Great white shark
2007 Schools Wikipedia Selection. Related subjects: Insects, Reptiles and Fish
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| Carcharodon carcharias (Linnaeus, 1758) |
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The great white shark (Carcharodon carcharias), also known as 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 about 6 metres (20 ft) and weighing almost 2,000 kilograms (4,400 lb), the great white shark is the world's largest known predatory fish. It is the only known 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 12 and 24° C (54° to 75° F), 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 and Adriatic Seas. The densest known population is found around Dyer Island, South Africa where up to 31 different white sharks have been documented by Michael Scholl of the White Shark Trust in a single day. It can be also found in tropical waters like those of the Caribbean and has been recorded off Mauritius. It is a pelagic fish, but recorded or observed mostly in coastal waters in the presence of rich game like fur seals, sealions, 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 metres (4,200 ft), but is most often found close to the surface.
In a recent study white sharks from California were shown to migrate to an area between Baja California and Hawaii, 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 to up to 900 metres (3,000 ft). After they arrive, they change behaviour and do short dives to about 300 m (1,000 ft) for up to 10 minutes. It is still unknown why they migrate and what they do there; it might be seasonal feeding or possibly a mating area.
In a similar study a 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 kilometres 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). It is pale to dark grey and has a white stomach.
Great whites, like many other sharks, have rows of teeth behind the main ones, allowing any that break off to be rapidly replaced. Their teeth are unattached to the jaw and are retractable, like a cat's claws, moving into place when the jaw is opened. Their teeth also rotate on their own axis (outward when the jaw is opened, inward when closed). The teeth are linked to pressure and tension-sensing nerve cells. This arrangement seems to give their teeth high tactile sensitivity. A great white'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. However great whites often feed on stingrays and swallow the 'sting' as well, the barbed sting often getting stuck in the shark's guts. There are anecdotal reports of the sting working its way out through the shark's side. Correspondingly, a tooth causes the shark no major harm.
Size
The average length of a full grown great white is 4 to 4.8 metres (13.3 to 15.8 ft), with a weight of 680 to 1,100 kilograms (1,500 to 2,450 lbs), females generally being larger than males. But the question of the maximum size of a great white shark has been subject to much debate, conjecture, and misinformation. Richard Ellis and John E. McCosker, both academic shark experts, devote a full chapter in their book The Great White Shark ( 1991) to analysis of various accounts of extreme size.
Today, most experts contend that the great white's "normal" maximum size is about 6 metres (20 ft), with a maximum weight of about 1,900 kilograms (4,200 lb). Any claims much beyond these limits are generally regarded as doubtful, and are closely scrutinized.
For some decades many ichthyological works, as well as the Guinness Book of World Records, listed two great whites as the largest individuals caught: an 11 metre (36 ft) great white captured in south Australian waters near Port Fairy in the 1870s, and an 11.3 metre (37.6 ft) shark trapped in a herring wier in New Brunswick, Canada in the 1930s. While this was the commonly accepted maximum size, reports of 7.5 to 10 metre (25 to 33.3 ft) great whites were common and often deemed credible.
Some researchers questioned the reliability of both measurements, noting they were much larger than any other accurately-reported great white. The New Brunswick shark may have been a wrongly-identified basking shark, as both sharks have similar body shapes. The question of the Port Fairy shark was settled in the 1970s, when J.E. Reynolds examined the shark's jaws and "found that the Port Fairy shark was of the order of 5 m (17 feet) in length and suggested that a mistake had been made in the original record, in 1870, of the shark's length.
Ellis and McCosker write that "the largest White Sharks accurately measured range between 19 and 21 ft [about 5.8 to 6.4 m], and there are some questionable 23-footers [about 7 m] in the popular — but not the scientific — literature". Furthermore, they add that "these giants seem to disappear when a responsible observer approaches with a tape measure." (For more about legendary exaggerated shark measurements, see the submarine).
The largest specimen Ellis and McCosker endorse as reliably measured was 6.4 metres (21.3 ft) long, caught in Cuban waters in 1945 (though confident in their opinion, Ellis and McCosker note, however, that other experts have argued this individual might have been a few feet shorter). Click here to see a photo of this Cuban shark.
There have since been claims of larger great whites, but, as Ellis and McCosker note, verification is often lacking and these extraordinarily large great whites have, upon examination, all proved of average size. For example, a female said to be 7.13 metres (over 23 ft) was fished in Malta in 1987 by Alfredo Cutajar. In their book, Ellis and McCosker agree this shark seemed to be larger than average, but they did not endorse the measurement. In the years since, experts eventually found reason to doubt the claim, due in no small part to conflicting accounts offered by Cutajar and others. A BBC photo analyst concluded that even "allowing for error ... the shark is concluded to be in the 18.3 ft [5.5 m] range and NO WAY approaches the 23 ft [7 m] reported by Abela." (as in original)
According to the Canadian Shark Research Centre, the largest accurately measured great white shark was a female caught in August 1983 at Prince Edward Island off the Canadian (North Atlantic) coast and measured 6.1 metres (20.3 ft). The shark was caught by David McKendrick, a local resident from Alberton, West Prince.
The question of maximum weight is complicated by an unresolved question: when weighing a great white, does one account for the weight of the shark's recent meals? With a single bite, a great white can take in up to 14 kilograms (30 lb) of flesh, and can gorge on several hundred kilograms or pounds of food.
Ellis and McCosker write in regards to modern great whites that "it is likely that [great white] sharks can weigh as much as 2 tons", but also note that the largest recent scientifically measured examples weigh in at about 2 tonnes (1.75 short tons).
The largest great white recognized by the International Game Fish Association (IGFA) is one landed by Alf Dean in south Australian waters in 1959, weighing 1,208 kilograms (2,664 lb). Several larger great whites caught by anglers have since been verified, but were later disallowed from formal recognition by IGFA monitors for rules violations.
Diet
Great white sharks primarily eat fish, smaller sharks, turtles, dolphins, and pinnipeds such as seals and sea lions. Great Whites have also been known to eat garbage that can't be digested. In great white sharks above 3.41 metres (11 ft, 2 in) towards a diet consisting of a higher proportion of mammals has been observed. Sometimes, they swim so fast that they actually jump out of the water while chasing/attacking seals. They are apex predators; the only animals known to attack them as prey are other great whites, and orcas.
Great whites are warm-bodied (certain parts of the body running at higher temperatures, while the heart and gills run at sea-temperature), keeping most of their body up to 14° C above the surrounding water, which would suggest a high metabolism. Despite that, the few estimates that have been made suggest they are economical with their calories and can go weeks between meals. Due to problems keeping great whites in captivity, no concrete figures for this exist.
Behaviour
Great 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. Off Seal Island in South Africa studies have shown that the shark attacks most often 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 in average is 55% in the first 2 hours, it falls to 40% in late morning and after that the sharks stop hunting. Sometimes, they swim so fast that they actually jump out of the water while chasing/attacking seals. This is one of only a few sharks that can jump fully out of the water, the others are thresher shark, shortfin mako, longfin mako, spinner shark, basking shark, blacktip shark, salmon shark, porbeagle shark and the copper shark. This is the only shark 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, since 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 crystal-clear waters of Isla Guadalupe, Mexico).
Capabilities
Great whites, 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. This is equivalent to detecting a flashlight battery from 1,600 kilometres (1,000 miles) away. Thus a great white's sense of sight is useful, but the shark does not depend on it. A shark primarily uses its extra senses (i.e, electrosense and mechanosense) to locate prey from far off. Then, the shark uses smell and hearing to further verify that its target is food. At close range, the shark utilizes sight for the attack. The shark will often in ambush deliver a massive disabling bite and then back off to allow the prey to expire. This tactic allows the animal to avoid combat with dangerous prey, such as sea lions. It also has allowed occasional rescue of humans bitten by the animal, though it appears to attack humans mostly in error.
Reproduction
There is still a great deal that is unknown about great white behaviour, such as mating habits. Birth has never been observed, but several pregnant females have been examined. Great whites are ovoviviparous, the eggs developing in the female's uterus, hatching there and continuing to develop until they are born, at which point they are perfectly capable predators. The embryos can feed off unfecundated eggs. The delivery takes place in the period transitioning spring and summer.
The young, which number 8 or 9 (with a maximum of perhaps 14) for a single delivery, are about 1.5 metres (5 ft) long when born. Their teeth are provided with small side cusps. They grow rapidly, reaching 2 metres of length in the first year of life. 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.
A white shark can reproduce when a male's length is around 3.8 metres (12 ft) and a female's length is around 4 to 4.8 metres (13.3 to 15.8 ft). Their lifespan has not been definitively established, though many sources estimate 30 to 40 years. It would not be unreasonable to expect such a large marine animal to live longer however.
Relationship to humans
Shark attacks
More than any documented attack, Steven Spielberg's 1975 film Jaws provided the great white with the image of a "man eater" in the public mind. While great whites have been responsible for fatalities in humans, they typically do not target humans as prey: for example, in the Mediterranean Sea there were 31 confirmed attacks against humans in the last two centuries, only a small number of them deadly. Many incidents seem to be caused by the animals "test-biting" out of curiosity. Great white sharks are known to perform test-biting with buoys, flotsam, and other unfamiliar objects as well, and might grab a human or a surfboard with their mouth (their only tactile organ) in order to determine what kind of object it might be.
Other incidents seem to be cases of mistaken identity, in which a shark ambushes a bather or surfer, usually from below, believing the silhouette it sees on the surface is a seal. Many attacks occur in waters with low visibility, or other situations in which the shark's senses are impaired. It has been speculated that the species typically does not like the taste of humans, or at least that the taste is unfamiliar.
However some researchers have hypothesized that the reason the proportion of fatalities is low is not because sharks do not like human flesh, but because humans are often able to get out of the water after the shark's first bite. In the 1980s John McCosker noted that divers who dived solo and were attacked by great whites were generally at least partially consumed, while divers who followed the buddy system were normally pulled out of the water by their colleagues before the shark could finish its attack. Tricas and McCosker suggest that a standard attack modus operandi for great whites is to make an initial devastating attack on its prey, and then wait for the prey to weaken before going in to consume the ailing animal. A human's ability to get to land (or onto a boat) with the help of others is unusual for a great white's prey, and thus the attack is foiled.
Humans, in any case, are not healthy for great white sharks to eat because the sharks' digestion is too slow to cope with the human body's high ratio of bone to muscle and fat. Accordingly, in most recorded attacks, great whites have broken off contact after the first bite. Fatalities are usually caused by loss of blood from the initial limb injury rather than from critical organ loss or from whole consumption.
Biologist Douglas Long and Tyler B. write that the great white's "role as a menace is exaggerated; more people are killed in the U.S. each year by dogs than have been killed by White sharks in the last 100 years."
Many "shark repellents" have been tested, some using scent, others using protective clothing, but to date the most effective is an electronic beacon ( POD) worn by the diver/surfer that creates an electric field which disturbs the shark's sensitive electro-receptive sense organs, the Ampullae of Lorenzini.
Great white sharks in captivity
All attempts to keep a great white shark in captivity prior to August 1981 lasted 11 days or less. However, that month a great white broke previous records by lasting 16 days in captivity at SeaWorld San Diego before being released into the wild.
In 1984, shortly before opening day, the Monterey Bay Aquarium in Monterey, California housed its first great white, which died after 10 days. In July 2003, Monterey researchers captured a small female and kept it in a large, netted pen off Malibu for five days, where they had the rare success of getting the shark to feed in captivity before it was released. It was not until September 2004 that the aquarium made history by becoming the first aquarium in the world to place a great white on long-term exhibit. The young female, who was caught off the coast of Ventura, was kept in the aquarium's massive 1 million-gallon (3,800,000 L) Outer Bay exhibit for 198 days before her successful release back to the wild in March 2005. She was tracked for 30 days after her early morning release. On the evening of August 31, 2006 the aquarium introduced a second shark to the Outer Bay exhibit. The juvenile male, caught outside Santa Monica Bay on August 17 , had its first official meal in captivity (a large salmon steak) on September 8, 2006 and as of that date, the shark was estimated to be 1.72 metres (5 ft 8 in) and to weigh approximately 47 kilograms (104 lb).
Probably the most famous great white to be kept in captivity was a female named "Sandy", which in August 1980 became the first and only great white to be housed at the Steinhart Aquarium in San Francisco, California. She was returned to the wild because she would not eat anything given to her and constantly bumped against the walls.
Shark tourism and cage diving
Viewing sharks from the safety of a cage gives tourists an adrenaline rush and has become a booming industry. Common practice is to chum the water to draw in sharks for the tourists to view. These practices have raised the fear that as a result of this form of tourism, sharks are becoming accustomed to people in their environment and beginning to associate human activity with food - a potentially dangerous situation.
Tour companies respond that they are being made the scapegoats, as people try to find someone to blame for shark attacks on humans. Most point out that lightning tends to strike more often than sharks bite. Their position is that further research needs to be done before banning practices such as chumming which are said to alter sharks natural behaviour.
Conservation status
It is unclear how much a concurrent increase in fishing for great whites had to do with the decline of great white 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 is now considered endangered. Their reproduction is slow, with sexual maturity occurring at about nine years of age, such that population can take a long time to rise.
In 2005, a tagged great white named "Nicole" was recorded swimming from South Africa to Australia and back, a 22,000 kilometre round trip. Researchers believe it may have undertaken this journey to mate, and hope studies such as this will produce more effective conservation measures.
The Convention on International Trade in Endangered Species (Cites) 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.
Related species
Dental features and the extreme size of both the great white and the megalodon, Carcharodon megalodon, lead some scientists to believe they were closely related, however there is much doubt about this hypothesis and other scientists would place the megalodon and white shark as distant relatives - sharing the family Lamnidae but no closer relationship. Megalodon is only known from its teeth, and may have reached sizes of 12 metres (40 ft) or more, considerably larger than even the largest white sharks. From time to time it is suggested that megalodon might still exist. Megalodon teeth have been found from as recently as 10,000 to 12,000 years ago, though some have questioned the reliability of these estimates. However, while megalodon fossils are widespread and plentiful, no evidence has surfaced that the species is anything but extinct.
Other evidence suggests that the great white is more closely related to the mako shark than to the megalodon.