Sei whale

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 tropica...

Japan

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 kg (2,000 lb) of food each day. For an animal of its size, its preferred prey lies low within the food chain; this includes 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 different baleen 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). 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.

Very little is known about the sei whale social structure. They have been documented traveling alone or in pods of up to six individuals; larger groups may assemble at particularly abundant feeding grounds. 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 10 minutes. One whale would always leave the group during or immediately after such socializing bouts. The sei whale is among the fastest cetaceans. 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". It can reach speeds of up to 50–55 km/h (31–34 mph) over short distances. However, it is not a remarkable diver, reaching relatively shallow depths for 5 to 15 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.). When about to dive, the sei whale usually just sinks below the surface; only the dorsal fin and blowholes protrude. The whale is generally less active on water surfaces than other whale species; they rarely exhibit lobtail behaviour.

Mating occurs in temperate, subtropical seas during the winter. Gestation is estimated to vary around 103⁄4 months, 111⁄4 months, or one year. During the breeding period, a mating pair will remain together. A newborn is weaned from its mother at 6–9 months of age, when it is 8–9 m (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 ft) and females 13.3–13.5 m (44–44 ft) at sexual maturity, while in the Southern Hemisphere, males average 13.6 m (45 ft) and females 14 m (46 ft). The average age of sexual maturity of both sexes is 5.6–11.7 years. The whales can reach ages up to 70 years. The documented oldest living sei whale was 74 years old.

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). 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. Adults have 300–380 ashy-black baleen plates on each side of the mouth, up to 80 cm (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 (and highly calcified and stiff) baleen bristles, about 0.1 mm (0.0039 in), are the most reliable characteristic that distinguishes it from other rorquals. The sei whale looks very similar to other large rorquals, especially to i…

As of 2018, the global population is estimated to be 50,000 mature individuals, with an increasing population trend. In the North Atlantic, 12,000 whales were estimated. As of 1983, around 10,000 whales were estimated in the Southern Hemisphere, and by 2011, approximately 35,000 individuals inhabited the North Pacific. 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 1977 population estimates were 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 western Canadian waters, researchers with Fisheries and Oceans Canada observed five seis together in the summer of 2017, the first such sighting in over 50 years. 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. Mass death events for sei whales have been recorded for many years and evidence suggests endemic poisoning (red tide) may have caused mass deaths in prehistoric times. In June 2015, scientists flying over southern Chile counted 337 dead sei whales, in what is regarded as the largest mass beaching ever documented. The cause is not yet known; however, toxic algae blooms caused by unprecedented warming in the Pacific Ocean, known as the Blob, may be implicated.

The sei whale is listed by the IUCN Red List as endangered, and with an increasing population trend, as of 2018. The sei whale did not have meaningful international protection until 1970, when the International Whaling Commission 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, despite the acknowledged misreporting of whaling data by the Soviet Union. Northern Hemisphere populations are listed in 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 in 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, and also on Appendix II. The species is listed as endangered by the U.S. government National Marine Fisheries Service under the U.S. Endangered Species Act.

"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.