The bull shark is one of the most dangerous shark species in the world. Also known as the Zambezi and the Lake Nicaragua shark, it’s also one of the most versatile. It is often found in the murky waters of estuaries and river mouths and frequently comes into contact with humans.
Because of this proximity, the bull shark is responsible for more shark attacks on humans than almost any other, apart from the great white.
Can Bull Sharks live in freshwater? One of the main reasons the bull shark comes closer to shore than other species is its ability to tolerate freshwater.
Most sharks can exist only in saltwater habitats. If a great white shark strayed too far up a river system and into freshwater, the higher level of salinity inside its body would cause it to bloat.
As freshwater entered through the gills, it would dilute the salt levels inside the shark, causing its cells to expand and rupture.
However, the bull shark has adapted to live in both salt and freshwater environments. They use several processes to regulate their bodily fluids and internal salinity levels.
Can Bull Sharks Live in Freshwater?
Bull sharks can live quite comfortably in 100% fresh water and occur in tropical river systems all over the world.
Although they usually frequent estuaries and river mouths, they occasionally venture further inland. In 1937, fishermen in Alton, a city situated 15 miles north of St. Louis on the Mississippi River, caught the culprit responsible for stealing their catch. To their surprise, it turned out to be a bull shark!
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Bull sharks have also been found in freshwater river systems and lakes in other parts of the world, including South America and Africa. Bull sharks have even been caught as far as 2,500 miles from the Atlantic Ocean!
Bull sharks are notoriously aggressive in the ocean, but very few shark attacks have been reported in freshwater environments. This could be because it’s generally younger bull sharks that are attracted to the freshwater, and they aren’t large enough to take on prey the size of a human being.
Why Can Bull Sharks Survive in Freshwater?
Bull sharks have developed an intricate process that aids salt retention and helps them adapt to freshwater living.
The bull shark’s gills play a critical role, absorbing sodium and chloride from the environment to regulate internal salinity levels.
The kidneys are also vital, enabling salt to be reabsorbed into the bloodstream. Bull sharks also adjust their urine output in response to water salinity, urinating 20 times more in freshwater than in the ocean.
The last quiver in the bull shark’s biological bow is an unusual rectal gland located just in front of the caudal fin.
All sharks possess this gland which, scientists believe, is made of “specialized salt-secreting tissue.” In other words, it removes excess salinity from the shark’s body.
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This gland is present in all sharks, but its function isn’t fully understood. It appears to play a critical role in controlling the shark’s internal concentrations of water and salt.
In bull sharks, this has evolved to the extent that they can live for long periods in freshwater. Quite why they made such adaptations is unclear, although scientists believe it may be a kind of a survival mechanism.
Although mature bull sharks have few predators, young bull sharks can fall prey to other shark species, including the tiger and the sandbar. Neither of these species is euryhaline, so they can’t adjust to different salinities as the bull shark can. Living in freshwater would therefore protect younger bull sharks from predation.
How Long can Bull Sharks Live in Freshwater?
Theoretically, bull sharks should be able to survive in freshwater for their entire lives, but there is no evidence to prove this. It appears that the bull shark moves into more coastal areas to breed and reproduce.
Bull sharks that were trapped when a dam was constructed on the Rio Bayano river in 1976 survived for around four years. This proved that they could “survive in freshwater for long periods,” but whether they could live there forever is still up for debate.
Can Bull Sharks Live in Freshwater Forever?
One study of the bull shark distribution in the Indian River Lagoon in Florida found that young bull sharks, measuring between 90 to 190cm (35” to 75”), occupied the freshwater river system all year round.
Adult-sized sharks, on the other hand, were only found in freshwater in the late spring and early summer. Not only that, but all these mature sharks were female, and the majority were either pregnant or carrying eggs.
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Very young or neonate bull sharks were also found in the river systems during spring and summer but, when the temperatures dropped in autumn, moved to more brackish waters.
When they found the bodies of bull sharks in Lake Bayano, researchers initially thought they’d starved to death, but this seems unlikely.
Studies of bull sharks’ feeding habits indicate they’re opportunistic and indiscriminate feeders that “will consume almost any kind of animal matter.” Therefore, the chances of a bull shark starving to death are decidedly slim.
Quite why the Lake Bayano bull sharks died so young is unclear. It may suggest that bull sharks aren’t suited to permanent freshwater residency despite their physiological adaptations.
Are There Bull Sharks in Freshwater Lakes?
Bull sharks occur in just one freshwater lake – Lake Nicaragua in Central America. Covering over 8,000 ㎢, this vast lake is often referred to as “the sweet sea” and resembles the oceans with its waves and islands.
Until recently, the so-called Lake Nicaragua shark was thought to be a distinct, land-locked species of freshwater shark. There appeared to be no way for an 11-foot-long shark to traverse the shallow river that connects the lake to the Caribbean Sea, suggesting it was stuck in a freshwater environment.
This was clear evidence that a bull shark could survive in a lake – or was it?
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Can a Bull Shark Survive in a Lake?
A bull shark can survive in a lake but doesn’t appear to live there permanently.
By tagging the bull sharks in Lake Nicaragua, researchers have discovered that they travel back and forth between the lake and the sea relatively frequently. This movement is called diadromous migration.
Besides Bull Sharks, What Other Shark Species Can Live in Freshwater?
Aside from the bull shark, which frequents tropical rivers all over the world, five other shark species can survive in freshwater.
#1 Ganges Shark
As its name suggests, the Ganges shark is found in the Ganges and Brahmaputra Rivers of Bangladesh and India. Critically endangered, the Ganges shark is similar to the bull shark in appearance but notably different in its physiology.
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Unlike the euryhaline bull shark that moves between different levels of salinity, the Ganges is a freshwater shark that rarely ventures into the open ocean. Their range rarely extends beyond tidal areas where there’s a combination of muddy waters and a low salinity.
There is little information available about the rare and secretive Ganges shark, although scientists think both the Borneo and Irrawaddy River Shark may belong to the same genus.
#2 Northern River Shark
There are thought to be just 250 mature Northern River sharks in the world, all of which spend their time in the bays, estuaries, and tidal rivers of Papua New Guinea and Australia.
Preferring murky waters and muddy bottoms, this unusual species relies on electrical impulses to find its prey.
The head of the Northern River shark is covered with special electroreceptors known as ampullae of Lorenzini. These sensing organs enable the shark to detect changes in water temperature and “sense electric fields in the water.”
Like young bull sharks, it appears to be the newborn and juvenile Northern River Sharks that frequent the freshwater, while the mature sharks remain in more coastal environments.
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#3 Speartooth Shark
Like the bull shark, the speartooth is a euryhaline species that can tolerate freshwater as easily as it does seawater. It inhabits tropical rivers, estuaries, and coastal marine waters in Australia and Papua New Guinea.
Its behavior is similar to that of the bull shark, with the juveniles venturing much further up the river system than the adults.
#4 Pondicherry Sharks
This shark species is so rare it was thought to be extinct for over 40 years. Sometimes known as “the lost shark,” there was no evidence of the Pondicherry’s existence until one was discovered at a Sir Lankan fish market in 2019.
The Pondicherry shark is so rare we know very little about the species, except that it appears to migrate between salt and freshwater areas only outside the breeding system.
#5 Greenland Shark
The enormous Greenland shark is a constant source of surprises. It lives for hundreds of years, can withstand depths over 7,000 feet and frigid waters of just 28℉. Not only that, but Greenland sharks can also tolerate freshwater!
There have been reports of Greenland sharks in the cold, deep waters of Canada’s Saguenay and St Lawrence rivers. Very little is known about how long the Greenland shark can survive in freshwater, or about its migratory patterns to and from these river systems.
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The bull shark is one of a handful of shark species that can tolerate freshwater and traverse between river systems and the ocean. Known as a euryhaline shark, the bull shark has a special renal gland that aids salt retention and enables it to survive in different salinities.
As few other shark species have this ability, it seems likely that the bull shark developed it as a survival mechanism. Younger bull sharks can effectively “hide” in freshwater, out of reach of saltwater species like the sandbar and tiger shark.
Although the bull shark can survive in freshwater, it doesn’t appear to stay there forever, preferring to move towards more coastal zones as it matures.
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Nicky is a British adventurer and animal lover who spends her time exploring the natural world and writing about her experiences. Whether on horseback, underwater, running, hiking or just standing with a fishing rod in hand, she embraces everything her adopted home of South Africa has to offer.