Bull Shark
The
bull shark,
Carcharhinus leucas, also known as the
bull whaler,
Zambezi shark or unofficially known as
Zambi in Africa and
Nicaragua shark in Nicaragua, is a shark common worldwide in warm, shallow waters along coasts and in rivers.
The bull shark is well known for its unpredictable, often aggressive
behavior. Many scientists agree that since bull sharks often dwell in
shallow waters, they may be more dangerous to humans than any other
species of sharks, and that they join tiger sharks and great white
sharks as the three most likely sharks to attack humans.
Unlike most other marine sharks, bull sharks tolerate fresh water.
They can travel far up rivers. As a result, they are probably
responsible for the majority of shark attacks on humans that take place
near the shore, including many attacks attributed to other species.
However, bull sharks are not true freshwater sharks (unlike the river
sharks of the genus Glyphis).
Taxonomy
The name, “bull shark”, comes from the shark’s stocky shape, broad,
flat snout and aggressive unpredictable behavior. In India, the bull
shark is often called the Sundarbans or Ganges shark. In Africa it is
also commonly called the Zambezi River shark or just Zambi. Its wide
range and diverse habitats result in many other local names, for example
Lake Nicaragua shark, Fitzroy Creek whaler, Van Rooyen’s shark, cub
shark, shovelnose shark, freshwater whaler.
Distribution and habitat
The bull shark is found all over the world in many different areas
and has been known to travel long distances. The bull shark is common in
the coastal areas of warm oceans, in rivers and lakes, and occasionally
streams if they are deep enough in both salt and fresh water. It is
found to a depth of 150 m, but does not usually swim deeper than 30 m.
In the Atlantic it is found from Massachusetts to southern Brazil, and
from Morocco to Angola. In the Indian Ocean it is found from South
Africa to Kenya, India, and Vietnam to Australia. It is estimated that
there are more than 500 bull sharks in the Brisbane River and greater
numbers still in the canals of the Gold Coast in Queensland, Australia.
In the Pacific Ocean, it can be found from Baja California to Ecuador.
The shark has been reported 4,000 km (2,220 mi) up the Amazon River
at Iquitos in Peru, and has been found as far up the Mississippi River
as Illinois and Missouri. It is also found in the fresh water Lake
Nicaragua, and in the Ganges and Brahmaputra rivers of West Bengal and
Assam in eastern India and adjoining Bangladesh. It can live in almost
any water including water with a high salt content as in St. Lucia
Estuary in South Africa. After Hurricane Katrina hit in 2005, a large
number of bull sharks were sighted in Lake Ponchartrain. Bull sharks
have occasionally been seen in Mississippi River as far North as St.
Louis. Even more rare, due to cooler waters, bull sharks have made their
way up the Illinois River and into Lake Michigan such as an encounter
off the coast of Chicago, Illinois.
Freshwater tolerance
Only 43 species of elasmobranch in ten genera and four families have
been reported to enter fresh water, of which the bull shark is the best
known. Other species that enter rivers include the stingrays
(Dasyatidae, Potamotygonidae and others) and sawfishes (Pristidae). Some
skates (Rajidae), smooth dogfishes (Triakidae), and sandbar sharks
(Carcharhinus plumbeus) regularly enter estuaries. The ability of
elasmobranchs to enter fresh water is limited because their blood is
normally at least as salty (in terms of osmotic strength) as seawater,
through the accumulation of urea and trimethylamine oxide, but bull
sharks living in fresh water reduce the concentration of these solutes
by up to 50%. Even so, bull sharks living in fresh water need to produce
twenty times more urine than those in salt water.
Until the 1970s, researchers thought the sharks in Lake Nicaragua
were a separate species because there was no way for the sharks to move
in or out. It was discovered that they were jumping along the rapids
just like salmon. Bull sharks tagged inside the lake were later caught
in the open ocean.
Anatomy and appearance
Bull sharks are large and stout. Males can reach 2.12 m (7 ft) and
weigh 90.91 kg (200 lb). Females can be much larger:up to 3.49 m (11.5
ft) and 318 kg (700 lb). Bull sharks are wider than other sharks of
comparable length, and are grey on top and white below. The second
dorsal fin is smaller than the first.
Diet
Since bull sharks are carnivores, their diet includes fish, other
sharks, rays, dolphins, turtles, birds, molluscs, echinoderms, and
crustaceans. Bull sharks have been known to use the bump-and-bite
technique when attacking their prey. This type of hunting behaviour has
been observed when researchers entered the water with relatively calm
bull sharks, and the sharks suddenly became violent and began to bump
the researchers. This behaviour was seen in the documentary Anatomy of a
Sharkbite, which aired on the Discovery Channel in 2003, during Shark
Week. Dr. Erich Ritter was severely wounded by a bull shark using this
attack technique. This attack was not listed as being a case of mistaken
identity, because the waters during the time of the attack were clear,
and no noticeable weather patterns were affecting the sharks. This
attack may have been a case of territoriality, in which the bull sharks
were very fierce toward intruders. Recently, Dr. Ritter concluded that
the attack was provoked by a piece of chum that had been thrown away
from him, but was taken by a remora and brought back in his direction.
The remora caused the bull sharks to get excited and swirl up the sand.
In the resulting cloud of sand, one of the sharks bit him.
Behaviour
Bull sharks are solitary hunters. They often cruise through shallow
waters. They can suddenly burst into speed and can be highly aggressive,
even attacking a racehorse in the Brisbane River in the Australian
state of Queensland. They are extremely territorial and will attack
other animals – including humans – that enter their territory. Along
with the great white, tiger and oceanic whitetip sharks, bull sharks are
among the four species considered the most dangerous to humans, and is
probably the most dangerous of the four species. One or more bull sharks
may have been responsible for the Jersey Shore shark attacks of 1916,
and which inspired the movie Jaws.
Many experts think the bull shark is responsible for most of the
deaths around the Sydney Harbour inlets in the past. Most of these
attacks were previously thought to be great whites. In India the bull
shark cruises up the Ganges River where it has killed and attacked a
large number of people. It also eats the corpses that the local
population floats on the river. Many of these attacks have been wrongly
blamed on the Ganges shark, Glyphis gangeticus, a fairly rare species
that is probably the only other shark that can live comfortably in both
saltwater and freshwater. The grey nurse shark was also blamed in the
sixties and seventies.
Reproduction
Bull sharks breed in the summer, often in the brackish water of river
mouths. After gestating for about a year, a bull shark may give birth
to as many as 13 live young (they are viviparous). The young are about
70 cms (28 in) at birth and take 10 years to reach maturity.
Threats
Bull sharks are apex predators, and rarely have to fear being
attacked by other animals. Humans are their biggest threat. Larger
sharks, such as the tiger shark and great white, may attack them.
Crocodiles, such as the saltwater crocodile may also eat them if they
enter their territories.
