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, and, along with tiger sharks and great white sharks, are among the three shark species most likely to attack humans.
Unlike most sharks, bull sharks tolerate fresh water and can travel far up rivers. As a result, they are probably responsible for the majority of near-shore shark attacks, including many attacks attributed to other species. However, bull sharks are not true freshwater sharks (unlike the river sharks of the genus Glyphis).
The name, "bull shark", comes from the shark's stocky shape, broad, flat snout and aggressive unpredictable behavior. 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, for example "Lake Nicaragua shark", "Fitzroy Creek whaler", "Van Rooyen's shark", "cub shark", "shovelnose shark", and "freshwater whaler".
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). 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. 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. 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. Bull sharks have occasionally gone up the Mississippi River as far north as St. Louis.
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.
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. They can jump along the rapids of the San Juan River (which connects Lake Nicaragua and the Caribbean Sea), almost like salmon. 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.
Anatomy and appearance
Bull sharks are large and stout. Males can reach 2.1 m (6 ft 11 in) and weigh 91 kg (200 lb). Females can be much larger: up to 4 m (13 ft 1 in) and 318 kg (700 lb). 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.
Bull sharks eat fish, other sharks, dolphins, rays, turtles, seabirds, molluscs, echinoderms, crustaceans, and virtually any other animal. 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.
Erich Ritter was severely wounded by such a bull shark. This attack does not appear to be a case of mistaken identity, because the water was clear, and no triggering weather was present. This attack may have been a case of territoriality, in which bull sharks attack intruders. Ritter concluded that the attack was provoked by chum originally thrown far from him, and then brought back in his direction by a remora. The remora excited the bull sharks, disturbing the sand. In the resulting sand cloud, one of the sharks bit him.
Bull sharks are typically solitary hunters, 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. 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. 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.
The bull shark is responsible for attacks around the Sydney Harbour inlets. 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 that can live comfortably in both saltwater and freshwater. The grey nurse shark was also blamed during the sixties and seventies.
Bull sharks mate during late summer and early autumn, often in the brackish water of river mouths. After gestating for 12 months, a bull shark may give birth to 4-10 live young. They are viviparous. The young are about 70 cm (27.6 in) at birth and take 10 years to reach maturity.
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. Saltwater crocodiles have been well-documented as regularly preying on bull sharks in the rivers and estuaries of Northern Australia. 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.
- Outline of sharks
- List of sharks
- List of fatal, unprovoked shark attacks in the United States by decade
Notes and references
- ^ Crist, R. 2002. "Carcharhinus leucas" (On-line), Animal Diversity Web. Accessed March 12, 2007 at animaldiversity.ummz.umich.edu
- ^ a b c d "Bull shark". National Geographic. http://animals.nationalgeographic.com/animals/fish/bull-shark.html.
- ^ a b "Bull shark". Florida Museum of Natural History. http://www.flmnh.ufl.edu/fish/Gallery/Descript/bullshark/bullshark.htm. Retrieved 2006-09-08.
- ^ Allen, Thomas B. (1999). The Shark Almanac. New York: The Lyons Press. ISBN 1-55821-582-4.
- ^ a b "Biology of Sharks and Rays". http://www.elasmo-research.org/education/ecology/fresh-bull.htm.
- ^ a b c "Carcharhinus leucas". University of Michigan Museum of Zoology, Animal Diversity Web. http://animaldiversity.ummz.umich.edu/site/accounts/information/Carcharhinus_leucas.html. Retrieved 2006-09-08.
- ^ Berrett, Nick (2008-11-14). "Canal shark shock". Redcliffe & Bayside Herald. Quest Community Newspapers. http://redcliffe-and-bayside-herald.whereilive.com.au/news/story/canal-shark-shock/. Retrieved 2009-03-26.
- ^ Bull shark (Carcharhinus leucas)
- ^ High number of sharks reported in Lake Pontchartrain.
- ^ a b Fresh Waters: Unexpected Haunts. elasmo-research.org. Accessed 2008-04-06.
- ^ Crist, R. 2002. Carcharhinus leucas. Animal Diversity Web. Accessed 2008-04-06
- ^ Bull sharks take to Louisiana swamp, WAFB-TV, CBS, 2009
- ^ http://sascblog.blogspot.com/2009/03/sasc-zambezi-shark-expedition.html
- ^ Anatomy of a Sharkbite. [Television production]. Discovery Channel. 2003.
- ^ "Shark mauls horse in Brisbane River". Sydney Morning Herald. 2005-03-23. http://www.smh.com.au/news/National/Shark-mauls-horse-in-Brisbane-Rivers/2005/03/23/1111525216327.html.
- ^ Handwerk, Brian. "Great Whites May Be Taking the Rap for Bull Shark Attacks". National Geographic News. http://news.nationalgeographic.com/news/2002/08/0802_020802_shark.html. Retrieved 2007-02-01.
- ^ Quinn, Ben (15 March 2009). "Shark attacks bring panic to Sydney's shore". The Guardian (London). http://www.guardian.co.uk/world/2009/mar/15/australia-sydney-shark-attacks-race. Retrieved November 2009.
- ^ a b McAuley, R. B.; C. A. Simpfendorfer, G. A. Hyndes, R. C. J. Lenanton (30 January 2007). "Distribution and reproductive biology of the sandbar shark, Carcharhinus plumbeus (Nardo), in Western Australian waters". Mar. Freshwater Res. 58 (1): 116–126. doi:10.1071/MF05234. http://www.publish.csiro.au/view/journals/dsp_journal_fulltext.cfm?nid=126&f=MF05234. Retrieved 2 December 2009. "The proportion of mature males with running spermatozoa increased from 7.1% in October to 79 and 80% in January and March, respectively, suggesting that mating activity peaks during late summer and early autumn.".
- ^ "No Bull: Saltwater Crocodile Eats Shark". UnderwaterTimes.com. 2007-08-13. http://www.underwatertimes.com/news.php?article_id=84173256109. Retrieved 2008-06-15.
- Simpfendorfer & Burgess (2000). Carcharhinus leucas. 2006. IUCN Red List of Threatened Species. IUCN 2006. www.iucnredlist.org. Retrieved on 11 May 2006. Database entry includes justification for why this species is near threatened
- Carcharhinus leucas (TSN 160275). Integrated Taxonomic Information System. Retrieved on 23 January 2006.
- Froese, Rainer, and Daniel Pauly, eds. (2005). "Carcharhinus leucas" in FishBase. 09 2005 version.
- Bull shark, Carcharhinus leucas MarineBio"
- Sunday Herald Sun, Sunday, April 23, 2005