The sei whale ( SAY, Norwegian: [sæɪ]; Balaenoptera borealis) is a baleen whale. It is one of ten rorqual species, and the third-largest member after the blue and fin whales. It can grow to 19.5 m (64 ft) in length and weigh as much as 28 t (28 long tons; 31 short tons). Two subspecies are recognized: B. b. borealis and B. b. schlegelii. The whale's ventral surface has sporadic markings ranging from light grey to white, and its body is usually dark steel grey in colour. It is among the fastest of all cetaceans, and can reach speeds of up to 50–55 km/h (31–34 mph) over short distances. 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, subpolar waters in summer to temperate, subtropical waters in winter with a lifespan of 70 years. It is a filter feeder, with its diet consisting primarily of copepods, krill, and other zooplankton. It is typically solitary or can be found in groups numbering half a dozen. During the breeding period, a mating pair will remain together. Sei whale vocalizations usually last approximately half a second, and occurs at 240–625 hertz. Following large-scale commercial whaling during the late 19th and 20th centuries, when over 255,000 whales were killed, the sei whale is now internationally protected. It is listed as endangered on the IUCN Red List, though populations are increasing. The Northern Hemisphere population is listed under CITES Appendix II, which indicates they are not threatened with extinction, while the Southern Hemisphere population is listed under CITES Appendix I, indicating that they are threatened and are given the highest levels of protection.
Etymology "Sei whale" is an anglicization of the Norwegian seihval, meaning "pollock whale". The species was so called because it "appeared off the coast of Norway at the same time each year as the pollock that came to feed on the abundant plankton". In the Pacific, the whale has been called the Japan finner; "finner" was a common term used to refer to rorquals. It has also been referred to as the lesser fin whale because it somewhat resembles the fin whale.
Taxonomy On 21 February 1819, Swedish-born German naturalist Karl Rudolphi initially identified a 9.8 m (32 ft) whale stranded near Grömitz, in Schleswig-Holstein, as Balaena rostrata (Balaenoptera acutorostrata). In 1823, the French naturalist Georges Cuvier described 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 16 m (52 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, British zoologist William Henry Flower named a 14 m (46 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, 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, 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. Whole genome sequencing suggests that sei and blue whales are closely related, with gray whales as a sister group. This study also found significant gene flow between minke whales and the ancestors of the blue and sei whale. Two subspecies have been identified—the northern sei whale (B. b. borealis) and southern sei whale (B. b. schlegelii).
Description
The sei 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. Sei whales have a solitary ridge extending from the tip of the rostrum to the paired blowholes that are a distinctive characteristic of baleen whales. Its 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 cm (15–35 in) and averages 53–56 cm (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.
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