Sei Whale

The sei whale  Balaenoptera borealis, is a baleen whale, the fourth-largest rorqual after the blue whale, the fin whale and the humpback whale. 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 and subpolar waters in summer to winter intemperate and subtropical waters.

Reaching 19.5 metres (64 ft) long and weighing as much as 28 tonnes (28 long tons; 31 short tons), the sei whale daily consumes an average of 900 kilograms (2,000 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 255,000 whales were taken, the sei whale is now internationally protected, although limited hunting occurs under a controversial research program conducted by Japan. As of 2008, its worldwide population was about 80,000, nearly a third of its pre-whaling population.

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, a name originally applied to Bryde's whales by early Japanese whalers. Later, as modern whaling shifted to Sanriku — where both species occur — it was confused for the sei whale. Now the term only applies to the latter species. It has also been referred to as the lesser fin whale because it somewhat resembles the fin whale. 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".

Sei whales usually travel alone or in pods of up to six individuals. Larger groups may assemble at particularly abundant feeding grounds. Very little is known about their social structure. 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 ten minutes. One whale would always leave the group during or immediately after such socializing bouts. 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 60 seconds (range: 45–90 sec.). Unlike the fin whale, the sei whale tends not to rise high out of the water as it dives, usually just sinking below the surface. The blowholes and dorsal fin are often exposed above the water surface almost simultaneously. The whale almost never lifts its flukes above the surface, and are generally less active on water surfaces than close related Bryde's whales; it rarely breaches.

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 kilograms (2,000 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 foreuphausiids, 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 speciesEuphausia 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). Some of these fish are commercially important. 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.