| Bull Sharks: More information about this shark species.
Diving with Bull Sharks
Diving with Bull Sharks in South Africa and Mozambique is quite safe - if you respect the sharks and stick to the rules. No scuba diver has ever been attacked by a Bull Shark while diving, and we intend to keep it that way.
We expect to start seeing Bull Sharks from September, with the most sightings from November to May. Sightings in other months are rare. The animals are not fed, baited or conditioned at all - the Bull Sharks are completely wild.
The Bull Sharks are curious of nature - therefore we often see them as soon as we hit the water because they have been waiting to find out if it is a fishing boat or not (clever buggers). The splash of divers doing backward rolls into the water causes the Bull Sharks to scatter at first.
They often come very close to inspect the divers on both the descent and ascent, but easily lose interest. The most enjoyable sightings however are while the divers are busy handing their kit up on the semi-rigid boat - the activity on the surface causes the Bull Sharks to investigate and gives us the chance to snorkel while they circle below us.
Classification
Kingdom: Animalia (animals)
Phylum: Chordata
SubPhylum: Vertebrata (vertebrates)
Class: Chondrichthyes (cartilaginous fish)
Subclass: Elasmobranchii (sharks and rays)
Order: Carcharhiniformes
Family: Carcharhinidae
Genus: Carcharhinus
Species: leucas
Taxonomy
The bull shark gets its name from its stout appearance and pugnacious reputation - it was first described by Valenciennes in Muller & Henle (1839) as Carcharias (Prionodon) leucas, and later changed to the currently valid name Carcharhinus leucas. The genus name Carcharhinus is derived from the Greek "karcharos" = sharpen and "rhinos" = nose. It has also appeared in the literature as Carcharias (Prionodon) zambezensis, Carcharhinus zambezensis, Prionodon platyodon, Squalus platyodon, Squalus obtusus, Eulamia nicaraguensis, Carcharias azureus, Carcharias spenceri, Galeolamna (Bogimba) bogimba, Galeolamna greyi mckaili, and Carcharhinus vanrooyeni. The French know the shark as requin bouledogue, and the Spanish as tiburon sarda. It is known by many different common names throughout its range including Zambezi shark, Van Rooyen's shark (Africa); Ganges shark (India); Nicaragua shark (Central America); freshwater whaler, estuary whaler, and Swan River whaler (Australia); shovelnose shark, square-nose shark, river
shark, slipway grey shark, ground shark and cub shark.
Distinctive Features
The Bull Shark can be recognised by a combination of characteristics including a stout body, short blunt snout, triangular serrated teeth in the upper jaw and no fin markings as an adult. This species has a second dorsal
fin about one third the height of the first, a small eye, and no skin ridge between the two dorsal fins. Bull sharks have relatively small eyes as compared to other carcharhinid sharks, which suggests that vision may not be
as important a hunting tool for this species which often occurs in turbid waters.
Coloration
Bull sharks are pale to dark grey above, fading to white on their underside. In younger individuals the fins have black tips which fade to a dusky colour as they grow.
Dentition
Upper jaw teeth of the bull shark are broad, triangular, and heavily serrated. Lower jaw teeth have a broad base, and are narrow and triangular with fine serrations. Anterior teeth are erect and nearly symmetrical, while posterior teeth become more oblique in shape. The teeth are located in rows which rotate into use as needed. The first two rows are used in obtaining prey, the other rows rotate into place as they are needed. As teeth are lost, broken, or worn down, they are replaced by new teeth that rotate into place.
Distribution
Bull sharks occur in tropical to subtropical coastal waters worldwide as well as in numerous river systems and some freshwater lakes. In Southern Africa Bull Sharks have killed cattle 200km up the Zambezi River and are commonly found in Lake St Lucia.
Habitat
The bull shark prefers to live in shallow coastal waters less than 100 feet deep (30 m), but ranges from 3-450 feet deep (1-150 m). It commonly enters estuaries, bays, harbours, lagoons, and river mouths. It is the only shark species that readily occurs in freshwater, and apparently can spend long periods of time in such environs. It is not likely that the bull shark's entire life cycle occurs within a freshwater system, however. There is evidence that they can breed in freshwater, but not as regularly as they do in estuarine and marine habitats. Juvenile bull sharks enter low salinity estuaries and lagoons as readily as adults do, and use these shallow areas as nursery grounds. They can also tolerate hyper saline water as high as 53 parts per thousand.
Diet
This is an aggressive species that is considered dangerous to humans. Some authors consider that the Bull Shark may be more dangerous than the Great White Shark and the Tiger Shark. This is because of the Bull Shark's omnivorous diet and habitat preferences. This species may be found in murky water, where the splashing of a swimmer could be mistaken for a struggling fish. Bull Sharks are solitary hunters. Bony fishes and small sharks make up the vast majority of the bull shark's diet. They also regularly consume stingrays and juvenile sharks including small individuals of their own species in their inshore nursery habitats. Other food items occasionally reported in bull sharks include sea turtles, dolphins, crabs, shrimp, sea birds, squid, and dogs.
Reproduction
Bull Sharks reproduce viviparous, which means that like mammals, they give birth to live young that have been nourished by a placenta. After a gestation period of about a year, a litter of 1-13 pups is born. These pups measure about 28 inches long at birth. Young baby bull sharks are found frequently in briny water, in protected bays near the mouths of rivers.
The maximum reported length of the bull shark is 11.5 feet (350 cm), weighing over 500 pounds (230 kg). Size at birth is around 29 inches (75 cm). Females grow larger than males, averaging 7.8 feet (240 cm) as adults, weighing around 285 pounds (130 kg). This is the result of a longer lifespan of about 16 years, compared to 12 years for males. Males average 7.3 feet (225 cm) and weigh 209 pounds (95 kg). Growth rates calculated from captive bull sharks were estimated to be about 11 inches (28 cm) per year in the first years of life, slowing to half that rate after about 4 years of age. Mating and birthing occurs year-round in the warmest parts of the bull shark's range. Females often bear mating scars on their dorsum. Coastal lagoons, river mouths, and other low-salinity estuaries are common nursery habitats.
Predators
Adult bull sharks are unlikely to have any natural predators. Young bull sharks, however, can fall prey to large tiger sharks, sandbar sharks, and other bull sharks. A crocodile in South Africa was also reported to have consumed a bull shark.
Parasites
Parasites of the bull shark include Pandarus sinuatus and Perissopus dentatus. These copepods parasitize the body surface of this shark.
Importance to Humans
Though the bull shark is not a targeted species in most commercial fisheries, it is regularly captured on bottom longline gear. In the south-eastern U.S. commercial shark fishery the bull shark comprises less than 1% of the catch. It is more often targeted in small fisheries because of its abundance in near shore environments. The meat is either used for fish meal or sold in local markets for human consumption. The fins are sold to Asia for shark fin soup. The hide is good quality for leather.
Recreationally, the bull shark is considered a popular game fish in the south-eastern U.S. and South Africa, and is often fished for by rod and reel from shore, piers, and bridges. According to the International Game Fish Association (IGFA), the largest bull shark caught on rod and reel weighed 771 lb. 9 oz. (347 kg) and was caught near Cairns, Australia.
Danger to Humans
According to the International Shark Attack File (ISAF) bull sharks are historically responsible for at least 69 unprovoked attacks on humans around the world, 17 of which resulted in fatality. In reality this species is likely responsible for many more, and has been considered by many experts to be the most dangerous shark in the world. It's large size, occurrence in freshwater bodies, and greater abundance in close proximity to numerous human populations in the tropics makes it more of a potential threat than either the white shark or tiger shark. Since the bull shark occurs in numerous Third World regions including Central America, Mexico, India, east and west Africa, the Middle East, Southeast Asia, and South Pacific Islands, attacks are often not reported. The bull shark is also not as easily identifiable as the white or tiger shark, so is likely responsible for a large percentage of attacks with unidentified culprits.
Conservation
Though the bull shark is not a targeted species, it is captured in fisheries around the world. Due to this shark's life history and environmental requirements, which bring it in close proximity to human populations, it could potentially be heavily impacted by human activities in these inshore regions. The inshore nursery grounds of this species could be particularly threatened. The Natal Sharks Board report that the average size of bull sharks caught in their beach nets have significantly declined in recent years, which does not bode well for South African populations of the species. The bull shark is not legally protected in any part of its range.
Further research is necessary on this unique species of shark so more can be understood of its biological, ecological, and fisheries significance.
The bull shark is currently listed by the World Conservation Union (IUCN) as "Near Threatened", but does not meet criteria to be considered endangered or vulnerable at this time. The IUCN is a global union of states, governmental agencies, and non-governmental organizations in a partnership that assesses the conservation status of species.
Sources
www.sharks.com
www.enchantedlearning.com
www.austmus.gov.au
www.flmnh.ufl.edu
www.boattalk.com
www.conservationinstitute.org
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