The whales reach lengths of up to 20 metres (66 ft) long and weigh up to 45 tonnes (50 tons). It consumes an average of 900 kilograms (2,000 lb) of food each day, primarily copepods and krill, and other zooplankton. It is among the fastest of all cetaceans, and can reach speeds of up to 50 kilometres per hour (31 mi/hr, 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 hunting of the species between the late-nineteenth and late-twentieth centuries when over 238,000 individuals were taken, the Sei Whale is now an internationally protected species, although limited hunting still occurs under controversial research programmes conducted by Iceland and Japan. As of 2006, the worldwide population of the Sei Whale was about 54,000, about a fifth of its pre-whaling population.
The word Sei comes from the Norwegian word sei 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.
Sei Whales are rorquals (family Balaenopteridae), a family of the baleen whales that includes the Humpback Whale, the Blue Whale, Bryde's Whale, the Fin Whale and the Minke Whale. Rorquals take their name from the Norwegian word røyrkval, meaning "furrow whale", because members of the family have a series of longitudinal pleats or grooves located below the mouth and continuing along the underside of the body. The family Balaenopteridae is believed to have diverged from the other families of the 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.
Two subspecies have been identified—the Northern Sei Whale (Balaenoptera borealis borealis) and Southern Sei Whale (Balaenoptera borealis schleglii). The two subspecies are geographically isolated from each other and their ranges do not overlap.
The Sei Whale is the third largest member of the Balaenopteridae family, 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 and 15 metres (40–50 ft) and weigh 20–30 tonnes (22–33 tons). The Southern Sei Whale is larger than the Northern Sei Whale, and females are considerably larger than males. The largest known Sei Whale measured 20 metres (66 ft), and weighed between 40 and 45 tonnes (44–50 tons). The largest specimens taken off Iceland were slightly longer than 16 metres (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 compared to other whales, with a length of only 9%–10% of the total body length, and pointed at the tips. It has single ridge extending from the tip of the snout 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 ectoparisitic 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 (10–24 in), and is set 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 of Sei Whales. The tail is thick and the fluke, or lobe, is relatively small in relation to the size of the whale's body.
This rorqual is a filter feeder, using its baleen plates to obtain its food from the water 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. An adult has 300–380 ashy-black baleen plates on each side of the mouth, each about 48 centimetres (19 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 very fine bristles of the Sei Whale's baleen (about 0.1 mm, 0.004 in) are cited as the most reliable feature distinguishing it from all other baleen whales.
The Sei Whale looks similar to other large baleen whales. The best way to distinguish between it and the Bryde's Whale, apart from differences in each whale's baleen, is by the presence of lateral ridges on the dorsal surface of the Bryde's Whale's head. Large Sei Whales can be confused with Fin Whales unless the Fin Whale's asymmetrical head colouration is clearly seen; the right side of the lower jaw of the Fin Whale is white, and the left side is grey. When viewed from the side, the upper edge of the Sei Whale's head has a small arch between the tip of the snout and the eye, whereas the Fin Whale's profile is relatively flat.
Sei Whales usually travel alone or in small groups of up to six individuals. Larger groups have been seen at particularly abundant feeding grounds. Very little is known about their social structure. Males and females may form a bond, but there is insufficient research to know this for certain.
The Sei Whale is among the fastest of all cetaceans. It can reach speeds of up to 50 kilometres per hour (31 mi/hr, 27 knots) over short distances. However, it is not a remarkable diver, diving only to relatively shallow depths for between five to fifteen minutes. Between these dives, the whale swims near the surface 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, or leaps high out of the water.
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, it is notable because for the most part, its preferred foods lie relatively low in the food chain, including zooplankton and small fish. The whale's preference for zooplankton has been determined from stomach analyses and direct observations of feeding behaviour. It has also been determined from the analysis of fecal matter collected near Sei Whales, which appears as a thin brown cloud in the water. The feces is collected in nets and DNA material in the wastes is separated out and individually identified, to be matched with known species. The whale competes for food with a variety of other species, including clupeid fishes (herring and its relatives), basking sharks, and Right Whales.
In the North Atlantic, the Sei Whale 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, the Sei Whale 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 is known to eat 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 in its diet are commercially important. Off central California, the whale has been observed feeding 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.
A newborn weans from its mother at 6–9 months of age when it is 11–12 metres (36–39 ft) in length, so weaning takes place on the feeding grounds in summer or autumn. Females reproduce every 2–3 years, with as many as 6 foetuses being 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 metres (40 ft) for males and 13 metres (50 ft) for females. The whales can reach ages of up to 65 years.
Sei Whales are found worldwide, although they are only rarely found in polar or tropical waters. The difficulty of distinguishing Sei Whales at seas from their close relatives, Bryde's Whales and in some cases from Fin Whales, has created confusion about their distributional limits and frequency of occurrence, especially in warmer waters where Bryde's Whales are most common.
In the North Atlantic, the range of the Sei Whale extends from southern Europe or northwestern Africa to Norway in the eastern North Atlantic, and from the southern United States to Greenland in the western. 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 St. 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, the Sei Whale is found from 20°N–23°N latitude in the winter, and from 35°N–50°N latitude in the summer. Approximately 75% of the total population of Sei Whales in the North Pacific is found east of the International Date Line, but there is a significant lack of information regarding the overall distribution of the whales in the North Pacific. 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–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 that the whale moves 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 trends in the migration patterns of the Sei Whale, exact migration routes are not known and scientists cannot readily predict exactly where groups will appear from one year to the next. A particular location may one year see an influx of many whales and none for several years afterwards. F.O. Kapel noted a correlation between the occasional appearance of the Sei Whale west of Greenland and the occasional incursion of relatively warm waters from the Irminger Current into that area. Some evidence from tagging data indicates that individual Sei Whales return off the coast of Iceland on an annual basis.
The development of explosive harpoons and steam-powered catcher boats in the late nineteenth century allowed the exploitation of previously unobtainable large whales by commercial whalers. Because of their quick speed and elusiveness, and later because of their comparatively small yield of oil and meat compared with other large whales, the Sei Whale was initially not methodically hunted. When stocks of the more commercially attractive Right Whales, Blue Whales, Fin Whales, and Humpback Whales became depleted, Sei Whales were hunted in earnest, particularly in the 1950s through the 1970s.
In Iceland, a total of 2,574 whales were taken from the Hvalfjörður whaling station between 1948 and 1985. Since the late 1960s or early 1970s, the Sei Whale has been second only to the Fin Whale as a preferred target of Icelandic whalers, with the demand for high-quality meat taking precedence over that for whale oil, which was once the main target of whalers.
Small numbers of Sei Whales 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.
Off the coast of North America, Sei Whales were hunted in waters 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 killed in British Columbia waters between 1962 and 1967. Between 1957 and 1971, California shore stations processed 386 whales. Commercial whaling for Sei Whales ended in the eastern North Pacific in 1971.
The Sei Whale did not have meaningful protection at the international level until 1970, when catch quotas for the North Pacific began to be set on a species basis by the International Whaling Commission (IWC). Prior to catch quotas, whalers were limited only by their ability to locate the whales. The Sei Whale was given complete protection from commercial whaling in the North Pacific in 1976, and quotas on Sei Whales were introduced in the North Atlantic in 1977. Southern hemisphere stocks were protected from whaling in 1979. Facing mounting evidence that several whale species worldwide were threatened with extinction, the IWC voted to implement a moratorium on commercial whaling beginning in 1986, at which time all legal whaling for Sei Whales stopped.
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, categorised as "endangered". Populations in the Northern Hemisphere are listed as CITES Appendix II, indicating that they are not necessarily threatened with extinction, but may become so if they are not listed. Populations in the Southern Hemisphere are listed as CITES Appendix I, indicating that they are threatened with extinction if trade is not halted.
Japanese scientists kill approximately 50 Sei Whales each year for this purpose. The research is conducted by the Institute of Cetacean Research (ICR) in Tokyo, a privately-funded, non-profit institution. The main focus of the research is to examine what Sei Whales eat and to determine the level of 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 that "...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 necessity of the research, saying that it is known that Sei Whales feed primarily on squid and plankton not hunted by humans, and only rarely on fish. They say that the programme is "nothing more than a plan designed to keep the whaling fleet in business, and the need to use whales as the scapegoat for over-fishing by humans." The scientific quality of the research obtained under the scientific whaling programme has been criticised as being very poor; at the 2001 meeting of the IWC Scientific Committee, 32 scientists submitted a document expressing their belief that the Japanese programme lacked scientific rigour and would not meet minimum standards of academic review that are widely used in science world-wide.
A study in 1977 produced a population estimate for the Pacific Ocean of 9,110, based upon the catch and CPUE data. This figure is disputed as outdated by Japanese whaling interests, which in 2002 claimed that the population of Sei Whales in the western North Pacific was over 28,000 whales, a figure not accepted by the scientific community. In California waters, there was only one confirmed and five possible sightings from 1991 to 1993 aerial and ship surveys, and there were no confirmed sightings off Oregon and Washington. Prior to commercial whaling activities, there were an estimated 42,000 Sei Whales in the North Pacific. By the end of the period of exploitation (1974), the numbers of Sei Whales in the North Pacific had been reduced to between 7,260 and 12,620 whales.
In the Southern Hemisphere, Sei Whale abundance estimates range between 9,800 and 12,000 whales, based upon the history of catches and CPUE in the southern oceans. The IWC reported an estimate of 9,718 whales based upon survey data between 1978 and 1988. Prior to commercial whaling, there were an estimated 65,000 Sei Whales living in the Southern Hemisphere.