North Pacific Sei Whales by the IWC Scientific Committee was in 1974 (IWC 1977). The exploitable population (animals of legally catchable size) was estimated to have declined from 42,000 in 1963 to 8,600 in 1974 (Tillman 1977). Over 40,000 Sei Whales were caught during this period (IWC 2006). A 75% reduction in Sei Whale catch rates of Californian whaling stations during the 1960s is consistent with an ocean-wide decline (Rice 1977). Exploitation ceased in 1975. The extent to which the population has recovered since then is unclear.
Hakamada et al. (2004) gave an estimate of 4,100 animals from one area of the western North Pacific, but attempts to extrapolate this to produce an estimate for the entire western North Pacific have not been accepted. It is likely that there has been some increase in the population since the end of exploitation, but given the tendency for sei whale distribution to change from year to year, it is hard to interpret the limited survey data that currently exist.
Sei Whales appear not to have formerly been uncommon in the eastern North Pacific. However, a preliminary estimate for the US west coast based on surveys in 1996 and 2001 is only 56 whales (Barlow 2003). In the Eastern Tropical Pacific, all potential sei/Bryde’s whales that could be specifically identified were found to be Bryde’s, suggesting that Sei Whales are now rare (Wade and Gerrodette 1993).
Southern Hemisphere. Over 200,000 Sei Whales are recorded taken by modern whaling in the southern hemisphere during 1905–1979. The IWC arbitrarily divides southern hemisphere Sei Whales, along with other southern hemisphere baleen whales, into six longitudinally defined management Areas. Sei Whales in all of the six Areas have been classified as Protection Stocks by the IWC since 1978. The greatest catches were during 1960–72, when they exceeded 5,000 in every year, topping at nearly 20,000 in 1964 alone. Most catches were taken in summer by pelagic fleets operating south of 40°S, but winter catches were also taken from land stations in Brazil, Peru and South Africa (and smaller numbers in Chile, where there is ambiguity with Bryde’s Whales).
The last assessment of southern hemisphere Sei Whales by the IWC Scientific Committee was conducted in 1979 (IWC 1980). Some extra information on these assessments is given by Horwood (1987). Based on catches/sightings per unit effort from Japanese catch and scouting vessels respectively, the “exploitable” stock size (Sei Whales of legal size, approximately 2/3 of total population), excluding Area II (South Atlantic sector), was estimated to have declined from about 64,000 in 1960 to about 11,000 by 1979. The population model used failed to produce an estimate for Area II, because nearly all the catch for this area was taken in just the two seasons 1964/65 and 1965/66, with nearly 30,000 Sei Whales being taken in these two years, yet the abundance indices suggested a more continuous decline during the 1960s and 1970s (IWC 1980). This suggests that the management Areas do not correspond to biological populations.
Other available evidence also suggests a severe and rapid decline in Sei Whale stocks during the 1960s and 1970s. The catch and sighting rates from winter whaling operations off Brazil and South Africa show even sharper declines. Abundance indices from the Brazilian whaling ground for Sei Whales during 1966–81 (da Rocha 1983) declined by ca. 90% during 1966–72.
No recovery of sei whales in Brazilian waters has been detected since that time (Zerbini et al. 1997, Andriolo et al. 2001). Catch and sightings indices off Durban, South Africa for 1965–74 (Best 1976) declined by over 95%. Tag-recapture data from pelagic whaling suggested an approximately 6-fold decline between 1962–76 (IWC 1980, Fig. 1).
IWC (1996, Table 2) gives a total population estimate for Sei Whales south of 30°S of about 10,000 based on a combination of IDCR and Japanese Scout Vessel (JSV) sightings data. It is difficult to know how much reliance to place on this estimate, because no variance is given, but it is consistent with a severe decline. No more recent summer data are available, except for the area south of 60°S, which is outside the main summer range of Sei Whales. In the absence of dedicated surveys in Sei Whale habitat, and resulting abundance estimates, it is not possible to assess whether there has been any increase in southern hemisphere sei whales since the cessation of whaling.
Generation time and maximum rate of increase. The generation time is estimated to be 23.3 years (Taylor et al. 2007). The 3-generation time window for applying the decline criterion (A) is 1937–2007.
For assessment purposes, the IWC Scientific Committee has used a natural mortality rate of 0.06, an age at first reproduction declining (with stock depletion) from 12-13 years to 10-11 years, and an annual pregnancy rate increasing (with stock depletion) from 0.27 to 0.37-0.39 (IWC 1980; Horwood 1980). Assuming a maximum annual pregnancy rate of 0.40, a minimum age of first reproduction of 10 years, and no juvenile mortality, the maximum rate of increase is 2.7% p.a. Horwood and Millward (1987) also conclude that the maximum rate of increase for sei whale populations is less than 3% per year.
Population assessment. Because the available published assessments for this species are not up to date, an updated population assessment is conducted here to enable assessment of the population reduction over the period 1937-2007 relative to the A criterion. While the available data do not permit a scientifically rigorous estimation of the extent of population reduction, it is reasonable to use conventional population assessment methods to provide a crude indication of the extent of possible reduction relative to the criteria. A conventional deterministic age-structured model with an age at first capture (“recruitment”) (ar) and an age at first reproduction (am), and linear density-dependence was applied to the North Pacific, North Atlantic and Southern Hemisphere regions separately. The results are provided in the linked PDF document, and constitutes an integral part of this assessment.
Global Short Term Trend: Unknown
Comments: Numbers have rebounded slightly if at all since most whaling was stopped by international treaty (Matthews and Moseley 1990).
Reports of other human-caused deaths of Sei Whales are rare. Two fatal ship strikes (Sei Whales found dead on the bows of ships) were reported on the US East Coast during 2000–2004 (Cole et al. 2006). It is hard to extrapolate from known cases to an estimated total, but Sei Whales appear to be at relatively low risk of human impacts, probably because of their largely offshore distribution.
Rice (1961, 1974) reported a pathological condition in several North Pacific sei whales which resulted in deterioration or loss of baleen; however, the current frequency of this condition, and its impact (if any) on the population, are unknown.
Degree of Threat: B : Moderately threatened throughout its range, communities provide natural resources that when exploited alter the composition and structure of the community over the long-term, but are apparently recoverable
Comments: Populations in all oceans have been depleted by overexploitation.
Management Requirements: A draft recovery plan for the North Pacific and North Atlantic stocks was available in August 1998 (www.nmfs.gov/tmcintyr/prot_res.html/).
Global Protection: None. No occurrences appropriately protected and managed
Comments: Whale is protected by the IWC, the US MMPA, and the US ESA.
Needs: Enforce ban on harvest set by the IWC and of the MMPA and ESA.
Relevance to Humans and Ecosystems
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.
Comments: Long exploited by shore-based and pelagic whalers, with large numbers taken beginning in the late 1800s; several thousand (probably low 10,000s) were harvested annually in the 1960s and early 1970s; see IUCN (1991) for review of exploitation history. Initially hunted for oil; now desired mainly for meat for human consumption.
IUCN Red List Category
- IUCN (2008) Cetacean update of the 2008 IUCN Red List of Threatened Species.
The sei whale (// or //), Balaenoptera borealis, is a baleen whale, the third-largest rorqual after the blue whale and the fin whale. It inhabits most oceans and adjoining seas, and prefers deep offshore waters. It avoids polar and tropical waters and semi-enclosed bodies of water. The sei whale migrates annually from cool and subpolar waters in summer to winter in temperate and subtropical waters.
Reaching 19.5 metres (64 ft) long and weighing as much as 28 tonnes (28 long tons; 31 short tons), the sei whale daily consumes an average of 900 kilograms (2,000 lb) of food, primarily copepods, krill, and other zooplankton. It is among the fastest of all cetaceans, and can reach speeds of up to 50 kilometres per hour (31 mph) (27 knots) over short distances. The whale's name comes from the Norwegian word for pollock, a fish that appears off the coast of Norway at the same time of the year as the sei whale.
Following large-scale commercial whaling during the late nineteenth and twentieth centuries, when over 255,000 whales were taken, the sei whale is now internationally protected, although limited hunting occurs under a controversial research program conducted by Japan. As of 2008[update], its worldwide population was about 80,000, nearly a third of its pre-whaling population.
- 1 Etymology
- 2 Taxonomy
- 3 Description
- 4 Anatomy
- 5 Life history
- 6 Range and migration
- 7 Whaling
- 8 Conservation status
- 9 Population estimates
- 10 See also
- 11 References
- 12 Further reading
- 13 External links
Sei is the Norwegian word for pollock, also referred to as coalfish, a close relative of codfish. Sei whales appeared off the coast of Norway at the same time as the pollock, both coming to feed on the abundant plankton. The specific name is the Latin word borealis, meaning northern. In the Pacific, the whale has been called the Japan finner; "finner" was a common term used to refer to rorquals. In Japanese, the whale was called iwashi kujira, or sardine whale, a name originally applied to Bryde's whales by early Japanese whalers. Later, as modern whaling shifted to Sanriku — where both species occur — it was confused for the sei whale. Now the term only applies to the latter species. It has also been referred to as the lesser fin whale because it somewhat resembles the fin whale. The American naturalist Roy Chapman Andrews compared the sei whale to the cheetah, because it can swim at great speeds "for a few hundred yards", but it "soon tires if the chase is long" and "does not have the strength and staying power of its larger relatives".
On 21 February 1819, a 32-ft whale stranded near Grömitz, in Schleswig-Holstein. The Swedish-born German naturalist Karl Rudolphi initially identified it as Balaena rostrata (=Balaenoptera acutorostrata). In 1823, the French naturalist Georges Cuvier described and figured Rudolphi's specimen under the name "rorqual du Nord". In 1828, Rene Lesson translated this term into Balaenoptera borealis, basing his designation partly on Cuvier's description of Rudolphi's specimen and partly on a 54-ft female that had stranded on the coast of France the previous year (this was later identified as a juvenile fin whale, Balaenoptera physalus). In 1846, the English zoologist John Edward Gray, ignoring Lesson's designation, named Rudolphi's specimen Balaenoptera laticeps, which others followed. In 1865, the British zoologist William Henry Flower named a 45-ft specimen that had been obtained from Pekalongan, on the north coast of Java, Sibbaldius (=Balaenoptera) schlegelii — in 1946 the Russian scientist A.G. Tomilin synonymized S. schlegelii and B. borealis, creating the subspecies B. b. schlegelii and B. b. borealis. In 1884-85, the Norwegian scientist G. A. Guldberg first identified the "sejhval" of Finnmark with B. borealis.
Sei whales are rorquals (family Balaenopteridae), baleen whales that include the humpback whale, the blue whale, the Bryde's whale, the fin whale, and the minke whale. Rorquals take their name from the Norwegian word røyrkval, meaning "furrow whale", because family members have a series of longitudinal pleats or grooves on the anterior half of their ventral surface. Balaenopterids diverged from the other families of suborder Mysticeti, also called the whalebone whales or great whales, as long ago as the middle Miocene. Little is known about when members of the various families in the Mysticeti, including the Balaenopteridae, diverged from each other.
The sei whale is the third-largest balaenopterid, after the blue whale (up to 180 tonnes, 200 tons) and the fin whale (up to 70 tonnes, 77 tons). In the North Pacific, adult males average 13.7 m (45 ft) and adult females 15 m (49 ft), while in the North Atlantic adult males average 14 m (46 ft) and adult females 14.5 m (47.5 ft). In the Southern Hemisphere they average 14.5 (47.5 ft) and 15 m (49 ft), respectively. In the Northern Hemisphere, males reach up to 17.1 m (56 ft) and females up to 18.6 m (61 ft), while in the Southern Hemisphere males reach 18.6 m (61 ft) and females 19.5 m (64 ft) — the authenticity of an alleged 22 m (72 ft) female caught fifty miles northwest of St. Kilda in July 1911 is doubted. The largest specimens taken off Iceland were a 16.15 m (53 ft) female and a 14.6 m (48 ft) male, while the longest off Nova Scotia were two 15.8 m (52 ft) females and a 15.2 m (50 ft) male. The longest measured during JARPN II cruises in the North Pacific were a 16.32 m (53.5 ft) female and a 15 m (49 ft) male. The longest measured by Discovery Committee staff were an adult male of 16.15 m (53 ft) and an adult female of 17.1 m (56 ft), both caught off South Georgia. Adults usually weigh between 15 and 20 metric tons — a 16.4 m (53.7 ft) pregnant female caught off Natal in 1966 weighed 37.75 metric tons (minus 6% for loss of fluids during flensing). Females are considerably larger than males. At birth, a calf typically measures 4.4-4.5 m (14.4-14.7 ft) in length.
The whale's body is typically a dark steel grey with irregular light grey to white markings on the ventral surface, or towards the front of the lower body. The whale has a relatively short series of 32–60 pleats or grooves along its ventral surface that extend halfway between the pectoral fins and umbilicus (in other species it usually extends to or past the umbilicus), restricting the expansion of the buccal cavity during feeding compared to other species. The rostrum is pointed and the pectoral fins are relatively short, only 9%–10% of body length, and pointed at the tips. It has a single ridge extending from the tip of the rostrum to the paired blowholes that are a distinctive characteristic of baleen whales.
The whale's skin is often marked by pits or wounds, which after healing become white scars. These are now known to be caused by "cookie-cutter" sharks (Isistius brasiliensis). It has a tall, sickle-shaped dorsal fin that ranges in height from 38–90 centimetres (15–35 in) and averages 53–56 centimetres (21–22 in), about two-thirds of the way back from the tip of the rostrum. Dorsal fin shape, pigmentation pattern, and scarring have been used to a limited extent in photo-identification studies. The tail is thick and the fluke, or lobe, is relatively small in relation to the size of the whale's body.
Adults have 300–380 ashy-black baleen plates on each side of the mouth, up to 80 centimetres (31 in) long. Each plate is made of fingernail-like keratin, which is bordered by a fringe of very fine, short, curly, wool-like white bristles. The sei's very fine baleen bristles, about 0.1 millimetres (0.004 in) are the most reliable characteristic that distinguishes it from other rorquals.
The sei whale looks similar to other large baleen whales. The best way to distinguish between it and Bryde's whale, apart from differences in baleen plates, is by the presence of lateral ridges on the dorsal surface of the Bryde's whale's rostrum. Large individuals can be confused with fin whales, unless the fin whale's asymmetrical head coloration is clearly seen. The fin whale's lower jaw's right side is white, and the left side is grey. When viewed from the side, the rostrum appears slightly arched (accentuated at the tip), while fin and Bryde's whales have relatively flat rostrums.
Sei whales usually travel alone or in pods of up to six individuals. Larger groups may assemble at particularly abundant feeding grounds. Very little is known about their social structure. During the southern Gulf of Maine influx in mid-1986, groups of at least three sei whales were observed "milling" on four occasions - i.e. moving in random directions, rolling, and remaining at the surface for over ten minutes. One whale would always leave the group during or immediately after such socializing bouts. The sei whale is among the fastest cetaceans. It can reach speeds of up to 50 kilometres per hour (27 kn) over short distances. However, it is not a remarkable diver, reaching relatively shallow depths for five to fifteen minutes. Between dives, the whale surfaces for a few minutes, remaining visible in clear, calm waters, with blows occurring at intervals of about 60 seconds (range: 45–90 sec.). Unlike the fin whale, the sei whale tends not to rise high out of the water as it dives, usually just sinking below the surface. The blowholes and dorsal fin are often exposed above the water surface almost simultaneously. The whale almost never lifts its flukes above the surface, and are generally less active on water surfaces than close related Bryde's whales; it rarely breaches.
This rorqual is a filter feeder, using its baleen plates to obtain its food by opening its mouth, engulfing or skimming large amounts of the water containing the food, then straining the water out through the baleen, trapping any food items inside its mouth.
The sei whale feeds near the surface of the ocean, swimming on its side through swarms of prey to obtain its average of about 900 kilograms (2,000 lb) of food each day. For an animal of its size, for the most part, its preferred foods lie unusually relatively low in the food chain, including zooplankton and small fish. The whale's diet preferences has been determined from stomach analyses, direct observation of feeding behavior, and analyzing fecal matter collected near them, which appears as a dilute brown cloud. The feces are collected in nets and DNA is separated, individually identified, and matched with known species. The whale competes for food against clupeid fish (herring and its relatives), basking sharks, and right whales.
In the North Atlantic, it feeds primarily on calanoid copepods, specifically Calanus finmarchicus, with a secondary preference for euphausiids, in particular Meganyctiphanes norvegica and Thysanoessa inermis. In the North Pacific, it feeds on similar zooplankton, including the copepod species Neocalanus cristatus, N. plumchrus, and Calanus pacificus, and euphausiid species Euphausia pacifica, E. similis, Thysanoessa inermis, T. longipes, T. gregaria and T. spinifera. In addition, it eats larger organisms, such as the Japanese flying squid, Todarodes pacificus pacificus, and small fish, including anchovies (Engraulis japonicus and E. mordax), sardines (Sardinops sagax), Pacific saury (Cololabis saira), mackerel (Scomber japonicus and S. australasicus), jack mackerel (Trachurus symmetricus) and juvenile rockfish (Sebastes jordani). Some of these fish are commercially important. Off central California, they mainly feed on anchovies between June and August, and on krill (Euphausia pacifica) during September and October. In the Southern Hemisphere, prey species include the copepods Neocalanus tonsus, Calanus simillimus, and Drepanopus pectinatus, as well as the euphausiids Euphausia superba and Euphausia vallentini and the pelagic amphipod Themisto gaudichaudii.
Mating occurs in temperate, subtropical seas during the winter. Gestation is estimated to vary around 103⁄4 months, 111⁄4 months, or one year, depending which model of foetal growth is used. The different estimates result from scientists' inability to observe an entire pregnancy; most reproductive data for baleen whales were obtained from animals caught by commercial whalers, which offers only a single snapshot of fetal growth. Researchers attempt to extrapolate conception dates by comparing fetus size and characteristics with newborns.
A newborn is weaned from its mother at 6–9 months of age, when it is 8–9 metres (26–30 ft) long, so weaning takes place at the summer or autumn feeding grounds. Females reproduce every 2–3 years, usually to a single calf. In the Northern Hemisphere, males are usually 12.8-12.9 m (42-42.3 ft) and females 13.3-13.4 m (43.6–44 ft) at sexual maturity, while in the Southern Hemisphere males average 13.6 m (44.6 ft) and females 14 m (46 ft). The average age of sexual maturity of both sexes is 8–10 years. The whales can reach ages of up to 65 years.
The sei whale makes long, loud, low-frequency sounds. Relatively little is known about specific calls, but in 2003, observers noted sei whale calls in addition to sounds that could be described as "growls" or "whooshes" off the coast of the Antarctic Peninsula. Many calls consisted of multiple parts at different frequencies. This combination distinguishes their calls from those of other whales. Most calls lasted about a half second, and occurred in the 240–625 hertz range, well within the range of human hearing. The maximum volume of the vocal sequences is reported as 156 decibels relative to 1 micropascal (μPa) at a reference distance of one metre. An observer situated one metre from a vocalizing whale would perceive a volume roughly equivalent to the volume of a jackhammer operating two metres away.
In November 2002, scientists recorded calls in the presence of sei whales off Maui. All the calls were downswept tonal calls, all but two ranging from a mean high frequency of 39.1 Hz down to 21 Hz of 1.3 second duration – the two higher frequency downswept calls ranged from an average of 100.3 Hz to 44.6 Hz over 1 second of duration. These calls closely resembled and coincided with a peak in "20- to 35-Hz irregular repetition interval" downswept pulses described from seafloor recordings off Oahu, which had previously been attributed to fin whales. Between 2005 and 2007, low frequency downswept vocalizations were recorded in the Great South Channel, east of Cape Cod, Massachusetts, which were only significantly associated with the presence of sei whales. These calls averaged 82.3 Hz down to 34 Hz over about 1.4 seconds in duration. This call has also been reported from recordings in the Gulf of Maine, New England shelf waters, the mid-Atlantic Bight, and in Davis Strait. It likely functions as a contact call.
BBC News quoted Roddy Morrison, a former whaler active in South Georgia, as saying, "When we killed the sei whales, they used to make a noise, like a crying noise. They seemed so friendly, and they'd come round and they'd make a noise, and when you hit them, they cried really. I didn't think it was really nice to do that. Everybody talked about it at the time I suppose, but it was money. At the end of the day that's what counted at the time. That's what we were there for."
Range and migration
Sei whales live in all oceans, although rarely in polar or tropical waters. The difficulty of distinguishing them at sea from their close relatives, Bryde's whales and in some cases from fin whales, creates confusion about their range and population, especially in warmer waters where Bryde's whales are most common.
In the North Atlantic, its range extends from southern Europe or northwestern Africa to Norway, and from the southern United States to Greenland. The southernmost confirmed records are strandings along the northern Gulf of Mexico and in the Greater Antilles. Throughout its range, the whale tends to avoid semi-enclosed bodies of water, such as the Gulf of Mexico, the Gulf of Saint Lawrence, Hudson Bay, the North Sea, and the Mediterranean Sea. It occurs predominantly in deep water, occurring most commonly over the continental slope, in basins situated between banks, or submarine canyon areas.
In the North Pacific, it ranges from 20°N to 23°N latitude in the winter, and from 35°N to 50°N latitude in the summer. Approximately 75% of the North Pacific population lives east of the International Date Line, but there is little information regarding the North Pacific distribution. Two whales tagged in deep waters off California were later recaptured off Washington and British Columbia, revealing a possible link between these areas, but the lack of other tag recovery data makes these two cases inconclusive. In the Southern Hemisphere, summer distribution based upon historic catch data is between 40°S and 50°S latitude in the South Atlantic and southern Indian Oceans and 45°S and 60°S in the South Pacific, while winter distribution is poorly known, with former winter whaling grounds being located off northeastern Brazil (7° S) and Peru (6° S). The majority of the "sei" whales caught off Angola and Congo, as well as other nearby areas in equatorial West Africa, are thought to have been predominately misidentified Bryde's whales. For example, Ruud (1952) found that 42 of the "sei whale" catch off Gabon in 1952 were actually Bryde's whales, based on examination of their baleen plates. The only confirmed historical record is the capture of a 14 m (46 ft) female, which was brought to the Cap Lopez whaling station in Gabon in September 1950. During cetacean sighting surveys off Angola between 2003 and 2006, only a single confirmed sighting of two individuals was made in August 2004, compared to 19 sightings of Bryde's whales.
In general, the sei whale migrates annually from cool and subpolar waters in summer to temperate and subtropical waters for winter, where food is more abundant. In the northwest Atlantic, sightings and catch records suggest the whales move north along the shelf edge to arrive in the areas of Georges Bank, Northeast Channel, and Browns Bank by mid to late June. They are present off the south coast of Newfoundland in August and September, and a southbound migration begins moving west and south along the Nova Scotian shelf from mid-September to mid-November. Whales in the Labrador Sea as early as the first week of June may move farther northward to waters southwest of Greenland later in the summer. In the northeast Atlantic, the sei whale winters as far south as West Africa such as off Bay of Arguin, off coastal Western Sahara and follows the continental slope northward in spring. Large females lead the northward migration and reach the Denmark Strait earlier and more reliably than other sexes and classes, arriving in mid-July and remaining through mid-September. In some years, males and younger females remain at lower latitudes during the summer months.
Despite knowing some general migration patterns, exact routes are incompletely known and scientists cannot readily predict exactly where groups will appear from one year to the next. F.O. Kapel noted a correlation between appearances west of Greenland and the incursion of relatively warm waters from the Irminger Current into that area. Some evidence from tagging data indicates individuals return off the coast of Iceland on an annual basis. An individual satellite tagged off Faial, in the Azores, traveled more than 4,000 km (2,500 miles) to the Labrador Sea via the Charlie-Gibbs Fracture Zone (CGFZ) between April and June 2005. It appeared to "hitch a ride" on prevailing currents, with erratic movements indicative of feeding behavior in five areas, in particular the CGFZ, an area of known high sei whale abundance as well as high copepod concentrations. Seven whales tagged off Faial and Pico from May to June in 2008 and 2009 made their way to the Labrador Sea, while an eighth individual tagged in September 2009 headed southeast – its signal was lost between Madeira and the Canary Islands.
The development of explosive harpoons and steam-powered whaling ships in the late nineteenth century brought previously unobtainable large whales within reach of commercial whalers. Initially their speed and elusiveness, and later the comparatively small yield of oil and meat partially protected them. Once stocks of more profitable right whales, blue whales, fin whales, and humpback whales became depleted, sei whales were hunted in earnest, particularly from 1950 to 1980.
In the North Atlantic between 1885 and 1984, 14,295 sei whales were taken. They were hunted in large numbers off the coasts of Norway and Scotland beginning in the late nineteenth and early twentieth centuries, and in 1885 alone, more than 700 were caught off Finnmark. Their meat was a popular Norwegian food. The meat's value made the hunting of this difficult-to-catch species profitable in the early twentieth century.
In Iceland, a total of 2,574 whales were taken from the Hvalfjörður whaling station between 1948 and 1985. Since the late 1960s to early 1970s, the sei whale has been second only to the fin whale as the preferred target of Icelandic whalers, with meat in greater demand than whale oil, the prior target.
Small numbers were taken off the Iberian Peninsula, beginning in the 1920s by Spanish whalers, off the Nova Scotian shelf in the late 1960s and early 1970s by Canadian whalers, and off the coast of West Greenland from the 1920s to the 1950s by Norwegian and Danish whalers.
In the North Pacific, the total reported catch by commercial whalers was 72,215 between 1910 and 1975; the majority were taken after 1947. Shore stations in Japan and Korea, processed 300–600 each year between 1911 and 1955. In 1959, the Japanese catch peaked at 1,340. Heavy exploitation in the North Pacific began in the early 1960s, with catches averaging 3,643 per year from 1963 to 1974 (total 43,719; annual range 1,280–6,053). In 1971, after a decade of high catches, it became scarce in Japanese waters, ending commercial whaling in 1975.
Off the coast of North America, sei whales were hunted off British Columbia from the late 1950s to the mid-1960s, when the number of whales captured dropped to around 14 per year. More than 2,000 were caught in British Columbia waters between 1962 and 1967. Between 1957 and 1971, California shore stations processed 386 whales. Commercial Sei whaling ended in the eastern North Pacific in 1971.
A total of 152,233 were taken in the Southern Hemisphere between 1910 and 1979. Whaling in southern oceans originally targeted humpback whales. By 1913, this species became rare, and the catch of fin and blue whales began to increase. As these species likewise became scarce, sei whale catches increased rapidly in the late 1950s and early 1960s. The catch peaked in 1964-65 at over 20,000 sei whales, but by 1976, this number had dropped to below 2,000 and commercial whaling for the species ended in 1977.
Since the moratorium on commercial whaling, some sei whales have been taken by Icelandic and Japanese whalers under the IWC's scientific research programme. Iceland carried out four years of scientific whaling between 1986 and 1989, killing up to 40 sei whales a year. Japanese scientists catch about 50 sei whales each year for this purpose. The research is conducted by the Institute of Cetacean Research (ICR) in Tokyo, a privately funded, nonprofit institution. The main focus of the research is to examine what they eat and to assess the competition between whales and fisheries. Dr. Seiji Ohsumi, Director General of the ICR, said,
- "It is estimated that whales consume 3 to 5 times the amount of marine resources as are caught for human consumption, so our whale research is providing valuable information required for improving the management of all our marine resources."
He later added,
- "Sei whales are the second most abundant species of whale in the western North Pacific, with an estimated population of over 28,000 animals. [It is] clearly not endangered."
Conservation groups, such as the World Wildlife Fund, dispute the value of this research, claiming that sei whales feed primarily on squid and plankton which are not hunted by humans, and only rarely on fish. They say that the program is
- "nothing more than a plan designed to keep the whaling fleet in business, and the need to use whales as the scapegoat for overfishing by humans."
At the 2001 meeting of the IWC Scientific Committee, 32 scientists submitted a document expressing their belief that the Japanese program lacked scientific rigour and would not meet minimum standards of academic review.
The sei whale did not have meaningful international protection until 1970, when the International Whaling Commission (IWC) first set catch quotas for the North Pacific for individual species. Before quotas, there were no legal limits. Complete protection from commercial whaling in the North Pacific came in 1976.
Quotas on sei whales in the North Atlantic began in 1977. Southern Hemisphere stocks were protected in 1979. Facing mounting evidence that several whale species were threatened with extinction, the IWC established a complete moratorium on commercial whaling beginning in 1986.
In the late 1970s, some "pirate" whaling took place in the eastern North Atlantic. There is no direct evidence of illegal whaling in the North Pacific, although the acknowledged misreporting of whaling data by the Soviet Union means that catch data are not entirely reliable.
The species remained listed on the IUCN Red List of Threatened Species in 2000, categorized as "endangered". Northern Hemisphere populations are listed as CITES Appendix II, indicating they are not immediately threatened with extinction, but may become so if they are not listed. Populations in the Southern Hemisphere are listed as CITES Appendix I, indicating they are threatened with extinction if trade is not halted.
The Sei whale is listed on both Appendix I and Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS). It is listed on Appendix I as this species has been categorized as being in danger of extinction throughout all or a significant proportion of their range and CMS Parties strive towards strictly protecting these animals, conserving or restoring the places where they live, mitigating obstacles to migration and controlling other factors that might endanger them and also on Appendix II as it has an unfavourable conservation status or would benefit significantly from international co-operation organised by tailored agreements.
Sei whale is covered by the Memorandum of Understanding for the Conservation of Cetaceans and Their Habitats in the Pacific Islands Region (Pacific Cetaceans MOU) and the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ACCOBAMS).
The current population is estimated at 80,000, nearly a third of the pre-whaling population. A 1991 study in the North Atlantic estimated only 4,000. Sei whales were said to have been scarce in the 1960s and early 1970s off northern Norway. One possible explanation for this disappearance is that the whales were overexploited. The drastic reduction in northeastern Atlantic copepod stocks during the late 1960s may be another culprit. Surveys in the Denmark Strait found 1,290 whales in 1987, and 1,590 whales in 1989. Nova Scotia's population estimates are between 1,393 and 2,248, with a minimum of 870.
A 1977 study estimated Pacific Ocean totals of 9,110, based upon catch and CPUE data. Japanese interests claim this figure is outdated, and in 2002 claimed the western North Pacific population was over 28,000, a figure not accepted by the scientific community. In California waters, there was only one confirmed and five possible sightings by 1991 to 1993 aerial and ship surveys, and there were no confirmed sightings off Oregon coasts such as Maumee Bay and Washington. Prior to commercial whaling, the North Pacific hosted an estimated 42,000. By the end of whaling, the population was down to between 7,260 and 12,620.
In the Southern Hemisphere, population estimates range between 9,800 and 12,000, based upon catch history and CPUE. The IWC estimated 9,718 whales based upon survey data between 1978 and 1988. Prior to commercial whaling, there were an estimated 65,000.
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Names and Taxonomy
Comments: Rice (1998) recognized northern and southern subspecies (B. borealis borealis and B. borealis schlegelii, respectively).
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