Can Bull Sharks Live in Freshwater? Rivers and Lakes Explained

How do bull sharks survive in fresh water?

Most sharks cannot survive in fresh water at all. A shark’s blood is kept about as salty as seawater, mainly by high levels of a waste chemical called urea. In fresh water, a body that salty would draw in water through the gills by osmosis, the movement of water toward the saltier side, while precious salts leaked out. An ordinary shark would quickly swell, lose its salts and die.

The bull shark is euryhaline, meaning it can handle a wide range of salinities. When it enters fresh water, several organs work together:

• Liver: it produces less urea, so the blood becomes much less salty, although still saltier than the river.
• Kidneys: they pump out large amounts of very dilute urine to get rid of the extra water, while taking back useful salts.
• Rectal gland: this gland normally removes excess salt at sea. In fresh water it slows down, so the shark keeps the salts it needs.
• Gills: they take up sodium and chloride from the surrounding water.

Fresh water also changes buoyancy. It is less dense than seawater, so a shark sinks more easily and must spend more energy staying up. Bull sharks caught in fresh water have been found to have lower-density, oilier livers, which may help offset that cost. For a wider look at how fish manage salt and water, see Do fish drink water?

The range of their tolerance is remarkable. Bull sharks can also cope with very salty water, up to about 53 parts per thousand, half again as salty as normal seawater, and they live in hypersaline lagoons such as the St Lucia estuary in South Africa.

Why do bull sharks swim up rivers?

The main reason is breeding. Pregnant females move into estuaries, lagoons and river mouths to give birth, and their pups grow up in these low-salinity nurseries. Most larger sharks, including tiger sharks and great white sharks that would eat young bull sharks, avoid such water, and there is plenty of food.

Tolerance changes with age. Newborn and young bull sharks are found mostly in the freshest parts of nurseries, while older sharks spend more time in saltier water. In Florida, young bull sharks grow up in places such as the Caloosahatchee River and the Indian River Lagoon. Tracking studies in the Caloosahatchee found that groups of juveniles moved downriver together when conditions changed, often riding the tide to save energy.

Adults may also follow prey upriver, or wander into rivers and harbors during floods and warm seasons.

How far up rivers have bull sharks gone?

Upriver records in the United States are rare. Apart from the 1937 Alton report, a bull shark was found at the Rush Island power plant south of St. Louis in 1995. Bull sharks are much more common in brackish waters near the coast, such as Lake Pontchartrain in Louisiana and the lower Potomac River in Maryland.

Lake Nicaragua’s famous sharks

Lake Nicaragua is the largest lake in Central America, and for centuries it was known for its sharks. Scientists once thought they were a unique landlocked species and named them Carcharhinus nicaraguensis. In 1961, comparisons showed they were simply bull sharks.

Tagging in the 1960s by American zoologist Thomas Thorson then proved that the sharks were not trapped. Sharks tagged in the lake were later caught in the Caribbean Sea, and sharks tagged at sea turned up in the lake, some after only 7–11 days. To make the trip, they swim up the San Juan River, passing through a series of rapids. Heavy fishing in the 20th century later greatly reduced the number of sharks reaching the lake.

Bull sharks in Australian rivers and a golf course lake

Australia has some of the best-known freshwater bull sharks. The Brisbane River in Queensland is an important nursery, and estimates have suggested it holds hundreds of young bull sharks. During the 2011 Brisbane floods, bull sharks were reported in flooded streets of the suburb of Goodna. They also live in the canals of the Gold Coast, visit Sydney Harbour in summer, and in Western Australia the species is known as the Swan River whaler, after the river that flows through Perth.

The strangest case is a golf course lake in Carbrook, Logan City, south of Brisbane. Floods of the Logan and Albert rivers in 1996 left several bull sharks trapped in a lake of about 21 hectares (51 acres). They survived there for about 17 years, growing large and becoming a local attraction, until they disappeared after more flooding around 2013. Researchers have described it as the longest known stay of bull sharks in such low-salinity water.

Can bull sharks live in fresh water forever?

Bull sharks can clearly survive in fresh or nearly fresh water for many years, as the golf course sharks showed. But they are not true freshwater fish. They are not known to form permanent, cut-off freshwater populations, and adults normally return to coastal salt or brackish water. The constant work of controlling salt and staying afloat makes fresh water a costly place to live.

For people, the main concern is that bull sharks can turn up in rivers, harbors and canals where swimmers do not expect sharks. Bites in fresh water are rare, but they do happen, especially in murky water after heavy rain. The wider picture is covered in Are bull sharks the most dangerous shark?

Other sharks and rays that enter fresh water

More than 40 species of sharks and rays have been recorded in fresh or brackish water, but only a handful can stay long. The river sharks of the genus Glyphis, such as the Ganges shark and the speartooth shark, live in rivers and estuaries in parts of Asia and northern Australia and are rare and threatened. Sawfish also move far up tropical rivers.

The champions are the South American freshwater stingrays (family Potamotrygonidae), which live their whole lives in rivers such as the Amazon and Orinoco and can no longer survive in the sea. Many other fish, from salmon to eels, also move between salt and fresh water, but among sharks, the bull shark is in a class of its own.