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===Size===
===Size===
While the average length of a Great White is 4 to 5 m (13 to 16 ft), females generally being larger than males, the question of the maximum size of Great White sharks has been subject to much debate, conjecture, and misinformation. [[Richard Ellis (biologist)|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.
While the average length of a Great White is 4 to 5 m (13 to 16 ft), females generally being larger than males, the question of the maximum size of Great White sharks has been subject to much debate, conjecture, and misinformation. [[Richard Ellis (biologist)|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. Sharks have no bones, they are cartilage fish, making the history of their evolution difficult to identify.


Today, most experts contend that the Great White's "normal" maximum size is about 6 m (20 ft), with a maximum weight of about 1900 kg (4200 lb). Any claims much beyond these limits are generally regarded as doubtful, and are closely scrutinized.
Today, most experts contend that the Great White's "normal" maximum size is about 6 m (20 ft), with a maximum weight of about 1900 kg (4200 lb). Any claims much beyond these limits are generally regarded as doubtful, and are closely scrutinized.

Revision as of 22:03, 4 June 2006

Great White Shark
File:Large white shark.jpg
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Chondrichthyes
Subclass:
Elasmobranchii
Order:
Lamniformes
Family:
Lamnidae
Genus:
Carcharodon

Smith, 1838
Species:
C. carcharias
Binomial name
Carcharodon carcharias
File:Great White Shark distribution.png
Great White shark range (in blue)
For information about the band, see Great White (band). For information about the movie, see Great White (movie).

The Great White Shark (Carcharodon carcharias), also known as White Pointer, White Shark, White Death, or Amaletz, is an exceptionally large lamniform shark found in coastal surface waters in all major oceans. Reaching lengths of about 6 meters (20 feet) and weighing almost 2,000 kilograms (4,000 pounds), the Great White is the world's largest predatory fish. They are the only known surviving species of their genus, Carcharodon.

Distribution and Habitat

Great White Sharks live in almost all the cold or temperate waters of the planet, with greater concentrations in the southern coasts of Australia, in South Africa, California, and to a degree in the Central Mediterranean and the Adriatic Sea. 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 White Shark Trust in a single day[citation needed]. It can be also found in tropical waters like those of the Caribbean and has been recorded off Mauritius. It is also a pelagic fish, but recorded or observed mostly in coastal waters in the presence of rich game like otariids, cetaceans, other sharks and large bony fish species. It is considered an open-ocean dweller and is found from the surface to as deep as 1280 meters, but is most often seen near the surface.

Anatomy and appearance

An illustration of the Great White shark

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 shark, but unlike most other sharks). It is pale to dark gray and has a white belly.

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 tensor-sensing nerve cells. This arrangement seems to give their teeth high tactile sensitivity.

Size

While the average length of a Great White is 4 to 5 m (13 to 16 ft), females generally being larger than males, the question of the maximum size of Great White sharks 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. Sharks have no bones, they are cartilage fish, making the history of their evolution difficult to identify.

Today, most experts contend that the Great White's "normal" maximum size is about 6 m (20 ft), with a maximum weight of about 1900 kg (4200 lb). Any claims much beyond these limits are generally regarded as doubtful, and are closely scrutinized.

For some decades, many standard ichthyology reference books listed an 11 m (36 feet) Great White captured in south Australian waters near Port Fairy in the 1870s as the largest individual. While this was the commonly accepted maximum size, reports of 7.5 to 10 m (25 to 30 ft) Great Whites were common and often deemed credible.

Some researchers questioned the reliability of this measurement of the Port Fairy shark, noting it was much larger than any other accurately reported Great White. The question was settled in the 1970s, when J.E. Reynolds examined the Port Fairy shark's jaws and "found that the Port Fairy shark was of the order of 5 m (17 feet) in length".[1]

Ellis and McCosker write that "the largest White Sharks accurately measured range between 19 and 21 feet [about 5.8 to 6.4 meters], and there are some questionable 23-footers [about 7 meters] 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 measurements, see The Submarine (shark)).

The largest specimen Ellis and McCosker endorse as reliably measured was 6.4 m (21 ft) long, caught in Cuban waters in 1945 (they note, however, that other experts have argued this individual might have been a few feet shorter). 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 meters (over 23 feet) 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 ft [5.5m] range and NO WAY approaches the 23 ft [7 m] reported by Abela." (as in original) [2]

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 coast (North Atlantic) and measured 6.1 m (20 ft). The shark was caught by David McKendrick a local resident from Alberton, West Prince[citation needed].

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 kg (30 lb) of flesh, and can gorge on several hundred pounds or kilograms of food.

Ellis and McCosker write that "it is likely that [Great White] sharks can weigh as much as 2 tons", but also note that the largest verified examples weigh in at about 1.75 short tons (1.6 metric tons).

The largest Great White recognized by the International Game Fish Association is one landed by Alf Dean in south Australian waters in 1959, weighing 1208 kg (2664 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. They are apex predators; the only animals known to attack them are other Great Whites, sperm whales and orcas.

Great Whites are partially warm-blooded, 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're economical with their calories and can go weeks between meals. Due to problems keeping Great Whites in captivity, no concrete figures for this exist.

Behavior

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 the bottom. Sometime 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, Blacktip reef 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 have 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.

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 occasional 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.

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 writes 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." [3]

Many "shark repellents" have been tested, some using scent, others using protective clothing, but to date the most effective is an electronic beacon worn by the diver/surfer that creates an electric field which disturbs the shark's sensitive electro-receptive sense, the Ampullae of Lorenzini.

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. 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 behavior, 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 8–9 (with a maximum of perhaps 14) for a single delivery, are about 1.5 m (5 ft) long when born. Their teeth are provided with small side cusps. They grow rapidly, reaching 2 meters of length in the first year of life. Almost nothing, however, is known about how and where the Great White mates. It should be noted that there is some evidence that points to the near-soporific effect as the result of a large kill (such as a large whale) possibly inducing mating.

A White Shark can reproduce when a male's length is around 3.8 meters and a female's length is around 4.5 to 5 meters. Their lifespan has not been definitively established, though many sources estimate 30–40 years. It would not be unreasonable to expect such a large marine animal to live longer however.

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. [4]

In 1984, 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. [5] 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 Orange County, was kept in the aquarium's massive 1 million-gallon 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 release. [6]

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. [7]

Conservation status

It is unclear how much a consummate 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, 22,000 kilometers round trip. Researchers believe it may have undertaken this journey to mate, and hope studies such as this will produce more effective conservation measures. [8]

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. 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.

Dental features and the extreme size of both the White Shark 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 18 m (59 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 10,000–12,000 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.

See also

References

  • "Carcharodon carcharias". Integrated Taxonomic Information System. 23 January. {{cite web}}: Check date values in: |date= and |year= / |date= mismatch (help)
  • Froese, Rainer; Pauly, Daniel (eds.). "Carcharodon carcharias". FishBase. October 2005 version.
  • Biology of the White Shark, a Symposium. Gretchen Sibley editor; Jeffrey A. Siegel, Camm C. Swift assistant editors. Los Angeles: Southern California Academy of Sciences, 1985. Memoirs of the Southern California Academy of Sciences, volume 9.
  • Great White Shark, Carcharodon carcharias at marinebio.org

http://sfgate.com/cgi-bin/article.cgi?f=/c/a/2003/08/20/MN292542.DTL

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