Sharks are largely associated with the ocean. While most shark species are strictly confined to saltwater, a select few exhibit a remarkable ability to venture into, and even thrive within, freshwater systems.
The Primary Freshwater Shark
The bull shark (Carcharhinus leucas) is the most well-known shark species capable of inhabiting freshwater. These sharks are found globally in warm, shallow coastal waters. They are euryhaline, tolerating a wide range of water salinities, from fully marine to entirely freshwater.
Bull sharks are robust, stocky predators with a blunt snout. Their movement between environments is often linked to reproduction, as pregnant females utilize estuaries and river mouths as safe nursery grounds for their young. Juvenile bull sharks spend considerable time in these lower-salinity environments, benefiting from fewer large marine predators. Adult bull sharks also travel far inland, sometimes thousands of miles upstream, in pursuit of prey.
How Sharks Adapt to Freshwater Environments
The ability of bull sharks to transition between saltwater and freshwater is primarily due to osmoregulation. This physiological mechanism allows them to maintain a stable internal salt and water balance despite drastic changes in external salinity. Several organs work in concert to achieve this.
When in freshwater, the bull shark’s rectal gland reduces its activity to conserve sodium and chloride. Their kidneys produce large volumes of dilute urine to expel excess water absorbed from the freshwater environment. The liver also modulates urea production in response to environmental salinity changes, helping to maintain the shark’s internal osmolality. Gills assist in the uptake of essential ions like sodium and chloride from the surrounding freshwater.
Global Freshwater Shark Habitats
Bull sharks are found in tropical and subtropical coastal waters worldwide, entering numerous freshwater systems. They have been documented traveling significant distances up major rivers, including more than 4,000 kilometers (2,500 miles) up the Amazon River in South America and over 1,700 miles up the Mississippi River in the United States, reaching as far north as Illinois.
Freshwater habitats include Lake Nicaragua in Central America, where a bull shark population accesses the ocean through a river system. In Africa, they are found in the Zambezi River and have been sighted in the St. Lucia Estuary. Locations also include the Ganges and Brahmaputra Rivers in India and Bangladesh, and the Tigris River in Iraq. Bull sharks have occasionally been found in landlocked areas, such as a golf course pond in Queensland, Australia, after flood events.
Other Sharks and Freshwater Encounters
While the bull shark is the most well-known for its freshwater capabilities, other sharks have been observed in these environments. The genus Glyphis, or river sharks, includes species like the speartooth shark (Glyphis glyphis) and the Ganges shark (Glyphis gangeticus). The Ganges shark is found exclusively in freshwater rivers and estuaries of India and Bangladesh. River sharks are rare and endangered, with less known about their biology than bull sharks.
The Greenland shark, primarily a deep-water marine species, has been found in Canada’s St. Lawrence and Saguenay rivers. Other freshwater shark sightings are often temporary or accidental, like bull sharks entering flooded areas or rivers after storms. Many “freshwater sharks” sold in the aquarium trade are bony fish with shark-like appearances, not true sharks.