|Scientific Name:||Carcharhinus leucas|
|Species Authority:||(Müller & Henle, 1839)|
Carcharias leucas Müller & Henle, 1839
|Red List Category & Criteria:||Near Threatened ver 3.1|
|Assessor/s:||Simpfendorfer, C. & Burgess, G.H.|
|Reviewer/s:||Musick, J.A. & Fowler, S.L. (Shark Red List Authority)|
This assessment is based on the information published in the 2005 sharks status survey (Fowler et al. 2005).
The Bull Shark (Carcharhinus leucas) is a common tropical and subtropical species that occurs in marine, estuarine and freshwater. It is the only species of shark that can exist for long periods in freshwater and penetrates long distances up large rivers. It is caught in fisheries throughout its range, but it is rarely a target species. Its occurrence in estuarine and freshwater areas makes it more vulnerable to human impacts and habitat modification.
|Range Description:||The Bull Shark has a worldwide distribution in tropical and warm temperate areas, with seasonal appearances in cool, temperate waters (Garrick 1982, Compagno 1984, Last and Stevens 1994). This has resulted in multiple descriptions and numerous common names for the species (including Zambezi Shark, Swan River Shark and Lake Nicaragua Shark) from throughout its range (see Compagno (1984) for a full list of synonyms).
Primarily the Bull Shark is an inhabitant of continental shelf waters to a depth of about 150 m (but mostly less than 30 m), but it commonly moves into estuarine and fresh waters. It has been documented as travelling large distances up rivers (Thorson 1972), including the Amazon, Gambia, Ganges, Mississippi, San Juan (and Lake Nicaragua), Tigris and Zambezi. It also has been observed to tolerate hypersaline conditions up to 53 parts per thousand (ppt) (sea water is approximately 35 ppt). Although mostly a continental species, there are insular records from the Philippines and the South Pacific islands of New Caledonia, Fiji and Rangiroa (Compagno et al. 1989).
Native:Australia (Queensland); Bangladesh; Bolivia, Plurinational States of; Botswana; Brazil; Colombia; Costa Rica; Ecuador; Fiji; Gambia; Guinea; Guyana; India; Iran, Islamic Republic of; Iraq; Malawi; Mexico; Mozambique; Nepal; New Caledonia; Nicaragua; Peru; Philippines; Senegal; South Africa; Tanzania, United Republic of; United States (Arkansas, Florida, Illinois, Iowa, Kentucky, Louisiana, Minnesota, Mississippi, Missouri, Tennessee, Wisconsin); Venezuela, Bolivarian Republic of; Zimbabwe
|FAO Marine Fishing Areas:||
Atlantic – eastern central; Atlantic – northwest; Atlantic – southwest; Atlantic – western central; Indian Ocean – eastern; Indian Ocean – western; Pacific – eastern central; Pacific – southeast; Pacific – southwest; Pacific – western central
|Range Map:||Click here to open the map viewer and explore range.|
|Habitat and Ecology:||
The Bull Shark is one of the few shark species that can tolerate long periods of time in freshwater, often penetrating long distances up freshwater rivers that connect to the ocean. For example, Thorson (1971) reported the movement of Bull Sharks from the ocean through the Rio San Juan system and into Lake Nicaragua, a large freshwater lake. C. leucas move into estuarine or fresh waters to give birth to their young (Springer 1963), while neonates and young juveniles appear to use these habitats as nursery areas (Snelson et al. 1984).
Bull Sharks grow up to about 340 cm in total length. The young are born at between 56?81 cm. Males mature at 157?226 cm and females at 180?230 cm (Compagno 1984b).
The diet of this species is relatively diverse, including turtles, birds, dolphins, terrestrial mammals, crustaceans, echinoderms, teleost fishes and elasmobranchs (Last and Stevens 1994). However, the most diverse diet is restricted to larger individuals that are capable of consuming larger prey. The most commonly eaten prey items are teleost fishes and elasmobranchs. In the coastal lagoons of Florida, Snelson and Williams (1981) recorded a wide array of species in the diet including jacks, snook, tarpon, mullets, catfish, croaker, stingrays and sandbar sharks, but noted that saltwater catfish and stingrays (Dasyatis spp.) were most commonly eaten.
Reproduction is by placental viviparity. Litter sizes range from 1?13 (Compagno 1984b), with most between 6?8 (Pattillo et al. 1997). The gestation period is 10?11 months, with birth normally occurring in late spring and summer (Clark and von Schmidt 1965, Bass et al. 1973, Branstetter 1981). In warmer areas (e.g., Nicaragua) breeding (and hence parturition) may occur year?round (Castro 1983). The length of the reproductive cycle has not been published, but is probably biennial (Compagno in prep. b).
Thorson and Lacy (1982) and Branstetter and Stiles (1987) have provided age and growth data for this species. Thorson and Lacy (1982) used tag recapture information to estimate the growth rates of Bull Sharks in Lake Nicaragua. They estimated that the growth rates in the first two years of life were 18 and 16 cm per year, respectively. Growth subsequently slowed to 11?12 cm per year and finally to 9?10 cm per year. They estimated that females would live up to 16 years and males to 12 years.
Branstetter and Stiles (1987) used vertebral ageing techniques to estimate growth parameters for animals from the northern Gulf of Mexico. They estimated the von Bertalanffy parameters to be L?=285 cm, k=0.076 year-1 and t = -3.0 years. The oldest estimated male was 21.3 years and the oldest female 24.2 years. Their estimates of growth for early years were similar to those of Thorson and Lacy (1982), but growth in later years was thought to be much slower (4?5 cm per year), accounting for the larger maximum ages. Ages at maturity based on Branstetter and Stiles (1987) data are 14?15 years for males and 18+ years for females.
A number of migratory habits have been documented for this shark. Pregnant females migrate to estuarine areas to give birth. The juveniles remain in these areas until temperatures drop below optimum levels and then migrate to warmer offshore waters. A general migration along the United States east coast is also observed, with movement northwards during the summer as water temperatures rise and southwards again as temperatures cool in the north (Castro 1983).
Smith et al. (1998) have reported the results of demographic analysis for C. leucas. The technique that these authors used estimated the rebound potential (r2M, similar to the intrinsic rate of increase) from litter size, age at maturity, maximum age and natural mortality. They estimated natural mortality to be 0.166 year-1 based on a maximum age of 27 years. The estimated the rebound potential was 0.027?0.039 year-1.
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.
This shark is also exploited by large aquariums. The species is good for public display, adapting well to life in a tank and providing a good example of a larger, aggressive shark. With the number of public aquaria rising worldwide there is an increasing demand for this and other species of sharks for display. While populations are healthy the needs of aquaria can probably be met without affecting the wild population. However, if a wild subpopulation is depleted this may not be the case and aquaria need to have responsible collection policies that will not result in further pressure being placed on a species. At present there is no evidence that collecting for aquariums has any impact on the wild population of Bull Shark.
|Conservation Actions:||No specific management or conservation programmes are known for this species. It is managed in the U.S. east coast shark fisheries as part of the ?large coastal? groups of species. The current quota for this group is 1,285 t/year (1997), but Bull Sharks make up only small percentage of this group (see above). Recent closures of coastal waters in several states in the southern US to gillnetting have removed pressure on the juveniles in estuarine and coastal nursery areas.|
|Citation:||Simpfendorfer, C. & Burgess, G.H. 2009. Carcharhinus leucas. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <www.iucnredlist.org>. Downloaded on 13 December 2013.|
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