It is well-known that sharks live in saltwater. Sure, there is such a strange thing as a river shark (like the Ganges shark) that seems to tolerate only fresh water (but, as has been proved, can migrate through saltwater, too). But all other sharks live solely in the ocean or in brackish nursing grounds to protect their young, right? Sadly this is wrong.
Because there is the Bull shark (Carcharhinus leucas). Even if their nursery habitats are in brackish water, older Bull sharks can tolerate high salinity as well as zero salinity, which enables them to live worldwide in coastal areas of tropical to subtropical oceans as well as in rivers and lakes.
S3 of a pregnant bull shark from Brunnschweiler J, Baensch H (2011). “Seasonal and Long-Term Changes in Relative Abundance of Bull Sharks from a Tourist Shark Feeding Site in Fiji“. PLOS ONE. DOI:10.1371/journal.pone.0016597. PMID 21346792. PMC: 3029404., CC by 2.5 )
Bull sharks are able to retain salt through their kidneys, liver, gills and a special gland near their tail (requiring a greatly increased production of highly dilute urine and is energetically very demanding). That’s the reason they survive and even thrive in fresh water as much upstream as 2,360 miles -3,800 kilometres- up the Mississippi River or 2,610 miles -4,200 kilometres- up the Amazon River. There is even a population of Bull sharks in the Lake Nicaragua in Central America, seemingly separate, but tagging showed that they wander between the lake and the Caribbean Sea back and forth, conquering 8 rapids on the way. Since a flood in the 1990s even a golf course lake in Queensland, Australia is the home to several bull sharks.
The Bull shark is a stout species with small eyes and a blunt snout (thus the name) and an average length of 7.9 ft – 2.4 m- (female) and 7.4 ft -2.25 m- (male), but there is a single record of a pregnant female specimen of 13 ft -4 m- in an African river. Their age of maturity seems to be varying according to geographic location, up to 14-15 years for males and 18 years for females. Bull sharks are ovoviviparous, like other requiem sharks.
Being opportunistic feeders, Bull sharks eat everything from bony fish and other sharks (even young Bull sharks), to turtles, birds, dolphins and terrestrial mammals like dogs and hippos, but also carcasses and (in case of Indian rivers) bodies. They have to (as mentioned above) in order to survive in fresh water with its zero salinity and significantly greater negative buoyancy. In the ocean their metabolism can slow down.
Based on their habitat, Bull sharks come in frequent contact with humans. In the clear waters of the Bahamas, for example, divers regularly interact with crowds of bull sharks without problems. Some researchers speculate that this non-threatening behavior may be because the sharks can clearly see people and recognize that they are not a typical food source. In murkier waters, however, incidents happen, and humans get bitten and sometimes die. Some Bull sharks being territorial and having virtually no tolerance for provocation doesn’t help matters, either.
Due to its habitat, but also its size, strength and teeth, the Bull shark has been considered by many experts to be the most dangerous shark in the world. If there is such a thing as a Man eater shark it would be the Bull shark and not the White shark. The latter may not in fact be responsible for many of the attacks pinned on the species (including the famous 1916 shark attacks in New Jersey that may have served as inspiration for Jaws), but the Bull shark with its nearly identical dentition. It’s amazing that we don’t have more incidents, and it just reconfirms that they really aren’t interested in us and usually an attack is a mistake.
The Bull shark is caught as bycatch in longline-fisheries, but more often targeted in small artisanal fisheries because of its abundance in nearshore environments and rivers for its meat, fins and skin. Additionally, it is a popular game fish. Due to pollution of their habitat in shallow costal waters, rivers and lakes, and overfishing of their nursery grounds, Bull sharks numbers have significantly declined.
Sources: here, here, here and here