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.
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.