Bull shark

Identification

Bull sharks are recognisable by their large size and girth, combined with a blunt, rounded snout and eyes that are proportionately smaller than other sharks. They have a large first dorsal fin, and grey colouration that fades to white on their underside. They have broad, triangular, heavily serrated teeth.

Bull sharks are a rarity among sharks in that they can tolerate great variations in water salinity, ranging from marine to brackish to fresh. Migrating back and forth among these waters, the gills, rectal glands, blood, hormone system and kidneys of these sharks are involved in a complex process called osmoregulation. They cope with freshwater by lowering the urea content in their tissues and removing excess water from their bodies as very dilute urine.

Special behaviour

Bull sharks are unusual amongst elasmobranchs as they readily move between marine and freshwater habitats. This is possible through a complex process known as osmoregulation, and involves specialised adaptations in their physiology. They cope with entering freshwater habitats by lowering the urea content in their tissues and disposing of excess water through higher urine output. It is thought that this ability gives them access to refuge habitats such as estuaries and rivers that are suitable for pupping.

Reproduction

Bull sharks are viviparous, meaning they nourish pups internally via a placenta before giving birth to independent offspring. They tend to have their pups in coastal, brackish habitats such as estuaries, and even up rivers. It is presumed this provides younger bull sharks greater protection from larger predators that cannot enter freshwater (although a different suite of predators, including crocodiles, pose alternative risks). Up to 13 pups are born in a single litter at no more than 0.8 m in size.

Habitat and geographical range

Bull sharks are found worldwide in tropical and sup-tropical seas. They primarily occupy turbid coastal waters and can even enter freshwater rivers and lakes, in some instances travelling more than 1,000 km inland. They will also undertake large-scale seasonal migrations (>2,000 km) both along coastlines and across open ocean. They are rarely found deeper than 300 m.

Diet

Globally, bull sharks have a varied diet that includes bony fish, other elasmobranchs, invertebrates and whale carcasses. However, there is evidence to suggest that although the population diet may be broad, individual bull sharks may specialise in particular prey. Diet will also vary as they mature, with younger bull sharks focusing more on fish, and larger bull sharks feeding more on sharks and rays.

Threats

Bull sharks are intentionally caught for their large fins, meat, skin and liver oil. They are also caught unintentionally as fisheries bycatch. Additionally, their coastal presence makes them susceptible to habitat loss and degradation when estuaries and mangrove habitats are developed or converted to aquaculture. Given that bull sharks have also been responsible for a number of fatal bites, several locations worldwide have implemented beach protection programmes that regularly catch them with nets and drum-lines. Despite these threats, bull sharks are considered Near Threatened by the IUCN as there is still insufficient data to identify clear population trends.

Relationship with humans

Bull sharks have a complicated relationship with people. Not only are they exploited through commercial and sports fisheries, but they are also responsible for a significant number of recorded shark bites. Their use of murky coastal habitats, and rivers, means they are more likely to come into contact with people than many other shark species and are often feared as a result. However, there is also a shift in perception as their large, coastal nature also makes them an attractive option for ecotour operators. There are now numerous locations in the world where people can dive and snorkel with them, including Fiji, The Bahamas, South Africa and Mexico, and this provides an alternative, more sustainable livelihood than fisheries.