The sei whale (pronounced /ˈseɪ/ or /ˈsaɪ/), 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 off-shore 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 20 meters (66 ft) long and weighing as much as 45 tonnes (44 LT; 50 ST), the sei whale daily consumes an average of 900 kilograms (1,984 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 238,000 whales were taken, the sei whale is now internationally protected, although limited hunting occurs under controversial research programmes conducted by Iceland and Japan. As of 2006, its worldwide population was about 54,000, about a fifth of its pre-whaling population.
The species was first officially described by French naturalist René Primevère Lesson in 1828, but an earlier description was given by Karl Rudolphi in 1822 [although he assumed it was a minke whale (Balaenoptera acutorostrala)], leading to occasional references to sei whales as Rudolphi's rorqual. Additional names include pollack whale, coalfish whale, sardine whale, or Japan finner. Additionally, it has been referred to as the lesser fin whale because it somewhat resembles the fin whale.
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, named for a fish that the whale has been observed to eat in the Pacific.
The sei was classified as Balaena rostraia, Balaena borealis, Bataenoptera laticeps, and Eulama physalus, among others, before Lesson's alternative Balaenoptera borealis was formalized.
Sei whales are rorquals (family Balaenopteridae), baleens that includes 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 below the mouth that continue along the body's underside. Balaenopteridae diverged from the other families of suborder Mysticeti, also called the whalebone whales or great whales, as long ago as the middle Miocene. However, 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 Balaenopteridae, after the blue whale (up to 180 tonnes, 200 tons) and the fin whale (up to 70 tonnes, 77 tons). Mature adults typically measure between 12–15 metres (39–49 ft) and weigh 20–30 tonnes (20–30 LT; 22–33 ST). The southern sei whale is larger than the northern. Females are considerably larger than males. The largest known sei whale measured 20 meters (66 ft), and weighed between 40–45 tonnes (39–44 LT; 44–50 ST). The largest specimens taken off Iceland were slightly longer than 16 meters (52 ft). At birth, a calf typically measures 4–5 metres (13–16 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 series of 32–60 pleats or grooves along the bottom of the body that allow the throat area to expand greatly during feeding. The snout is pointed and the pectoral fins are relatively short, with at only 9%–10% of body length, and pointed at the tips. It has a single ridge extending from snout tip 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 believed to be caused by ectoparasitic copepods (Penella spp.), lampreys (family Petromyzontidae), or possibly "cookie-cutter" sharks (Isistius brasiliensis). It has a tall, sickle-shaped dorsal fin that ranges in height from 25–61 centimetres (9.8–24 in), about two-thirds of the way back from the tip of the snout. 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, each about 48 centimeters (18.9 in) long. Each plate is made of fingernail-like keratin that frays out into whitish fine hairs on the ends inside the mouth near the tongue. The sei's very fine baleen bristles, about 0.1 millimetres (0.004 in) are the most reliable characteristic that distinguishes it from other baleen whales.
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 head. 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 upper edge of the sei's head has a small arch between snout tip and eye, while the fin whale's profile is relatively flat.
Sei whales usually travel alone or in groups of up to six individuals. Larger groups may assemble at particularly abundant feeding grounds. Very little is known about their social structure. Males and females may bond, but this is uncertain.
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 40–60 seconds. Unlike the fin whale, the sei whale tends not to rise high out of the water as it dives. The blowholes and dorsal fin are often exposed above the water surface simultaneously. The whale almost never extends its flukes above the surface, and it rarely breaches.
This rorqual is a filter feeder, using its baleen plates to obtain its food by opening its mouth, engulfing 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 (1,984 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 (krill), in particular Meganyctiphanes norvegica and Thysanoessa inermis. In the North Pacific, it feeds on similar zooplankton, including the copepod species Calanus cristatus, Calanus plumchrus, and Calanus pacificus, and euphausid species Euphausia pacifica, Thysanoessa inermis, Thysanoessa longipes, and Thysanoessa spinifera. In addition, it eats larger organisms, such as the Japanese flying squid, Todarodes pacificus pacificus, and small fish, including members of the Engraulis (anchovies), Cololabis (sauries), Sardinops (pilchards), and Trachurus (jack mackerels) genera. Some of these fish are commercially important. Off central California, the whale may 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 Calanus tonsus, Calanus simillimus, and Drepanopus pectinatus, as well as the euphausids Euphausia superba and Euphausia vallentini. Sei whales also eat sardines.
Mating occurs in temperate, subtropical seas during the winter. Gestation is estimated to vary around 10 3/4 months, 11 1/4 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 11–12 metres (36–39 ft) in length, so weaning takes place at the summer or autumn feeding grounds. Females reproduce every 2–3 years, with as many as six fetuses reported, but single births are far more common. The average age of sexual maturity of both sexes is 8–10 years, at a length of around 12 meters (39 ft) for males and 13 meters (43 ft) for females. 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 the 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 meter. An observer situated one meter from a vocalizing whale would perceive a volume roughly equivalent to the volume of a jackhammer operating two meters away.
Range and migration
Sei whales live in all oceans, although rarely in polar or tropical waters. The difficulty of distinguishing the 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–23°N latitude in the winter, and from 35°N–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 40–50°S latitude, while winter distribution is unknown.
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, 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 not 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.
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 1950-1980.
In the North Atlantic between 1885 and 1984, 14,295 sei whales were taken. They were hunted in large numbers off the coast of Norway and Scotland beginning in the late nineteenth and early twentieth centuries, and in 1885 alone, more than 700 were killed off Finnmark, Norway. 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 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 current population is estimated at 54,000, about one fifth 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 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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- National Audubon Society Guide to Marine Mammals of the World, Reeves, Stewart, Clapham and Powell, 2002, ISBN 0-375-41141-0
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