| Common thresher | |
|---|---|
| Conservation status | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Order: | Lamniformes |
| Family: | Alopiidae |
| Genus: | Alopias |
| Species: | A. vulpinus |
| Binomial name | |
| Alopias vulpinus (Bonnaterre, 1788) |
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| Range of the common thresher | |
| Synonyms | |
|
Alopecias barrae Perez Canto, 1886 |
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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.
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The common thresher was first described by French naturalist Pierre Joseph Bonnaterre as Squalus vulpinus, in the 1788 Tableau encyclopédique et méthodique des trois règnes de la nature. This species was later moved to the genus Alopias.[2] The specific epithet is from the Latin vulpes, meaning "fox", and some sources give the scientific name incorrectly as Alopias vulpes.[3] Other common names include Atlantic thresher, fox shark, grayfish, green thresher, sea fox, slasher, swiveltail, thintail thresher, thrasher/thresher, thresher shark, and whip-tailed shark.[4]
Morphologically, the common thresher and the pelagic thresher (A. pelagicus) are united in having thin tails, smaller eyes, and no lateral grooves on the head.[3] However, an allozyme analysis conducted by Blaise Eitner in 1995 found that the pelagic thresher is most closely related to the bigeye thresher (A. superciliosus), and that common thresher forms a separate clade on its own. Its closest relative in the family may be an unrecognized fourth thresher species from off southern California.[5]
The distribution of the common thresher is virtually circumglobal in warm waters. In the western Atlantic it occurs from Newfoundland to Cuba, though is rare north of New England, and from Brazil to Argentina. In the eastern Atlantic, it is found from Norway and the British Isles to Ghana and the Ivory Coast, including the Mediterranean Sea. In the Indo-Pacific region, it has been reported from South Africa, Tanzania, Somalia, Maldives, Chagos Archipelago, Gulf of Aden, Pakistan, India, Sri Lanka, Sumatra, Japan, Republic of Korea, Australia, New Zealand, and New Caledonia. It occurs off the Society Islands, Fanning Island, and Hawaiian Islands in the central Pacific, and from British Columbia to Baja California and Panama to Chile in the eastern Pacific.[2]
The common thresher is migratory, moving to higher latitudes following warm water masses. In the eastern Pacific, males travel further than females, reaching as far as Vancouver Island in the late summer and early fall. Juveniles tend to remain in warm nursery areas.[3] There appear to be separate populations with different life history characteristics in the eastern Pacific and western Indian Ocean and possibly elsewhere; this species is not known to make transoceanic movements.[6] In the northwestern Indian Ocean, males and females segregate by location and depth during the pupping season from January to May.[7] Analysis of mitochondrial DNA has revealed substantial regional genetic variation within common threshers in all three oceans. This reaffirms the idea that, despite being high mobile, sharks from different areas rarely interbreed.[8]
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.[9] Most individuals are encountered near the surface, but this species has been recorded to at least a depth of 550 m (1,800 ft).[2]
Common threshers are the largest species of thresher shark; individuals 2–5 m (6.6–16 ft) long and weighing 230 kg (510 lb) are not uncommon, and they can grow upwards of 6 m (20 ft) long.[7] The heaviest common thresher on record is a 4.8 m (16 ft) female weighing 510 kg (1,100 lb), caught in the English Channel on November 22, 2007.[10] The body is solidly built and torpedo-shaped, with very long, falcate (sickle-shaped) pectoral fins. The first dorsal and pelvic fins are large, while the second dorsal and anal fins are tiny. The distinctive upper caudal fin lobe is slender, bearing a moderate ventral notch near the tip, and can be nearly as long as the rest of the body.[9]
The head is fairly short and broad, with a strongly convex dorsal profile and a pointed conical snout. The eyes are of medium size. The mouth is relatively small, with short furrows at the corners. The teeth are small, smooth-edged, and knife-like, lacking the small lateral cusplets of the pelagic thresher. There are 20 tooth rows on either side of the upper jaw and 21 tooth rows on either side of the lower jaw. The five pairs of gill slits are short, with the last two placed over the pectoral fin bases. The dermal denticles are minute and overlapping. The common thresher is dark brown to gray to almost black above, with a metallic luster. Dark spots are present around the pelvic fins and the caudal peduncle, and there may be a white spot at the tip of the pectoral fins. The underside is white, extending in a characteristic patch over the pectoral fin bases to the "cheeks". This pattern stands in contrast to that of the similar pelagic thresher, whose dark dorsal coloration extends uninterrupted to and over the pectoral fins.[2][9]
Common threshers are active, strong swimmers; there are infrequent reports of them leaping completely out of the water.[11] Like the fast-swimming sharks of the family Lamnidae, the common thresher has a strip of aerobic red muscle along its flank that is able to contract powerfully and efficiently for long periods of time.[12] In addition, they have slow-oxidative muscles centrally located within their bodies and a blood vessel countercurrent exchange system called the rete mirabile ("wonderful net"), allowing them to generate and retain body heat. The temperature inside the red muscles of a common thresher averages 2°C (3.6°F) above that of the ambient seawater, though there is significant individual variation.[13] Unlike the pelagic and bigeye threshers, the common thresher lacks an orbital rete mirabile to protect its eyes and brain from temperature changes.[14]
Immature common threshers fall prey to larger sharks. Aside from observations of killer whales feeding on common threshers off New Zealand,[15] adults have no known natural predators. Known parasites of the common thresher include nine species of copepods in the genus Nemesis, which attach themselves to the sharks' gill filaments, damaging tissue and hindering respiration.[2] Other parasites include the protozoan Giardia intestinalis,[16] the digenean Campula oblonga (not usual host),[17] the tapeworms Paraorygmatobothrium exiguum and Sphyriocephalus tergetinus,[18][19] and the copepod Kroeyerina benzorum.[20]
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.[2] Off California, common threshers feed mostly on the northern anchovy (Engraulis mordax), with Pacific hake (Merluccius productus), Pacific sardine (Sardinops sagax), Pacific mackerel (Scomber japonicus), market squid (Loligo opalescens), and pelagic red crab (Pleuroncodes planipes) also being important food items. The sharks concentrate on a few prey species during cold water years, but become less discriminating during less productive, warmer El Niño periods.[21]
There are numerous accounts of common threshers using the long upper lobes of their tail fins to stun prey, and they are often snagged on longlines by their tails after presumably striking at the bait. In July 1914, shark-watcher Russell J. Coles reported seeing a thresher shark use its tail to flip prey fish into its mouth, and that one fish that missed was thrown a "considerable distance". On April 14, 1923, noted oceanographer W.E. Allen observed a 2 m (6.6 ft) thresher shark pursuing a California smelt (Atherinopsis californiensis) off a pier at the Scripps Institution of Oceanography. The shark overtook the small fish and swung its tail above the water like a "coachwhip" with "confusing speed", severely injuring its target. In the winter of 1865, Irish ichthyologist Harry Blake-Knox claimed to have seen a thresher shark in Dublin Bay use its tail to strike a wounded loon (probably a great northern diver, Gavia immer), which it then swallowed. Blake-Knox's account was subsequently disputed by other authorities, who asserted that the thresher's tail is not rigid or muscular enough to effect such a blow.[7]
Like other thresher sharks, common threshers are ovoviviparous. They give birth to litters of two to four (rarely six) pups in the eastern Pacific, and three to seven pups in the eastern Atlantic.[6] They are believed to reproduce throughout their range; one known nursery area is the Southern California Bight. Breeding occurs in the summer, usually July or August, and parturition occurs from March to June following a gestation period of nine months. The developing embryos are oophagous, feeding on eggs ovulated by the mother.[3] The teeth of small embryos are peg-like and non-functional, being covered by a sheath of soft tissues. As the embryos mature, their series of teeth become progressively more like those of adults in shape, though they remain depressed and hidden until shortly before birth.[22]
Newborn pups usually measure 114–160 cm (3.74–5.2 ft) long and weigh 5–6 kg (11–13 lb), depending on the size of the mother. The juveniles grow about 50 cm (1.6 ft) a year while adults grow about 10 cm (0.33 ft) a year.[2] The size at maturation appears to vary between populations. In the eastern North Pacific males mature at 3.3 m (11 ft) and five years old, and females at around 2.6–4.5 m (8.5–15 ft) and seven years old. They are known to live to at least 15 years of age and their maximum lifespan has been estimated to be 45–50 years.[3][6]
Despite their large size, common threshers pose little danger to humans, though they should be treated with respect as their powerful tails are easily capable of breaking bones. Most divers report that they are shy and difficult to approach underwater. The International Shark Attack File lists a single provoked attack by the thresher shark and four attacks on boats, which were probably incidental from individuals fighting capture. There is an unsubstantiated report of a common thresher acting aggressively towards a spearfisherman off New Zealand.[7]
Famed big-game angler Frank Mundus, in his book Sportsfishing for Sharks, recounted a tale in which a longline fisherman off the Carolinas leaned over the side of his boat to examine something large that he had hooked, and was decapitated by the caudal fin of a thresher shark estimated to be 5 m (16 ft) long. The head supposedly fell into the water and was never recovered. This account is considered highly improbable by most authors.[7]
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.[6]
In the United States, a drift gillnet fishery for the common thresher developed in southern California in 1977, beginning with 10 vessels experimenting with a larger-sized mesh. Within two years the fleet had increased to 40 vessels, and the fishery peaked in 1982 when 228 vessels landed 1,091 metric tons. The common thresher population rapidly collapsed from overfishing, with landings decreasing to less than 300 metric tons a year by the late 1980s and larger size classes disappearing from the population.[6][23] Common threshers are still taken commercially in the United States, with about 85% coming from the Pacific and 15% from the Atlantic. The largest catches remain from the California-Oregon gillnet fishery, which had shifted its focus to the more valuable swordfish (Xiphius gladius) but still take threshers as bycatch. Small numbers of Pacific threshers are also taken by harpoons, small-mesh driftnets, and longlines. In the Atlantic, threshers are primarily taken on longlines meant for swordfish and tuna.[24][25]
Common threshers are well regarded by sports fishers as one of the strongest fighting sharks alongside the shortfin mako shark (Isurus oxyrhinchus), and are ranked as game fish by the International Game Fish Association (IGFA). They are pursued by anglers using rod and reel off California, South Africa, and elsewhere. Frank Mundus has called thresher sharks "exceedingly stubborn" and "pound for pound, a harder fish to whip" than the mako.[7] Fishing for the common thresher is similar to that for the mako; the recommended equipment is a 24 kg (53 lb) rod and a big-game reel holding at least 365 m (400 yd) of 24 kg (53 lb) line. The ideal method is trolling with baitfish, either deep or allowing it to drift.[26][27]
All three thresher shark species were reassessed from Data Deficient to Vulnerable by the International Union for Conservation of Nature (IUCN) in 2007.[1] 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.[28] 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.[2][6]
The United States manages common thresher fisheries via regulations such as commercial quotas and trip limits, and recreational minimum sizes and retention limits. Shark finning is illegal under U.S. federal law. The Atlantic common thresher fishery is regulated by the National Marine Fisheries Service (NMFS) Highly Migratory Species Management Division through the 2006 Consolidated Atlantic Highly Migratory Species (HMS) Fishery Management Plan (FMP), and the Pacific common thresher fishery is regulated by the Pacific Fishery Management Council through the Fishery Management Plan (FMP) for U.S. West Coast Fisheries for Highly Migratory Species (HMS).[24][25] In the 1990s, after the depletion of common thresher stocks by the California gillnet fishery, the fleet was limited to 70 boats and restrictions were placed on season, operation range, and landings. There is evidence that the California subpopulation is recovering, and the potential population growth rate has been estimated to be 4–7% per year.[23]
The Greek philosopher Aristotle (384–322 BCE) wrote some of the earliest observations about the common thresher. In his Historia Animalia, he claimed that hooked threshers had a propensity for freeing themselves by biting through fishing lines, and that they protected their young by swallowing them. These "clever" behaviors, which have not been borne out by science, led the ancient Greeks to call it alopex (meaning "fox"), on which its modern scientific name is based.[7]
An oft-repeated myth about the common thresher is that they cooperate with swordfish to attack whales. In one version of events, the thresher shark circles the whale and distracts it by beating the sea to a froth with its tail, thereby allowing the swordfish to impale it in a vulnerable spot with its rostrum. In an alternate account, the swordfish positions itself beneath the whale, while the thresher leaps out of the water and lands on top of the whale, hammering it onto the swordfish's rostrum. Yet other authors describe the thresher "cutting huge gashes" in the side of the whale with its tail. Neither threshers nor swordfish however are known to feed on whales or indeed possess the dentition to do so. The story may have arisen from mariners mistaking the tall dorsal fins of killer whales, which do attack large cetaceans, for thresher shark tails. Swordfish bills have also been found embedded in blue and fin whales (likely accidents due to the fast-moving fish's inertia), and thresher sharks do exhibit some of the aforementioned behaviors independent of whales.[7][29]
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