Sharks rank among the most feared predators in the world, but they have a surprising weakness – most cannot tolerate fresh water. Approximately forty percent of bony fish, such as goldfish and trout, live in fresh water. Sharks, on the other hand, belong to a group of cartilaginous fish called elasmobranchs, only five percent of which can live in fresh water (1).
Sharks lose buoyancy in fresh water.
(Source: Wikimedia Commons)
Sharks are not adapted to live in fresh water. Fresh water dehydrates them, dulls their senses, and compromises their reproduction. It also creates extra drag for them, reduces their buoyancy, and essentially makes them sink. Fish that have evolved more recently have very efficient air bladders that are well suited for fresh water. In contrast, the relatively more ancient elasmobranchs rely significantly on their lipid-rich livers as flotation devices (2).
To study sharks in fresh water, researchers modeled the swimming mechanics of a bull shark, a species that lives partially in fresh water. They concluded that because of a loss of buoyancy, the shark spends about fifty percent more energy on staying afloat in fresh water than in salt water (1). Another study showed that bull sharks and sawfish another fresh water elasmobranch, were less buoyant than previously studied ocean species, despite having extra-fatty livers (2).
Fossil evidence suggests that freshwater sharks were more common long ago, and it is possible that flotation problems drove them to the sea.
Sources:
1. Weiler N (2015) Sharks sink in fresh water. Sciencemag.org. Retrieved from http://news.sciencemag.org/biology/2015/01/sharks-sink-fresh-water
2. Gleiss AC, et al. (2015) Mechanical challenges to freshwater residency in sharks and rays. The Journal of Experimental Biology. doi: 10.1242/jeb.114868