Oceanic whitetip shark
|Oceanic whitetip shark|
|Range of the oceanic whitetip shark|
The oceanic whitetip shark (Carcharhinus longimanus), also known as Brown Milbert's sand bar shark, brown shark, nigano shark, oceanic white-tipped whaler, and silvertip shark, is a large pelagic requiem shark inhabiting tropical and warm temperate seas. Its stocky body is most notable for its long, white-tipped, rounded fins.
This aggressive but slow-moving fish dominates feeding frenzies, and is a danger to shipwreck or air crash survivors. Recent studies show steeply declining populations because its large fins are highly valued as the chief ingredient of shark fin soup, and as with other shark species, the whitetip faces mounting fishing pressure throughout its range.
The oceanic whitetip shark, or lesser white shark was described in 1831 by naturalist René-Primevère Lesson, who named the shark Carcharhinus maou. It was next described by Cuban Felipe Poey in 1861 as Squalus longimanus. The name Pterolamiops longimanus has also been used. The species epithet longimanus refers to the size of its pectoral fins (longimanus translates from Latin as "long hands"). The oceanic whitetip shark has many common names in English: Brown Milbert's sand bar shark, brown shark, nigano shark, oceanic white-tipped whaler, and whitetip shark.
The rules of the International Commission on Zoological Nomenclature are that in general the first-published description has priority; therefore, the valid scientific name for the oceanic whitetip shark should be Carcharhinus maou. However, Lesson's name remained forgotten for so long that Carcharhinus longimanus remains widely accepted.
Distribution and habitat
The oceanic whitetip is found globally in deep, open water, with a temperature greater than 18 °C (64 °F). It prefers waters between 20 and 28 °C (68 and 82 °F) and tends to withdraw from areas when temperatures fall outside of this. It was once extremely common and widely distributed, and still inhabits a wide band around the globe; however, recent studies suggest that its numbers have drastically declined. An analysis of the US pelagic longline logbook data between 1992 and 2000 (covering the Northwest and Western Central Atlantic) estimated a decline of 70% over that period.
It is found worldwide between 45°N and 43°S latitude. In 2004, an oceanic whitetip was discovered dead on the west coast of Sweden—far beyond what was once considered the northern boundary of its range.
The shark spends most of its time in the upper layer of the ocean—to a depth of 150 m (490 ft)—and prefers off-shore, deep-ocean areas. According to longline capture data, increasing distance from land correlates to a greater population of sharks. Occasionally, it is found close to land, in waters as shallow as 37 m (120 ft), mainly around midocean islands such as Hawaii, or in areas where the continental shelf is narrow with access to nearby deep water. It is typically solitary, though gatherings have been observed where food is plentiful. Unlike many animals, it does not have a diurnal cycle, and is active both day and night. Its swimming style is slow, with widely spread pectoral fins. Despite its habitual isolation from members of its own species, pilot fish, dolphinfish, and remora may accompany it. In 1988, Jeremy Stafford-Deitsch reported seeing an individual accompanied by a shortfin pilot whale.
C. longimanus' most distinguishing characteristics are its long, wing-like pectoral and dorsal fins. The fins are significantly larger than most other shark species, and are conspicuously rounded. The shark's nose is rounded and its eyes are circular, with nictitating membranes.
C. longimanus has a 'typical', although somewhat flattened requiem shark body, often with a mildly humpbacked aspect. It is bronze, brown, bluish, or grey dorsally (the colour varies by region), and white ventrally (although it may occasionally have a yellow tint). The oceanic whitetip shark is a medium-sized requiem shark. The largest specimen ever caught measured 4 m (13 ft), an exceptionally large size considering few specimens are known to exceed a length of 3 m (9.8 ft). The maximum reported weight is 170 kg (370 lb). The female is typically larger than the male by 10 cm (3.9 in). Males attain sexual maturity at 1.7 to 1.9 m (5.6 to 6.2 ft) and females about 1.8 to 2.0 m (5.9 to 6.6 ft). In the Gulf of Mexico in the 1950s, the mean weight of oceanic whitetip sharks was 86.4 kg (190 lb). In the 1990s, the sharks of the species from the same area averaged only 56.1 kg (124 lb).
Most of its fins (dorsal, pectoral, pelvic, and caudal) have white tips (juvenile specimens and some adults may lack these). Along with white tips, the fins may be mottled, and young specimens can have black marks. A saddle-like marking may be apparent between first and second dorsal fins. The shark has several kinds of teeth. Those in the mandible (lower jaw) have a thin, serrated tip and are relatively small and triangular (somewhat fang-like). Between 13 and 15 teeth are on either side of the symphysis. The teeth in the upper jaw are triangular, but much larger and broader with entirely serrated edges— 14 or 15 occur along each side of the symphysis. The denticles lie flat and typically have between five and seven ridges.
C. longimanus feeds mainly on pelagic cephalopods and bony fish. However, its diet can be far more varied and less selective—it is known to eat threadfins, stingrays, sea turtles, birds, gastropods, crustaceans, and mammalian carrion. The bony fish it feeds on include lancetfish, oarfish, barracuda, jacks, dolphinfish, marlin, tuna, and mackerel. Its feeding methods include biting into groups of fish and swimming through schools of tuna with an open mouth. When feeding with other species, it becomes aggressive. Peter Benchley, author of Jaws, observed this shark swimming among pilot whales and eating their faeces.
The oceanic whitetip is usually solitary and slow-moving, and tends to cruise near the top of the water column, covering vast stretches of empty water scanning for possible food sources. Until the 16th century, sharks were known to mariners as "sea dogs" and the oceanic whitetip, the most common ship-following shark, exhibits dog-like behaviour when its interest is piqued: when attracted to something that appears to be food, its movements become more avid and it approaches cautiously but stubbornly, retreating and maintaining a safe distance if driven off, but ready to rush in if the opportunity presents itself. Oceanic whitetips are not fast swimmers, but they are capable of surprising bursts of speed. Whitetips commonly compete for food with silky sharks, making up for its comparatively leisurely swimming style with aggressive displays.
Groups often form when individuals converge on a food source, whereupon a feeding frenzy may occur. This seems to be triggered not by blood in the water or by bloodlust, but by the species' highly strung and goal-directed nature (conserving energy between infrequent feeding opportunities when it is not slowly plying the open ocean). The oceanic whitetip is a competitive, opportunistic predator that exploits the resource at hand, rather than avoiding trouble in favour of a possibly easier future meal.
Segregation by sex and size does not seem to occur. Whitetips follow schools of tuna or squid, and trail groups of cetaceans such as dolphins and pilot whales, scavenging their prey. Their instinct is to follow baitfish migrations that accompany ocean-going ships. When whaling took place in warm waters, oceanic whitetips were often responsible for much of the damage to floating carcasses.
Mating season is in early summer in the northwest Atlantic Ocean and southwest Indian Ocean, although females captured in the Pacific have been found with embryos year round, suggesting a longer mating season there. The shark is viviparous—embryos develop in utero and are fed by a placental sac. Its gestation period is one year. Litter sizes vary from one to 15 with the young born at a length around 0.6 m (24 in). Sexual maturity is reached around 1.75 m (69 in) for males and 2 m (80 in) for females.
Relationship with humans
The oceanic whitetip is a commercially important species for its fins, meat, and oil. It is eaten fresh, smoked, dried, and salted and its hide is used for leather. It is subject to fishing pressure throughout virtually its whole range—although it is more often taken as bycatch than by design, since it is drawn to longline bait that is intended for other species.
Famed oceanographic researcher Jacques Cousteau described the oceanic whitetip as "the most dangerous of all sharks". Despite the greater notoriety of the great white shark and other sharks habitually found nearer the shore, the oceanic whitetip is suspected to be responsible for many fatal shark bites on humans, as a result of predation on survivors of shipwrecks or downed aircraft. Such incidents are not included in common shark-bite indices for the 20th and 21st centuries, and as a result, the oceanic whitetip does not have the highest number of 'recorded' incidents; only five recorded bites as of 2009. In one incident, the torpedoing of USS Indianapolis on 30 July 1945, oceanic whitetips are believed to be responsible for many of the fatal bites of sailors who survived the initial sinking, though most reportedly died from exposure to the elements rather than from shark bites.
Also during World War II, the Nova Scotia, a steamship carrying about 1,000 people near South Africa, was sunk by a German submarine. With only 192 survivors, many deaths were attributed to the whitetip.
One particularly infamous oceanic whitetip was implicated in several bites on tourists in the Red Sea near Sharm El Sheikh, Egypt, in 2010. This oceanic whitetip was recognized individually by the bite mark taken out of its upper tail lobe. Accumulating evidence revealed this shark to have been conditioned to being hand fed. Upon associating the divers with an easy supply of food, it bit the divers and snorkelers where it had seen the fish being kept; fanny packs the divers carried. This caused the shark to target the divers' buttock and thigh regions in the hope of obtaining a meal. The 2010 Sharm El Sheikh bites resulted in one death and four injuries to humans. The attacks might have been instigated by the overfishing in that area of the Red Sea, effectively forcing the shark closer to shore where the bites took place.
Dr. Christopher Neff, a policy analyst at the University of Sydney, argues that terms like "attack" are laden with cultural stigmatization. Instead of the word "attack", he proposes labeling human-shark interactions, on a scale of extremity, as encounters, minor bites, moderate bites and fatal bites. The term "attack" is only appropriate in specific instances where specialists can confirm the predatory nature of the shark-human encounter, which is extremely difficult to do. While many encounters with oceanic whitetip sharks appear predatory in nature, without the verification of a scientific community in each instance, it is best to assume the accidental or nonpredatory intent of the encounters.
The oceanic whitetip has fared better in captivity than other large sharks of the open ocean, such as the mako and blue shark. Among five recorded captive oceanic whitetips, the three with time records all lived for more than a year in captivity. One of these, a female in Monterey Bay Aquarium's Outer-Bay exhibit, lived for more than three years during which it grew 0.3 m (1 ft). The two remaining lack a time record, but grew about 0.5 m (1.6 ft) during their time in captivity.
In 1969, Lineaweaver and Backus wrote of the oceanic whitetip: "[it is] extraordinarily abundant, perhaps the most abundant large animal, large being over 100 pounds [45 kg], on the face of the earth". Little further population study occurred until 2003, when the numbers were estimated to have dropped by as much as 70% in the Northwest and Western Central Atlantic between 1992 and 2000. Another study focusing on the Gulf of Mexico, using a mix of data from US pelagic longline surveys from the mid-1950s and observations from the late-1990s, estimated a decline in numbers in this location of 99.3% over this period. However, changes in fishing practices and data collection methods complicate estimates.
As a result of these findings, its status on the IUCN Red List was moved to "Vulnerable" globally (from "Lower Risk/Near Threatened") and "Critically Endangered" in the Northwest and Western Central Atlantic areas.
Under the 1995 UN Agreement on the Conservation and Management of Straddling Fish Stocks and Highly Migratory Fish Stocks, coastal and fishing states are specifically required to adopt measures to conserve listed species, but little progress is visible on the oceanic whitetip.
In March 2013, three endangered commercially valuable sharks, the hammerheads, the oceanic whitetip, and porbeagle were added to Appendix II of CITES, bringing shark fishing and commerce of these species under licensing and regulation.
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|External identifiers for Carcharhinus longimanus|
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- Introducing an ongoing study of the Oceanic Whitetip Shark in the Red Sea
- Oceanic Whitetip shark Carcharhinus longimanus at marinebio.org
- Whitetip Oceanic Shark profile, Florida Museum of Natural History.