Bull shark (Carcharhinus leucas)
|Also known as:||freshwater whaler, ground shark, river whaler, Swan River whaler, Zambezi shark|
|Size||Length: up to 340 cm (2)|
The bull shark is classified as Near Threatened (NT) by the IUCN Red List (1).
The bull shark (Carcharhinus leucas), named for its stocky body and ferocious reputation (3), is most famous for its remarkable ability to thrive in both saltwater and freshwater. It has a grey, robust body, a paler underside, and a blunt, rounded snout (2). The large, triangular first dorsal fin and moderately large second dorsal fin, as well as the other fins, have dusky tips but are not strikingly marked (4). Bull sharks have relatively small eyes, indicating that vision is not the most important sense required for hunting in its frequently turbid habitat (2).
Occurring along the continental coasts of all subtropical and tropical seas, the bull shark can often be found in freshwater rivers and lakes as well (2) (4).
The bull shark is most frequently found in coastal waters between 30 and 50 metres deep, but makes occasional deep dives to well over 100 metres; the maximum depth recorded is 204.4 metres for a bull shark from Fiji (4). It also commonly enters estuaries, bays and harbours, and penetrates far into freshwater, inhabiting rivers and lakes (2).
The biology of the bull shark is still little known but it shows extraordinary physiological adaptations that allow it to persist in both freshwater and saltwater. Bull sharks have been captured in places you would never imagine a shark to be found; in the foothills of the Peruvian Andes, 3,700 kilometres up the Amazon River; and in Lake Nicaragua, the largest lake in Central America. However, the bull shark may not be able to complete its entire life cycle in freshwater, and all sharks in freshwater require access to saltwater through rivers and estuaries (2) (3).
Its swims slowly and heavily, usually near the bottom, concealing its surprising agility and speed employed when attacking prey (2), and deceiving one into believing this may not be one of the most dangerous species of tropical shark, as it is frequently cited (2) (5) (6). Along with the great white and tiger shark the bull shark is responsible for the most accidents involving people (2); a result of its tendency to take large prey and the proximity of its habitat to the activities of humans (2). The bull shark’s broad and varied diet includes bony fishes, other shark species (even occasionally young bull sharks), sea turtles, birds, dolphins, and terrestrial mammals (2).
The bull shark is viviparous, giving birth to 1 to 13 young in each litter after a pregnancy of 10 to 11 months (2). The female gives birth in late spring and early summer in both hemispheres, in estuaries, river mouths, and very occasionally in freshwater lakes (2). Mating takes place at the same time of the year but it is unknown where exactly as it has never been directly observed (4).
Although the bull shark is rarely the target species of commercial fisheries, it is often caught as by-catch throughout its range (1), and its abundance in inshore habitats make it a target of artisanal fisheries (2) (3). When captured, the meat is consumed by humans or used in fishmeal, the hide is used for leather, the fins are used in shark-fin soup and the liver is utilised for its vitamin-rich oil (2). In certain areas, such as the Gulf of Mexico and South Africa, the bull shark is also a popular game fish (2). The bull shark’s inshore and freshwater habitat not only makes it an easier target of fisheries, but these are habitats that are particularly vulnerable to habitat modification and pollution caused by human activities (1).
Whilst the IUCN consider there to be insufficient information to determine the extent to which the bull shark may be threatened, its proximity to human populations, with their hunting and environmentally damaging activities, is likely to be greatly impacting populations (1). Further research is evidently required, to determine the conservation status of the bull shark, and subsequently enable appropriate conservation measures to be implemented to ensure the future of this unique shark.
For further information on sharks and their conservation:
For further information on the bull shark:
Bull Shark Biological Profile, Ichthyology Department, Florida Museum of Natural History:
Authenticated (26/03/08) by Dr Juerg Brunnschweiler.
- Artisanal fisheries: small-scale, traditional fisheries.
- Dorsal fin: the unpaired fin found on the back of the body of fish, or the raised structure on the back of most cetaceans.
- Viviparous: giving birth to live offspring that develop inside the mother’s body.
IUCN Red List (March, 2011)
- Compagno, L.J.V. (1984) FAO Species Catalogue. Vol 4: Sharks of the World. An annotated and illustrated catalogue of shark species known to date. Part 2: Carcharhiniformes. Food and Agriculture Organization of the United Nations, Rome.
Bull Shark Biological Profile, Ichthyology Department, Florida Museum of Natural History (September, 2007)
- Brunnschweiler, J. (2008) Pers. comm.
ReefQuest Centre for Shark Research (September, 2007)
- Carwardine, M. and Watterson, K. (2002) The Shark Watcher’s Handbook. BBC Worldwide Ltd, London.