Great White Shark
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The great white shark, Carcharodon carcharias, also known as great white, white pointer, white shark, or white death, is an exceptionally large lamniform shark found in coastal surface waters in all major oceans. Reaching lengths of more than 6 metres (20 ft) and weighing up to 2,240 kilograms (4,938 lb), the great white shark is arguably the world's largest known predatory fish. It is the only surviving species of its genus, Carcharodon.
Distribution and habitat
Great white sharks live in almost all coastal and offshore waters which have a water temperature of between 12C (54F) and 24C (75F), with greater concentrations off the southern coasts of Australia, off South Africa, California, Mexico's Isla Guadalupe and to a degree in the Central Mediterranean, Adriatic Seas and New Zealand, where they are a protected species. One of the densest known populations is found around Dyer Island, South Africa where much research on the shark is conducted. It can be also sometimes found in tropical waters like those of the Caribbean, and has been recorded off Mauritius, Madagascar, Kenya and the Seychelles. It is an epipelagic fish, but recorded or observed mostly in inland tributaries in the presence of rich game like fur seals, sea lions, cetaceans, other sharks and large bony fish species. It is considered an open-ocean dweller and is recorded from the surface down to depths of 1,280 m (4,200 ft), but is most often found close to the surface.
In a recent study, great white sharks from California were shown to migrate to an area between Baja California and Hawaii known as White Shark Cafe, where they spend at least 100 days of the year before they migrate back to Baja. On the journey out, they swim slowly and dive down to around 900 m (3,000 ft). After they arrive, they change behaviour and do short dives to about 300 m (980 ft) for up to 10 minutes. Another white shark tagged off the coast of South Africa swam to the southern coast of Australia and back within the space of a year. This had disproved traditional theories of white sharks being coastal territorial predators and opens up the possibility of interaction between white shark populations that were previously thought to be discrete from one another, and it is still unknown why they migrate and what they do at their destination; it might be seasonal feeding or possibly a mating area.
In a similar study a great white shark from South Africa was tracked swimming to the northwestern coast of Australia and back to the same location in South Africa, a journey of 20,000 km (12,000 mi) in under 9 months.
Anatomy and appearance
The great white shark has a robust large conical-shaped snout. It has almost the same size upper and lower lobes on the tail fin (like most mackerel sharks, but unlike most other sharks).
Great white sharks display countershading, having a white underside and a grey dorsal area (sometimes in a brown or blue shade) that gives an overall "mottled" appearance. The coloration makes it difficult for prey to spot the shark because it breaks up the shark's outline when seen from a lateral perspective. When viewed from above, the darker shade blends in with the sea and when seen from below casts a minimal silhouette against the sunlight.
Great white sharks, like many other sharks, have rows of teeth behind the main ones, allowing any that break off to be rapidly replaced. A great white shark's teeth are serrated and when the shark bites it will shake its head side to side and the teeth will act as a saw and tear off large chunks of flesh. Great whites often swallow their own broken off teeth along with chunks of their prey's flesh. These teeth frequently cause damage to the great white's digestive tract, often resulting in death from infection and blood loss.
A typical adult great white shark measures 4-4.8 metres (13-16 ft) and has a mass of 680-1,100 kilograms (1,500-2,400 lb), females generally being larger than males. The great white shark's "normal" maximum size is about 6 m (20 ft), with a "normal" maximum weight of about 1,900 kg (4,200 lb).
The maximum size of the great white shark has been hotly debated. Richard Ellis and John E. McCosker, both academic shark experts, devote a full chapter in their book, Great White Shark (1991), to analysing various accounts of extreme size.
For several decades, many ichthyological works, as well as the Guinness Book of World Records, listed two great white sharks as the largest individuals caught: an 11 m (36 ft) great white captured in Southern Australian waters near Port Fairy in the 1870s, and an 11.3 m (37 ft) shark trapped in a herring weir in New Brunswick, Canada in the 1930s. While this was the commonly accepted maximum size, reports of 7.5-10-metre (25-33 ft) great white sharks were common and often deemed credible.
The largest great white shark recognized by the International Game Fish Association (IGFA) is one landed by Alf Dean in south Australian waters in 1959, weighing 1,208 kg (2,660 lb). Several larger great white sharks caught by anglers have since been verified, but were later disallowed from formal recognition by IGFA monitors for rules violations.
Great white sharks, like all other sharks, have an extra sense given by the Ampullae of Lorenzini, which enables them to detect the electromagnetic field emitted by the movement of living animals. Every time a living creature moves it generates an electrical field and great whites are so sensitive they can detect half a billionth of a volt. Most fish have a less developed but similar ability in the horizontal line along their body.
To more successfully hunt fast moving and agile prey such as sea lions, the great white shark has developed adaptations that allow it to maintain a body temperature warmer than the surrounding water. One of these adaptations is a "rete mirabile" (Latin for "wonderful net"). This close web-like structure of veins and arteries, located along each lateral side of the shark, conserves heat by warming the cooler arterial blood with the venous blood that has been warmed by the working muscles. This keeps certain parts of the body (particularly the brain) at temperatures up to 14C (25F) above the surrounding water, while the heart and gills remain at sea-temperature. When conserving energy (a great white shark can go weeks between meals), the core body temperature can drop to match the surroundings. A great white shark's success in raising its core temperature is an example of gigantothermy. Therefore, the great white shark can be considered an endothermic poikilotherm, because its body temperature is not constant but is internally regulated.
Great white sharks are carnivorous, and primarily eat fish (including rays, tuna, and smaller sharks), dolphins, porpoises, whale carcasses and pinnipeds such as seals, fur seals and sea lions and sometimes sea turtles. Sea otters and penguins are attacked at times although rarely, if ever, eaten. Great whites have also been known to eat objects that they are unable to digest. In great white sharks above 3.41 metres (11 ft) a diet consisting of a higher proportion of mammals has been observed. These sharks prefer prey with high contents of energy-rich fat. Shark expert Peter Klimley used a rod-and-reel rig and trolled carcasses of a seal, a pig, and a sheep to his boat in the South Farallons. The sharks attacked all three baits but rejected the sheep carcass.
The great white is regarded as an apex predator with its only real threats from humans. Although their diets overlap greatly, great whites do not seem to directly compete with orcas and there are few reports of encounters between them. The average orca is larger and faster which is probably why the sharks do not bother to compete. However in one famous incident a female orca killed a subadult great white and her calf feasted on the shark's liver. Pods of dolphins can kill a great white shark through mobbing behaviour in which the dolphins ram the shark. Great whites are also sometimes preyed on by larger specimens.
Great white breaching to catch a Cape Fur SealGreat white sharks' reputation as ferocious predators is well-earned, yet they are not (as was once believed) indiscriminate "eating machines". They typically hunt using an "ambush" technique, taking their prey by surprise from below. Near the now-famous Seal Island, in South Africa's False Bay; studies have shown that the shark attacks most often occur in the morning, within 2 hours after sunrise. The reason for this is that it is hard to see a shark close to the bottom at this time. The success rate of attacks is 55% in the first 2 hours, it falls to 40% in late morning and after that the sharks stop hunting.
The hunting technique of the white shark varies with the species it hunts. When hunting Cape fur seals off Seal Island, South Africa, the shark will ambush it from below at high speeds and hit the seal at mid-body. They go so fast that they actually breach out of the water. The peak burst speed of these sharks is largely accepted in the scientific community to be above at least 40 kilometers per hour (25 mi/h), however further precision is still regarded as speculation and obviously depends on the individual shark. They have also been observed chasing their prey after a missed attack. The prey is usually attacked at the surface.
When hunting Northern elephant seals off California, the shark immobilizes the prey with a large bite to the hindquarters (which is the main source of the seal's mobility) and waits for the seal to bleed to death. This technique is especially used on adult male elephant seals which can be as large or larger than a great white and are potentially dangerous adversaries. Prey is normally attacked sub-surface. Harbour seals are simply grabbed from the surface and pulled down until they stop struggling. They are then eaten near the bottom. California sea lions are ambushed from below and struck in mid-body before being dragged and eaten.
When hunting dolphins and porpoises, white sharks attack them from above, behind or below to avoid being detected by their echolocation. Among the species targeted are dusky dolphins, harbour porpoises, Risso's dolphins and Dall's porpoises.
A new study from the University of New South Wales in Sydney, Australia, is using CT scans of a shark's skull and complex computer models to measure the maximum bite force of the great white. The study will reveal what forces and behaviours the carnivore's skull is adapted to handle and will help resolve competing theories about its feeding behaviour.
The behavior and social structure of the white shark is not well understood. In South Africa, white sharks have a dominance hierarchy depending on size, sex and squatter's rights: Females dominate males, larger sharks dominate smaller sharks, and residents dominate newcomers. When hunting, white sharks tend to space out and resolve conflicts with rituals and displays. White sharks rarely resort to combat although some individuals have been found with bite marks that match those of other white sharks. This suggests that when their personal space is intruded upon, white sharks give the intruder a warning bite. Another possibility is that white sharks softly bite to show dominance. Also, white sharks are cannibalistic.
The great white shark is one of only a few sharks known to regularly lift its head above the sea surface to gaze at other objects such as prey; this is known as "spy-hopping". This behaviour has also been seen in at least one group of blacktip reef sharks, but this might be a behaviour learned from interaction with humans (it is theorized that the shark may also be able to smell better this way, because smells travel through air faster than through water). They are very curious animals, and can display a high degree of intelligence and personality when conditions permit (such as in the clear waters off of Isla Guadalupe, Mexico).
There is still a great deal that is unknown about great white shark behaviour, such as their mating habits. Birth has never been observed, but several pregnant females have been examined. Great white sharks are ovoviviparous, the eggs developing in the female's uterus, hatching there and continuing to develop until they are born. The great white has an 11 month gestation period, with the shark pup's powerful jaws beginning to develop in the 1st month. The unborn sharks are thought to participate in a phenomenon known as intrauterine-cannibalism in which the strongest pups consume their weaker womb-mates. This behavior is known to occur in several other species of shark as well. The delivery is thought to take place in the period between spring and summer. When giving birth, the female has to go fast to prevent herself from eating her young after they are born.
Almost nothing, however, is known about how and where the great white mates. There is some evidence that points to the near-soporific effect resulting from a large feast (such as a whale carcass) possibly inducing mating.
Great White Sharks take around 15 years to reach sexual maturity. The lifespan of the great white has not been definitively established, although many sources estimate that great whites live 30 to over 100 years. It would not be unreasonable to expect such a slow maturing animal to live longer than other, faster maturing varieties.
It is unclear how much a concurrent increase in fishing for great white sharks had to do with the decline of great white shark population from the 1970s to the present. No accurate numbers on population are available, but populations have clearly declined to a point at which the great white shark is now considered endangered. Their reproduction is slow, with sexual maturity occurring at about 12-15 years of age, the population, therefore, can take a long time to rise.
The Convention on International Trade in Endangered Species (C.I.T.E.S.) has put the great white shark on its 'Appendix II' list of endangered species. The shark is targeted by fishermen for its jaws, teeth, and fins, and as a game fish. The great white shark, however, is rarely an object of commercial fishing, although its flesh is considered valuable. If casually captured (it happens for example in some tonnare in the Mediterranean), it is sold as smooth-hound shark. From April 2007 great white sharks were fully protected within 370 kilometers (200 nautical miles) of New Zealand and additionally from fishing by New Zealand-flagged boats outside this range. The maximum penalty for targeting white pointers is a $250,000 fine and up to six months in prison.
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