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Bull shark

The bull shark, Carcharhinus leucas, also known as the bull whaler, 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. Many scientists agree that since bull sharks often dwell in shallow waters, they may be more dangerous to humans than any other species of sharks, and that they join tiger sharks and great white sharks as the three most likely sharks to attack humans.

Unlike most other marine sharks, bull sharks tolerate fresh water. They can travel far up rivers. As a result, they are probably responsible for the majority of shark attacks on humans that take place near the shore, including many attacks attributed to other species. However, bull sharks are not true freshwater sharks (unlike the river sharks of the genus Glyphis).

Taxonomy

The name, "bull shark", comes from the shark's stocky shape, broad, flat snout and aggressive unpredictable behavior. In India, the bull shark is often called 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, for example Lake Nicaragua shark, Fitzroy Creek whaler, Van Rooyen's shark, cub shark, shovelnose shark, freshwater whaler.

Distribution and habitat

The bull shark is found all over the world in many different areas and has been known to travel long distances. The bull shark is common in the coastal areas of warm oceans, in rivers and lakes, and occasionally streams if they are deep enough in both salt and fresh water. It is found to a depth of 150 m, but does not usually swim deeper than 30 m. 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. It is estimated that 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. In the Pacific Ocean, it can be found from Baja California to Ecuador.

The shark has been reported 4,000 km (2,220 mi) up the Amazon River at Iquitos in Peru, and has been found as far up the Mississippi River as Illinois and Missouri. It is also found in the 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 almost any water including water with a high salt content as in St. Lucia Estuary in South Africa. After Hurricane Katrina hit in 2005, a large number of bull sharks were sighted in Lake Ponchartrain..Bull sharks have occasionally been seen in Mississippi River as far North as St. Louis. Even more rare, due to cooler waters, bull sharks have made their way up the Illinois River and into Lake Michigan such as an encounter off the coast of Chicago, Illinois.

Freshwater tolerance

Only 43 species of elasmobranch in ten genera and four families have been reported to enter fresh water, of which the bull shark is the best known. Other species that enter rivers include the stingrays (Dasyatidae, Potamotygonidae and others) and sawfishes (Pristidae). Some skates (Rajidae), smooth dogfishes (Triakidae), and sandbar sharks (Carcharhinus plumbeus) regularly enter estuaries. The ability of elasmobranchs 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 more urine than those in salt water.

Until the 1970s, researchers thought the sharks in Lake Nicaragua were a separate species because there was no way for the sharks to move in or out. It was discovered that they were jumping along the rapids just like salmon. Bull sharks tagged inside the lake were later caught in the open ocean.

Anatomy and appearance

Bull sharks are large and stout. Males can reach 2.12 m (7 ft) and weigh 90.91 kg (200 lb). Females can be much larger:up to 3.49 m (11.5 ft) and 318 kg (700 lb). Bull sharks are wider than other sharks of comparable length, and are grey on top and white below. The second dorsal fin is smaller than the first.

Diet

Since bull sharks are carnivores, their diet includes fish, other sharks, rays, dolphins, turtles, birds, molluscs, echinoderms, and crustaceans. Bull sharks have been known to use the bump-and-bite technique when attacking their prey. This type of hunting behaviour has been observed when researchers entered the water with relatively calm bull sharks, only to have the sharks suddenly become violent and begin to bump the researchers. This behaviour was seen in the documentary Anatomy of a Sharkbite, which aired on the Discovery Channel in 2003, during Shark Week

Dr. Erich Ritter was severely wounded by a bull shark using this attack technique. This attack was not listed as being a case of mistaken identity, because the waters during the time of the attack were clear, and no noticeable weather patterns were affecting the sharks. This attack may have been a case of territoriality, in which the bull sharks were very fierce toward intruders. Recently, Dr. Ritter concluded that the attack was provoked by a piece of chum that had been thrown away from him, but was taken by a remora and brought back in his direction. The remora caused the bull sharks to get excited and swirl up the sand. In the resulting cloud of sand, one of the sharks bit him.

Behavior

Bull sharks are solitary hunters. They often cruise through shallow waters. They can suddenly burst into speed and can be highly aggressive, even attacking a racehorse in the Brisbane River in the Australian state of Queensland. They are extremely territorial and will attack other animals – including humans – that enter their territory. Along with the great white, tiger and oceanic whitetip sharks, bull sharks are among the four species considered the most dangerous to humans, and is probably the most dangerous of the four species. One or more bull sharks may have been responsible for the Jersey Shore shark attacks of 1916, which inspired the movie Jaws..

Many experts think the bull shark is responsible for most of the deaths around the Sydney Harbour inlets in the past. Most of these attacks were previously thought to be great whites. In India the bull shark cruises up the Ganges River where it has killed and attacked a large number of people. It also eats the corpses that the local population floats 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 that can live comfortably in both saltwater and freshwater. The grey nurse shark was also blamed in the sixties and seventies.

Reproduction

Bull sharks breed in the summer, often in the brackish water of river mouths. After gestating for about a year, a bull shark may give birth to as many as 13 live young (they are viviparous). The young are about 70 cms (28 in) at birth and take 10 years to reach maturity.

Threats

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. Crocodiles, such as the saltwater crocodile may also eat them if they enter their territories.