You’re swimming in a lake with your friends when you notice a fin cutting through the water and heading straight for you. Is it a great white shark or are your friends shooting a prank video? Should you stand brave and tall so your friends don’t get the reaction they seek, or should you flee in a mad panic?
The bull shark is one shark in particular that you should be aware of that thrives in both salt and freshwater. Bull sharks have been tracked up the Mississippi River as far as Alton, Illinois which is 1,100 miles away from the ocean. Bull sharks can be aggressive.
In other words, can sharks live in freshwater and do you have anything to fear when swimming in lakes and rivers?
Can Sharks Live in Fresh Water?
We have some good news and bad news.
The good news is that most sharks can only live in saltwater and you won’t find them in freshwater rivers and lakes. The bad news is that there are some exceptions, and one of them is just as deadly as the great white.
Bull sharks (Carcharhinus leucas) are a type of requiem shark that thrive in brackish water and can also be found in freshwater systems.
They’re not alone, either. There are also river sharks known as “glyphis”, and depending on where you are in the world, you could encounter any of these ferocious sharks in fresh and brackish water.
The bull shark has several different names depending on where it’s located. These include the “Lake Nicaragua shark” and the “Zambezi shark”. It can be found all over the world, including in the United States.
Bull sharks have been known to swim up the Mississippi River, going over 700 miles from the ocean. They are stocky and strong with aggressive characteristics and they are thought to be responsible for a large number of near-shore shark attacks.
In fact, bull sharks are one of the three deadliest sharks in the world, along with the great white shark and the tiger shark.
Bull sharks live in warm and shallow waters and have very short tempers. They don’t tolerate provocation and will attack if they are threatened or annoyed. Despite this, the risk of being attacked by a bull shark is still very low.
Humans are not typical prey, and bull sharks mostly feed on small sharks, bony fish, sea turtles, dolphins, sea birds, crustaceans, and stingrays. They have even been known to eat other bull sharks.
Many experts believe that a bull shark was responsible for the 1916 New Jersey shark attacks in which 5 people were attacked (4 of which died) over a period of 12 days. These attacks are said to have inspired Jaws, although Jaws was a great white shark and not a bull shark.
Bull sharks are not considered “true river sharks” because while they can survive in freshwater systems, they migrate to saltwater to reproduce.
The shark species listed below are all true river sharks, as they can remain in freshwater systems.
The Ganges shark is so-named because it’s found in the Ganges river. It’s a rare shark currently listed as “critically endangered”, as habitat changes are driving it to extinction.
The Borneo river shark and the Irrawaddy river shark are both considered to be varieties of the Ganges shark.
The Ganges river shark is often considered to be a ferocious and aggressive shark. It has been described as a “man-eater”, and locals usually stay well clear. However, this reputation is likely the result of the Ganges shark being confused with the bull shark, which is more common, more aggressive, and also lives along the Ganges river system.
Northern River Sharks
The northern river shark is found in rivers and coasts across Papua New Guinea and Australia. It is a stocky shark that can grow to over 8 feet in length.
Classed as “vulnerable”, these sharks are struggling with many of the same issues as other shark species, including climate change, pollution, and general habitat destruction.
The speartooth shark is a rare species that’s classed as “vulnerable”. It lives around New Guinea and Australia and can be found in water with varying salinity.
These river sharks grow up to 8.5 feet and are very stocky.
Other River Sharks
In addition to the above, we know of two extinct species of river sharks.
The glyphis hastalis once lived around the coasts of Great Britain. It’s not the first country that springs to mind when you think about sharks, but there are actually over 40 species that live in the area (you’re unlikely to encounter any during a trip to the beach) and there may have been more in the past.
The glyphis pagoda, like the glyphis hastalis, is a long extinct river shark species thought to have existed during the Miocene period over 5.3 million years ago.
Should I Be Scared of the Bull Shark?
In the United States, the only river shark you need to worry about is the bull shark.
It can be a very aggressive shark and if it attacks, it will do serious damage. It’s rare for these creatures to attack humans and deaths are even rarer (there are roughly 15 shark attacks a year and 1 death every 2 years), but it’s concerning to know that they can live in freshwater.
There have been reports of these sharks living in golf courses in Australia and they are also seen in the Mississippi River.
Stenohaline vs Euryhaline
Stenohaline organisms are ones that can only survive in a narrow range of salinities while euryhaline organisms can adapt to many. Most sharks are in the former category, but river sharks are in the latter.
The word “stenohaline” actually comes to us from the Greek words meaning “narrow” and “salt”, while “euryhaline” means “wide and “salt”.
How Long Could a Shark Live in Freshwater?
Sharks need salt. It must be retained inside their bodies at all times. If not, it will lead to bloating, cell rupture, and death.
When they enter fresh water, their salt levels will dilute rapidly and they will die quickly. Some sharks will spend brief periods in freshwater to look for food, but others stay well clear.
Of course, river sharks are the exception. Their bodies are adapted to survive in fresh water and so they can remain indefinitely.