Here’s Why Sharks Prefer Salt Water

If you're afraid of sharks, there's a simple way to avoid them: steer clear of shark-infested oceans and choose a freshwater spot instead. But why? What, exactly, is keeping these freshwater places shark-free? A new study provides one explanation: most sharks would actually sink in fresh water.

Scott Sutherland of the Weather Network reports on a new study that shows that sharks have trouble staying afloat in fresher waters. A team of biologists from the United States and Australia wondered why so few shark species can be found in freshwater habitats. Prevailing theories suggested that something in the physical structure of sharks made it hard for them to survive in fresher waters.

The researchers used a hydromechanical model to study how shark physiology affects buoyancy. As Sutherland explains, sharks lack a swim bladder—the gas-filled organ that helps other bony fish float. But they do have extra large livers, which provide some buoyancy and help keep them sinking to the ocean floor.

But fresh water changes how much buoyancy the shark's liver can provide. The researchers found that when sharks swim in fresh water, they are two to three times less buoyant than when they swim in salt water. In order to keep afloat the same way they do in salt water, sharks would need livers eight times as large—unrealistic for an organ that’s already the largest in a shark’s body.

“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,” notes Sutherland. That means that the shark would have support a two-ton liver just to float in fresh water—that’s two times its existing mass.

So it seems that ocean-loving sharks aren’t likely to adapt to fresh water any time soon. But don’t think that means you’ll never see a fin in fresh water. There are a few river sharks, like bow sharks, that have adapted to less salty environments; their livers, the study found, are less dense than those of marine sharks, which helps them stay afloat.

Though the number of freshwater sharks are dwindling, they still can threaten humans. On second thought, perhaps the kiddie pool isn’t such a bad swimming spot after all.

Erin Blakemore | | READ MORE

Erin Blakemore is a Boulder, Colorado-based journalist. Her work has appeared in publications like The Washington Post, TIME, mental_floss, Popular Science and JSTOR Daily. Learn more at erinblakemore.com.