Bull shark

Bull shark
Conservation status
Near Threatened (IUCN 2.3)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordate
Class:	Chondrichthyes
Subclass:	Elasmobranchii
Order:	Carcharhiniformes
Family:	Carcharhinidae
Genus:	Carcharhinus
Species:	C. leucas
Binomial name
Carcharhinus leucas (Müller and Henle, 1839)
Range of bull shark

The bull shark, Carcharhinus leucas, also known as Zambezi shark or unofficially known as Zambi in Africa and Nicaragua shark in Nicaragua, is a shark common worldwide in warm, shallow waters along coasts and in rivers. The bull shark is well known for its unpredictable, often aggressive behavior. Since bull sharks often dwell in shallow waters, they may be more dangerous to humans than any other species of shark,^[1] and, along with tiger sharks and great white sharks, are among the three shark species most likely to attack humans.^[2]

Unlike most sharks, bull sharks tolerate fresh water and can travel far up rivers. They have even been known to travel as far up as Indiana in the Ohio River and Illinois in the Mississippi River, although there have been few recorded attacks. As a result, they are probably responsible for the majority of near-shore shark attacks, including many attacks attributed to other species.^[3] However, bull sharks are not true freshwater sharks (unlike the river sharks of the genus Glyphis).

Etymology

The name, "bull shark", comes from the shark's stocky shape, broad, flat snout and aggressive unpredictable behavior.^[2] In India, the bull shark may be confused with the "Sundarbans" or "Ganges shark". In Africa it is also commonly called the "Zambezi River shark" or just "Zambi". Its wide range and diverse habitats result in many other local names, including "Ganges River Shark", "Fitzroy Creek Whaler", "van Rooyen’s Shark", "Lake Nicaragua Shark",^[4] "river shark", "freshwater whaler", "estuary whaler", "Swan River Whaler",^[5] "cub shark", and "shovelnose shark".^[6]

Distribution and habitat

The bull shark lives all over the world in many different areas and travels long distances. It is common in coastal areas of warm oceans, in rivers and lakes, and occasionally salt and freshwater streams if they are deep enough. It is found to a depth of 150 metres (490 ft) but does not usually swim deeper than 30 metres (98 ft).^[7] In the Atlantic it is found from Massachusetts to southern Brazil, and from Morocco to Angola. In the Indian Ocean it is found from South Africa to Kenya, India, and Vietnam to Australia. There are more than 500 bull sharks in the Brisbane River and greater numbers still in the canals of the Gold Coast in Queensland, Australia. A large bull shark was caught in the canals of Scarborough, 2 hours north of the Gold Coast.^[8] In the Pacific Ocean, it can be found from Baja California to Ecuador.

The shark has traveled 4,000 kilometres (2,500 mi) up the Amazon River to Iquitos in Peru.^[9] It also lives in fresh water Lake Nicaragua, and in the Ganges and Brahmaputra rivers of West Bengal and Assam in eastern India and adjoining Bangladesh. It can live in water with a high salt content as in St. Lucia Estuary in South Africa. After Hurricane Katrina, many bull sharks were sighted in Lake Ponchartrain.^[10] Bull sharks have occasionally gone up the Mississippi River as far upstream as Alton, Illinois. ^[11]

Freshwater tolerance

The bull shark is the best known of 43 species of elasmobranch in ten genera and four families to have been reported in fresh water. Other species that enter rivers include the stingrays (Dasyatidae, Potamotrygonidae and others) and sawfish (Pristidae). Some skates (Rajidae), smooth dogfishes (Triakidae), and sandbar sharks (Carcharhinus plumbeus) regularly enter estuaries. Elasmobranchs' ability to enter fresh water is limited because their blood is normally at least as salty (in terms of osmotic strength) as seawater, through the accumulation of urea and trimethylamine oxide, but bull sharks living in fresh water reduce the concentration of these solutes by up to 50%. Even so, bull sharks living in fresh water need to produce twenty times as much urine as those in salt water.^[4]

Initially, scientists thought the sharks in Lake Nicaragua belonged to an endemic species, the Lake Nicaragua shark (Carcharhinus nicaraguensis). In 1961, following specimens comparisons, taxonomists synonymized them.^[12] They can jump along the rapids of the San Juan River (which connects Lake Nicaragua and the Caribbean Sea), almost like salmon.^[13] Bull sharks tagged inside the lake have later been caught in the open ocean (and vice versa), with some taking as little as 7–11 days to complete the journey.^[12]

Anatomy and appearance

A sketch of a bull shark

Bull sharks are large and stout. Females are larger than males. The bull shark can grow up to 3.5m (11 ft 6in). It also can weigh up to 230 kg (505 lbs). Bull sharks are wider than other requiem sharks of comparable length, and are grey on top and white below. The second dorsal fin is smaller than the first.

Diet

Most of a bull shark's diet consists of bony fish and smaller sharks, including other bull sharks.^[14] Bull sharks' diets can also include turtles, birds, dolphins, terrestrial mammals, crustaceans, and echinoderms.^[15] Bull sharks have been known to use the "bump-and-bite" technique to attack their prey. Relatively calm bull sharks can suddenly become violent and begin to bump divers.^[16]

Erich Ritter was severely wounded by such a bull shark.^[7] This attack does not appear to be a case of mistaken identity, because the water was clear, and no triggering weather was present. While Ritter himself initially admitted that the shark seemed to be trying to tow him out to deeper water, he later claimed that the shark clearly had no feeding intentions towards him. Ritter contended that the attack was provoked by chum originally thrown far from him, and then brought back in his direction by a remora. The remora theoretically excited the bull sharks, disturbing the sand. In the resulting sand cloud, Ritter contends that one of the sharks was confused and bit him.

Behavior

Bull shark (Bahamas)

Bull sharks are typically solitary hunters,^[7] but occasionally hunt in pairs. They often cruise through shallow waters. They can suddenly accelerate and can be highly aggressive, even attacking a racehorse in the Brisbane River in the Australian state of Queensland.^[17] They are extremely territorial and attack animals that enter their territory. Along with the great white and tiger sharks, bull sharks are among the three species most likely to attack humans.^[2] One or more bull sharks may have been responsible for the Jersey Shore shark attacks of 1916, which was the inspiration for Peter Benchley's novel Jaws.^[18]

The bull shark is responsible for attacks around the Sydney Harbour inlets.^[19] Most of these attacks were previously thought to be great whites. In India bull sharks swim up the Ganges River and have attacked people. It also eats human corpses that the local population float on the river. Many of these attacks have been wrongly blamed on the Ganges shark, Glyphis gangeticus, a fairly rare species that is probably the only other shark in India that can live comfortably in freshwater. The grey nurse shark was also blamed during the sixties and seventies.

Reproduction

Bull sharks mate during late summer and early autumn,^[20] often in the brackish water of river mouths. After gestating for 12 months, a bull shark may give birth to 4–10 live young.^[20] They are viviparous. The young are about 70 cm (27.6 in) at birth and take 10 years to reach maturity.

• A recent (September 2010) TV documentary claimed that cannibalism within the womb may occur, and that only one baby shark may survive a pregnancy.

Ecology

Bull sharks are apex predators, and rarely have to fear being attacked by other animals. Humans are their biggest threat. Larger sharks, such as the tiger shark and great white shark, may attack them.^[3] Saltwater crocodiles have been well-documented as regularly preying on bull sharks in the rivers and estuaries of Northern Australia.^[21] It is likely that other large crocodilians, such as the Nile crocodile and the American crocodile (both of whom share virtually all of their range with the bull shark) exhibit similar predatory behavior.

See also

• Outline of sharks
• List of sharks
• List of fatal, unprovoked shark attacks in the United States by decade

Notes and references

General references

External links

• Bull shark at the Open Directory Project