Freshwater Sharks: Big Fish Where You Least Expect Them

Placid lakes, rivers, estuaries and large streams make attractive vacation sites, but they can also attract non-human visitors, including sharks. The toothy elasmobranches may show up if the climate is tropical, warm or temperate. Shark freshwater hot spots include places like the Mississippi River in the U.S., the rivers of Natal in South Africa, the Tigris River system of southern Iraq, and multiple other freshwater bodies in India, China, Indonesia, West Africa, New Guinea, the Philippines and Australia. It’s unknown if any shark can live out its entire life in fresh water, but the miracle is that they can thrive at all under such conditions. 

How Sharks Survive in Fresh Water 

For most shark species, spending a day in fresh water would be like placing a human on the moon without a spacesuit. It could not survive due to the inhospitable surrounding environment. A process called osmosis is central to the problem. Osmosis is when a fluid moves through a semi-permeable membrane from a solution with a low solute concentration to a solution with a higher solute concentration, until there is an equal concentration of liquid on both sides of the membrane. The dissolved substances, in this case, primarily involve sodium and chloride.  

Since sharks evolved in salt water, they tend to have very salty bodies. Even sharks in fresh water contain more than twice the amount of salt and chloride as more common freshwater fishes. In theory, they should burst like an overfilled water balloon, given the osmosis effect, but they have come up with an effective answer to the problem — they urinate a lot.  

Ichthyologist Thomas Thorson studied bull sharks living in Lake Nicaragua and found these huge fish take in a lot of extra water, as expected, but they excrete much of it as dilute urine, at a rate of over 20 times that of typical saltwater sharks. That means their kidneys must work extra hard, utilizing additional energy. Like people who become accustomed to life in low oxygen regions, however, sharks in fresh water appear to adapt to what would seem to be formidable conditions. 

Sharks Adapted for Fresh Water   

Although a survey of freshwater sharks and rays in 1995 determined that 43 species of elasmobranches penetrate freshwater environments, relatively few sharks spend substantial time in these areas. Sharks that do frequent such regions include the river sharks and the aforementioned bull sharks, which, as their name suggests, possess stocky bodies and an often aggressive, unpredictable nature to match. 

The term “river sharks” refers to six rare species in the genus Glyphis. These include some of the world’s most endangered sharks, such as the Ganges, speartooth, Irrawaddy River, Bizant River, Borneo River and New Guinea River sharks. Aside from their pumped-up kidneys and other internal adaptations, these sharks tend to additionally possess certain telltale exterior features. Their snouts are often short and broad, containing small, widely spaced nostrils. Broad, serrated teeth fill the upper jaw, while the lower contains teeth that menacingly protrude, even when their mouths are fully shut. All of these characteristics likely help them to exist under shallow conditions. 

Why Freshwater Sharks Face Extinction 

River shark populations are at dangerous lows now. Bull shark numbers are higher, since they can often move between fresh- and saltwater environments. Species like the Ganges, though, which are more adapted to river and lake life, are almost prisoners within their more land-locked environments, since they must withstand both natural and human-induced problems. Natural problems include temperature, oxygen, mineral content and turbidity changes that continue to be influenced by climate change. Human activities involve dam building, modifications to water for irrigation and fisheries, and the introduction of pollutants into the water. 

Adding to the problem is the “bad boy” image of sharks lurking in places where humans frequent. As apex predators with no fear of being attacked by another animal, bull sharks have mistaken humans for prey over the years, leading to unfortunate consequences. In fact, a series of bull shark attacks on the Jersey Shore in 1916 is said to have inspired the 1970s movie thriller Jaws, which wound up featuring a great white instead of a bull shark. Humans are a much greater threat to sharks than they are to us, as evidenced by the 2006 ban that the government of Nicaragua imposed on freshwater fishing of bull sharks. An alarming decline in the bull shark population there prompted the ban.  

The World Conservation Union’s Shark Specialist Group recently stated “biological data is urgently needed for freshwater elasmobranches to make it possible to attempt management and conservation.” The scientists then added a jolting statement: “At present, there is a vacuum of information, and elasmobranches can easily drop to extinction without notice.” Time will only tell whether or not river sharks, which have been on Earth for millions of years, can survive into the next decade, much less century.