Bull Shark

IUCN

###IUCN Status

Near Threatened
###General

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.

###Habitat

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.
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