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 Bull
sharks, Carcharhinus leucas, occur in tropical to subtropical coastal
waters worldwide as well as in numerous river systems and some freshwater
lakes. They have been reported 3700 km (2220 mi) up the Amazon River in
Peru, and over 3000 km (1800 mi) up the Mississippi River in Illinois. A
population in Lake Nicaragua (Central America) was once thought to be
landlocked, but they gain access to the ocean through rivers and estuaries.
In the western Atlantic bull sharks migrate north along the coast of the
U.S. during summer, swimming as far north as Massachusetts, and then return
to tropical climates when the coastal waters cool.
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, harbors, 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 hypersaline water as high as 53 parts per thousand.
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.
 Bony
fishes and small sharks make up the vast majority of the bull shark's diet.
In the western Atlantic they commonly feed on mullet, tarpon, catfishes,
menhaden, gar, snook, jacks, mackerel, snappers, and other schooling fish.
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. Bull sharks often
appear sluggish as they slowly cruise along the bottom, but are quite quick
and effective at capturing smaller, agile prey, and are capable of burst
speeds of over 11 mph (19 km/h).
Male bull
sharks reach maturity at around 6.3 feet (192 cm), females at 6.7 feet (205
cm), at an age of 5-6 years. One to 13 pups are nourished internally by the
mother for 10-11 months, and they then give birth to live, free-swimming
young. In the Gulf of Mexico mating occurs during summer months, and the
pups are born the following April-June. 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.
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.
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