Last year’s horror flick, Shark Night 3D, tells the tale of a group of college friends who are attacked by sharks while vacationing near a lake. While the premise of the movies is that evildoers intentionally put sharks in the lake to make money, it made me wonder whether a shark could ever actually make its way inland to a lake.
First of all, the lake would need to be somehow connected to the ocean via a river or other body of water deep and wide enough to accommodate the large animal as it travels upstream. Secondly, most sharks can only tolerate saltwater, or at the very minimum, brackish water, so freshwater rivers and lakes are generally out of the question for species such as great white sharks, tiger sharks, and hammerhead sharks.
Bull sharks are the exception to this rule – they can tolerate brackish and even freshwater conditions because of their advanced ability to osmoregulate, or maintain a constant concentration of water in their bodies despite changing salinity levels in the water. This species is known to travel 60 miles upstream in warm rivers like the Mississippi and the Amazon.
North Carolina’s Neuse River has long been thought to harbor bull sharks, and Duke University graduate student Meagan Dunphy-Daly has proven it by tagging several of them there this summer. On one of her research trips, she encountered a 2.5 meter male bull shark swimming in water with a salinity of only 21 ppt (seawater is usually around 35 ppt). Why would a bull shark journey up the Neuse River? Perhaps in pursuit of the dolphins also swimming there, or maybe because the river provides a good nursery area for their pups.
Other than the bull shark, there are at least five species of “river sharks” in the genus Glyphis which have been observed in freshwater rivers in South and Southeast Asia and Australia, but they are extremely rare due to habitat degradation and little is known about them. These are the only purely freshwater sharks that have been discovered.
So, although it seems unlikely that you will ever encounter a shark in a freshwater lake, you might want to keep an eye out next time you're on a river.