Interesting Creatures… The Bull Shark [Carcharhinus leucas]

Apr 15, 2018 Features / Columnists, Interesting Creatures in Guyana

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The bull shark (Carcharhinus leucas), also known as the Zambezi shark (informally “zambi”) in Africa, and Lake Nicaragua shark in Nicaragua, is a requiem shark commonly found worldwide in warm, shallow waters along coasts and in rivers. These sharks are known for their aggressive nature, predilection for warm shallow water, and presence in brackish and freshwater systems including estuaries and rivers.

They are known to thrive in both salt and fresh water and can travel far up rivers. However, few freshwater human-shark interactions have been recorded. Larger-sized bull sharks are probably responsible for the majority of near-shore shark attacks, including many bites attributed to other species.

Unlike the river sharks of the genus Glyphis, bull sharks are not true freshwater sharks despite their ability to survive in freshwater habitats.

The name “bull shark” comes from the shark’s stocky shape, broad, flat snout, and aggressive, unpredictable behaviour.

Some of the bull shark’s closest living relatives do not have the capabilities of osmoregulation.

They however share numerous similarities with river sharks of the genus Glyphis, and other species in the genus Carcharhinus, but its phylogeny has not been cleared yet.

This fish species is large and stout, with females being larger than males. They can be up to 81 cm (2.66 ft) in length at birth. Adult female bull sharks average 2.4 m (7.9 ft) long and typically weigh 130 kg (290 lb), whereas the slightly smaller adult male averages 2.25 m (7.4 ft) and 95 kg (209 lb). While a maximum size of 3.5 m (11 ft) is commonly reported, a single record exists of a female specimen of exactly 4.0 m (13.1 ft). The maximum recorded weight of a bull shark was 315 kg (694 lb), but may be larger.

Bull sharks are wider and heavier than other requiem sharks of comparable length, and are grey on top and white below. The second dorsal fin is smaller than the first. The bull shark’s caudal fin is longer and lower than that of the larger sharks, and it has a small snout, and lacks an interdorsal ridge.

Bull sharks have a bite force up to 5,914 newtons (1,330 lbf), weight for weight the highest among all investigated cartilaginous fishes.

The bull shark is commonly found worldwide in coastal areas of warm oceans, in rivers and lakes, and occasionally streams if they are deep enough. It is found to a depth of 150 m (490 ft), but does not usually swim deeper than 30 m (98 ft).

The bull shark’s diet consists mainly of bony fish and small sharks, including other bull sharks, but can also include turtles, birds, dolphins, terrestrial mammals, crustaceans, echinoderms, and stingrays. They hunt in murky waters where it is harder for the prey to see the shark coming. Bull sharks have been known to use the bump-and-bite technique to attack their prey. After the first initial contact, they continue to bite and tackle prey until they are unable to flee.

The bull shark is known to be a solitary hunter, although brief moments exist in which the bull shark teams up with another bull shark to make hunting and to tricking prey easier.

Sharks are known to be opportunistic feeders, and the bull shark is no exception to this, as it is part of the Carcharhinus family of sharks. Normally, sharks eat in short bursts, and when food is scarce, they digest for a much longer period of time in order to avoid starvation. As part of their survival mechanism, bull sharks will regurgitate the food in their stomachs in order to escape from a predator. This is a distraction tactic; if the predator moves to eat the regurgitated food the bull shark can use the opportunity to escape.

Mating is during late summer and early autumn and often in freshwater or in the brackish water of river mouths. After gestating for 12 months, a bull shark may give birth to one to 13 live young.

They are viviparous, born live and free-swimming. The young are about 70 cm (27.6 in) at birth.

The bull shark does not rear its young; the young bull sharks are born into flat, protected areas. Coastal lagoons, river mouths, and other low-salinity estuaries are common nursery habitats.

The male bull shark is able to begin reproducing around the age of 15 years while the female cannot begin reproducing until the age of 18 years.

The size of a fully matured female bull shark to produce viable eggs for fertilization seems to be 175 cm to 235 cm. The courting routine between bull sharks has not been observed in detail as of yet. The male likely bites the female on the tail until she can turn upside down and the male can copulate at that point. At some points, the harassment of the male can become violent. Seeing scratches and other marks on a mature female which may be from the mating ritual is not uncommon.

Bull sharks have an unusual migratory pattern in comparison to other sharks. They are found in rivers all over the world.

The young bull sharks are free from predators while they grow up in the river before they go out to the sea to find mates. The ability to be able to survive in both fresh and salt water also gives another benefit that has been driven by evolution. Because the majority of sharks are only able to survive in salt water, the bull shark has evolved to have their offspring in the fresh water where other sharks cannot enter. Therefore, the freshwater acts as a protective area since the young are able to grow and mature without the threat of larger sharks preying on them. This is an explanation for the behaviour that is observed from the Bull sharks as to why there would be any reason for the adult bull shark to ever travel into a freshwater area despite being able to tolerate the high salinity of marine water. [Source: Wikipedia]

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