Common Thresher
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Scientific Name Alopias vulpinus
Conservation Status Vulnerable
Family Chordata
Habitat Mild Ocean
Food fish, fish, and more fish

The common thresher, Alopias vulpinus, is the largest species of thresher shark, family Alopiidae, attaining a maximum known length of 6 m (20 ft). Almost half of that length consists of the elongated upper lobe of its caudal fin. This structure, the source for many a fanciful tale about this shark through history, is employed by the thresher in a whip-like fashion to deliver incapacitating blows to its prey. The common thresher resembles (and has often been confused with) the pelagic thresher (A. pelagicus), which also has a streamlined body, short pointed snout, and modestly sized eyes. It can be distinguished from the latter species by the white of its belly extending in a band over the bases of its pectoral fins.

Common threshers inhabit both coastal and pelagic waters in tropical and temperate climates worldwide, from the surface to a depth of 550 m (1,800 ft). These sharks are seasonally migratory and follow warm water to higher latitudes in summer. The common thresher is a fast, strong swimmer that has been known to leap clear of the water. It possesses physiological adaptations that allow it to maintain an internal body temperature higher than that of the surrounding sea water. This species feeds primarily on small, schooling forage fishes. In common with other mackerel sharks, the common thresher is ovoviviparous with the unborn embryos being sustained by undeveloped eggs ovulated by their mother. Females give birth to litters of 2–7 pups following a gestation period of nine months.

Although large, the common thresher has relatively small teeth and a timid disposition, posing minimal danger to humans. They are highly valued by commercial fishers for their meat, fins, hide, and liver oil; large numbers are taken by longline and gillnet fisheries throughout its range. This shark is also esteemed by recreational big game anglers for the exceptional fight it offers on hook-and-line. The common thresher has a low rate of reproduction and cannot sustain heavy fishing pressure for long, a case in point being the rapid collapse of the thresher fishery off the U.S. state of California in the 1980s. With commercial exploitation increasing in many parts of the world, the International Union for Conservation of Nature (IUCN) has assessed this species as Vulnerable.

Common threshers are inhabitants of both continental waters and the open ocean. They tend to be most abundant in proximity to land, particularly the juveniles which frequent near-coastal habitats such as bays. Most individuals are encountered near the surface, but this species has been recorded to at least a depth of 550 m (1,800 ft).

Some 97% of the common thresher's diet is composed of bony fishes, mostly small schooling forage fish such as mackerel, bluefish, herring, needlefish, and lanternfish. Before striking, the sharks compact schools of prey by swimming around them and splashing the water with its tail, often in pairs or small groups. Threshers are also known to take large, solitary fishes such as lancetfish, as well as squid and other pelagic invertebrates.

The common thresher is widely caught by offshore longline and pelagic gillnet fisheries, especially in the northwestern Indian Ocean, the western, central, and eastern Pacific, and the North Atlantic. Participating countries include the former USSR, Japan, Taiwan, Spain, the United States, Brazil, Uruguay, and Mexico. The meat is highly prized for human consumption cooked, dried and salted, or smoked. In addition, their skin is made into leather, their liver oil is processed for vitamins, and their fins are used for shark fin soup. The United Nations Food and Agricultural Organization (FAO) reported a worldwide common thresher take of 411 metric tons in 2006.

All three thresher shark species were reassessed from Data Deficient to Vulnerable by the International Union for Conservation of Nature (IUCN) in 2007. The rapid collapse of the Californian subpopulation (over 50% within three generations) prompted concerns regarding the species' susceptibility to overfishing in other areas, where fishery data is seldom reported and aspects of life history and population structure are little-known. In addition to continued fishing pressure, common threshers are also taken as bycatch in other gear such as bottom trawls and fish traps, and are considered a nuisance by mackerel fishers as they become entangled in the nets.