Bull Shark, Lake Nicaragua Shark, Zambezi Shark.
Stout body. Head wide and bluntly rounded with a very short snout and small eyes. Broad triangular first dorsal fin; origin level with pectoral fin axil. First and second dorsal fins have short free rear tips. No interdorsal ridge. Subtle caudal keel. Dorsal colouration grey or grey/brown, occasionally with irregular darker blotches. Fins unmarked or slightly dusky towards tips/posterior margins; more noticeable in juveniles.
Maximum length at least 324cm. Size at birth 56-81cm.
It is estimated that bull sharks may live for more than 50 years.
A warm water species preferring temperatures of 26ºC or warmer. Found on inshore reefs and bays. It is a euryhaline species that travels thousands of kilometres up large rivers and into lakes that access the ocean. Surface to at least 152m.
The bull shark has a cosmopolitan distribution in tropical and sub-tropical nearshore environments. It has been recorded hundreds or thousands of kms in the Amazon, Mississippi, and other large rivers of the tropics.
The frequent use of estuarine and freshwater areas by the Bull Shark makes it more susceptible to deleterious human impacts than species of sharks occurring in other coastal or offshore areas. Bull sharks more frequently encounter humans while in waters of low salinity, and are thereby subjected to increased fishing pressure and environmental changes associated with habitat modification.
Bull Sharks are commonly caught in both commercial and recreational fisheries. Thorson (1982a) reported that a commercial fishery existed for C. leucas in Lake Nicaragua and the Rio San Juan river system in Central America. However, in most situations, Bull Sharks are not normally a fishery target species but are caught as bycatch or as part of a multi-species fishery. For example, in the US Atlantic region they are an important component of inshore ecosystems, but only comprise 1-6% of the large coastal shark catch for this area (Branstetter and Burgess 1997).
While the Bull Shark has been exploited commercially for its skin, liver oil and flesh, currently its fins are the major product driving demand for this and many other species. There are limited data on recreational catches of this species. The best data come from the Gulf of Mexico where Casey and Hoey (1985) reported that in 1978 C. leucas made up about 11% (by weight) of the recreational shark catch of around three million pounds (Casey and Hoey 1985). Recreational catches of large sharks in the Gulf of Mexico have decreased substantially since the 1970s, but Casey and Hoey’s results illustrate that recreational fishing may have a substantial impact on Bull Shark populations.
Beach protection programmes in KwaZulu-Natal, South Africa and Queensland, Australia also regularly catch Bull Sharks. Cliff and Dudley (1992) reported that between 1978 and 1990 the South African programme caught 59 Bull Sharks, 21% of which were released alive. Species identification problems occurred in the Queensland programme until the early 1990s, thus the importance of C. leucas can only be based on data from latter years. Gribble et al. (1998) reported that after identification was improved 16% of the sharks caught state-wide were Bull Sharks, with the majority caught in the central part of the state. The lack of historical data for the Queensland programme and the low abundance in the South African programme make it impossible to assess the impact of beach meshing on Bull Shark populations.
The location of nursery areas in estuarine and freshwater systems makes the species vulnerable to pollution and habitat modification, but there has been only limited study of these impacts on Bull Sharks. Canal developments have been prolific in some estuarine areas where the species is commonly found. It is not known whether these developments have negative impacts. In Florida, USA and the Gold Coast of Queensland, Australia, these developments have substantially altered the environment. Bull Sharks occur frequently in Gold Coast canals and the species has been responsible for a number of attacks on humans (Simpfendorfer unpubl.). The warm water effluent from power stations may also impact this shark. In Florida, USA, juveniles have been reported to be trapped in the warm water outfalls during winter when they would normally have migrated to warmer water areas (Snelson et al. 1984, C. Manire, Center for Shark Research, Mote Marine Laboratory pers. Comm.). The potential impacts of pollution and habitat modification need to be further investigated for this species.
Citations and References
Simpfendorfer, C. & Burgess, G.H. 2009. Carcharhinus leucas. The IUCN Red List of Threatened Species 2009: e.T39372A10187195. https://dx.doi.org/10.2305/IUCN.UK.2009-2.RLTS.T39372A10187195.en. Downloaded on 26 September 2020.
A viviparous species with yolk-sac placenta. 1-13 pups per litter, around 10 on average. Gestation is approximately 12 months. Mating occurs biannually.
Diet consists of a wide range of bony and cartilaginous fishes, and molluscs. Juveniles in Florida feed heavily on catfishes and stingrays. Larger animals are known to eat jacks, tarpon, mullet, and smaller sharks. Bull sharks are cannibalistic; the stomach of a 162cm specimen contained the head of an estimated 80-90cm bull shark.
Bull sharks mostly cruise close to substrate. They are known to travel over long distances, migrating into warmer water over the winter months. Bull sharks are well known for ability to tolerate fresh water. They have been found thousands of miles upriver but it is unclear if they are residents or nomadic in the world’s great tropical rivers.
Reaction to divers
Bull sharks seem to have quite sensitive hearing, making them difficult to approach in un-baited situations. CAUTION: In baited situations bull sharks quickly become very aggressive and potentially dangerous. They are responsible for numerous attacks on humans, some of which were fatal.
Bull sharks are seem by divers in numerous locations, both with and without the use of bait.
In southern Africa, there is a long-standing bull shark feed in the summer months at Protea Bank, south of Durban. Further north at Ponta Do Ouro, Mozambique, divers often encounter bull sharks during reef dives.
In Fiji’s Beqa Lagoon, there is a daily shark feed that attracts scores of large, aggressive bull sharks as well a s a few smaller species and the occasional big tiger shark.
On the Atlantic coast of Mexico, there is a well established bull shark feed at Playa del Carmen. The dive usually attracts around 2-12 sharks. The season runs from November until March at which point the sharks (most of which are gravid females) leave to deliver their young.
At Cabo Pulmo on the west side of the Sea of Cortez, Bull sharks are becoming increasingly more abundant. It is not uncommon to see five or six on a single dive.
In the Bahamas, bull sharks regularly show up at the great hammerhead feeds in Bimini from November until around April when the hammerheads begin their migration and the feeds slow down. Likely the bull sharks remain for much of the year. The operators make a point of not feeding any of the bull sharks during the hammerhead dive because they quickly become too aggressive. In fact, if the bulls start to show too much interest or approach too closely, the feeder will tap the metal bait crate with his feeding pole; an action that immediately scatters the bull sharks but has virtually no effect on the hammerheads.
At Big Game Club Marina on North Bimini Island, it is possible to enter a small shark cage that is tied to the dock. A feeder will then entice some of the resident bulls up to the bars. Although the cage itself is mostly for non-diving tourists, this is an excellent location to capture over/unders of bull sharks (see image gallery).
Bull sharks also show up at Tiger Beach; the name given to a group of dive sites on Little Bahama Bank north of Grand Bahama. The bulls do not remain there year round but we usually see a handful swimming around when we go each April: Tiger Beach Trip
Caribbean Reef Shark Although this species is more slender, divers consistently mistake large CRSs for bull sharks. It is distinguished by its longer snout and noticeably higher, shorter first dorsal fin.
Pigeye Shark Very similar looking species i.e. thick-bodied, with a short snout, small eyes, and a triangular first dorsal. Distinguished by the relative size of the dorsal fins. Pigeye first dorsal is greater than 3.5 x height of second dorsal. Bull first dorsal is less than 3.5 x height of second dorsal.