Sei whale
The sei whale 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 in length and weigh as much as. 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 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 by the IUCN Red List, despite increasing populations. 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 whale stranded near Grömitz, in Schleswig-Holstein, as Balaena rostrata. 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 female that had stranded on the coast of France the previous year. 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 specimen that had been obtained from Pekalongan, on the north coast of Java, Sibbaldius ''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, 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 and southern sei whale.
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, 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. It has a tall, sickle-shaped dorsal fin that ranges in height from and averages, 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 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, are the most reliable characteristic that distinguishes it from other rorquals.
The sei whale looks very similar to other large rorquals, especially to its smaller relative the Bryde's whale. Exceptional individuals may resemble a fin whale, which leads to confusion. They are usually differentiated from the fin whale by the colour of their head. Contrary to the fin whale's smooth rostrum, the sei whale's rostrum is curved.
Size
The sei whale is the third-largest balaenopterid, after the blue whale and the fin whale. Adults usually weigh between. They exhibit sexual dimorphism, with females outweighing and being longer than their male counterparts. At birth, a calf typically measures in length.In the Northern Hemisphere, males reach up to and females up to, while in the Southern Hemisphere males reach a maximum of and females of. The authenticity of an alleged female caught northwest of St. Kilda in July 1911 is doubted. Five specimens taken off Iceland exceeded in length. The longest measured during JARPN II cruises in the North Pacific were a female and a male.
In the North Pacific, adult males average and adult females. In the North Atlantic, the average length of an adult male is and of an adult female is. In the Southern Hemisphere, they average and, in males and females, respectively.
In the North Pacific, males weigh an average of and females. North Atlantic sei whale males average and females. Southern Hemisphere whales average in body weight. The sei whale is estimated to have weighed up to.
Life history
Surface behaviours
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 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. 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.Feeding
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 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, sardines, Pacific saury, mackerel, jack mackerel and juvenile rockfish. Off central California, they mainly feed on anchovies between June and August, and on krill 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.