Sink or Swim? Here's a big reason why you're not likely to find a shark in your local lake
Wednesday, January 21, 2015, 9:28 AM - If tuning in to watch Shark Week, or sitting down for a viewing of the movie Jaws has you crossing swimming off your 'to-do' list, here's some news that may have you heading back to the beach sooner than you thought - as long as that beach borders a body of fresh water, rather than salt water. Why? According to a new study, fresh water causes sharks to sink.
Over the course of evolution, in order to help with their mobility, species that live under water developed specific ways of staying buoyant. This make it easier for them to get around in bodies of water because they only have to work on moving forward and changing direction, and very little work has to be put into keeping themselves from sinking to the bottom.
In most bony fish (the most abundant type of fish in the world's oceans, lakes and rivers), this involves a specialized, gas-filled organ in their body called (amongst other things) the swim bladder. Sharks and other cartilaginous fish (also called elasmobranchs), though, don't have a swim bladder. Instead, they rely on the fact that their liver - which can represent up to a quarter of their body mass - is packed with fat cells that make it especially buoyant.
All aquatic creatures are more buoyant in salt water than in fresh water, simply because salt water is denser than fresh water. However, for sharks, this difference would require a fairly extreme bulk-up routine for them to make that transition between swimming in the oceans and swimming in a fresh water lake or river.
According to a recent study by American and Australian scientists, which used a sophisticated hydromechanical model to simulate a shark swimming through water, in order for a shark to remain buoyant in fresh water, its fatty liver would have to be around 8 times larger than it already is.
For reference, the liver of a fully-grown, 1-ton Great White Shark can tip the scales at up to 250 kilograms - over 3.5x the mass of the average male human! For this brute to remain buoyant in fresh water, by the researchers' model, the shark's liver would weigh in at around 2 tons - twice the mass of the shark itself!
Anything less and the animal simply wouldn't have enough buoyancy to maintain its mobility, and it would sink to the bottom of whatever lake or river it (unfortunately) wandered into.
One important thing to point out is that there are freshwater species of sharks, but a study of two such species - Carcharhinus leucas (or bull shark) and Pristis pristis (the largetooth sawfish) - only supported the researchers' findings. Despite the low density of their livers (revealing the organ's high fat-content), these sharks showed a very high negative buoyancy (the tendency to sink), and according to the study, the highest negative buoyancy of any sharks or other elasmobranchs studied so far.
The results of this study may shed light on why sharks and other similar species are far more likely to be found in the oceans than in bodies of fresh water.
RELATED VIDEO: Don't get too comfortable, though. Here's the story of a boy's encounter with a bull shark in Louisiana's brackish estuary, Lake Pontchartrain.