Science Fair Project Encyclopedia
Great White Shark
The Great White Shark (Carcharodon carcharias), also known as White Pointer, White Shark or Amaletz, is an exceptionally large lamniform shark found in coastal surface waters in all major oceans. Reaching lengths of about 6 metres (about 21 feet) and weights of about 1,800 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.
Great Whites have excellent eyesight and can see in color, and have highly-developed behaviors which are only now being researched. Their reputation as ferocious predators is well-earned, yet they are not (as once was believed) indiscriminate "eating machines". Great White sharks primarily eat fishes and pinnipeds such as seals and sea lions. Great Whites are apex predators ; the only animals known to attack them are other Great Whites, sperm whales, humans and orcas.
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 The Great White Shark (1991) to analysis of various accounts of extreme size.
Average length is 3 to 4 m, and females are generally larger than males.
For some decades, many standard ichthyology reference books listed a 36 foot (11.0 m) 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 25 to 30 ft (7.5 to 10 m) Great Whites were common and often deemed credible.
Some researchers questioned the reliability 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 examination of the shark's jaws revealed it was actually 16 to 17 ft (about 5 m) long, and the error assumed to probably be a typographical error.
Ellis and McCosker write that "the largest White Sharks accurately measured range between 19 and 21 ft (about 6 m), and there are some questionable 23-footers (7 m) in the popular — but not the scientific — literature." Furthermore, they add that "these giants seem to disappear when a responsible observer approach with a tape measure."
The largest specimen Ellis and McCosker endorse is a 21-footer (6.4 m) caught in Cuban waters in 1945. A 23-footer (7.0 m) was reported from Malta in 1987; Ellis and McCosker stop short of endorsing the measurement, and one analysis of the Malta shark concludes it was "in the 18ft [5.5 m] range".
Today, most experts contend that the Great White's maximum size is about 6 m (21 ft), with a maximum weight of about 1900 kg. Any claims much beyond these limits are generally regarded as doubtful.
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 30 lb (14 kg) 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 2664 lb (1208 kg). Several larger Great Whites caught by anglers have since been verified, but were later disallowed from formal recognition by IFGA monitors for rules violations.
Breeding, Behavior and Lifespan
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 young are about 1.5 m (5 ft) long when born. Almost nothing, however, is known about how and where the Great White mates.
Their lifespan is not known, but 25 to 30 years is a generally accepted estimate.
Attacks on Humans
While Great Whites have been responsible for many fatalities in humans, they typically do not target humans as prey. 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, grabbing 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.
Humans, in any case, aren't good 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 nearly all recorded attacks, Great Whites have broken off contact after the first bite. Fatalities are caused by loss of blood from the initial injury. Most attacks also occur in waters with low visibility, or in other cases in which the shark's senses are impaired.
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."
Many "shark repellents" have been tested, some using smell, others using protective clothing, but to date the most effective is an electronic beacon worn by the diver/surfer that emits a high frequency signal disturbing to the shark's electromagnetic sensors.
Great Whites, like many other sharks, have rows of teeth behind the main ones, allowing any that break off to be replaced rapidly. Their teeth are unattached to the jaw and are retractable, like a cat's claws, moving into place when the jaw is opened. This arrangement also seems to give their teeth high tactile sensitivity.
Occurrence in fiction
It is unclear how much the film Jaws, and a consummate increase in fishing for Great Whites, had to do with the decline of Great White populations 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 populations can take a long time to rise.
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.
These sharks have an extinct relative, the Megalodon (Carcharodon megalodon), which could possibly have reached sizes of 18 m (59 ft) or more, and is currently known only from its teeth. Megalodon is thought to have been similar to the White Shark, but substantially larger. 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.
- MarineBio: Great White shark, Carcharodon carcharias
- Review of the great white shark
- Envirofacts: Great White Shark
- Richard Ellis and John E. McCosker, The Great White Shark, Stanford University Press, 1991, ISBN 0804725292
The contents of this article is licensed from www.wikipedia.org under the GNU Free Documentation License. Click here to see the transparent copy and copyright details