When you think about an ocean, what pictures come to mind? The rolling waves crashing upon the shore? The sandy beach littered with beach umbrellas and chairs? A lot of people think about the sharks that swim out beyond the depths; or those that forage close to the shore.

When you think about a river or a lake, what pictures come to mind? The flowing river passing by the banks? The people in canoes, kayaks, or even riverboats flowing with the current? The sharks that might be living in the river waters? Wait…what?

Think you know all about sharks? What you learn today might make you a little bit more knowledgeable than you were yesterday.

For The Love Of Salt

The majority of shark species live in the ocean. This is due to their ancient evolution; they were born into the salt water and there they shall remain until the world stops turning. Freshwater has a bad effect on the shark; it dehydrates them, can dull their senses, and wreaks havoc on their reproductive system. And since they don’t have any flotation devices in their bodies (like more modern fish do); they sink. Their need to live happy and healthy in salt water comes down to one thing: science.

Brace Yourself For The Salty Science Of The Shark

All living things (including you, me, and sharks) have a mixture of salt and water in our bodies; it’s how we sustain life. Skin and other living tissue may look solid, but we all have tiny pores that allow water and salts to pass between our insides and the outside. One example of this is what happens when we sweat. Now, here’s where we get all science-like. If there is more water or salt on the outside of our bodies then there is on the inside, the molecules will shift themselves around constantly to create an equal balance; this is called diffusion. Though the ocean is mostly water, it is concentrated with salt. Because you and I (and most fish) have more fresh water inside us and less salt, we diffuse our fresh water insides to compensate for the salinity in the sea. This is why swimming in the ocean makes us feel so dehydrated and thirsty!

When it comes to a shark, the opposite is true. Inside the shark is a concentration of saltiness that is slightly higher than the ocean, so they keep their salt and their freshwater inside. Plus, they keep the freshwater by diffusing it in through their mouth, gills, and membranes. Any freshwater that is in excess is filtered right back out through the kidneys and excreted. Now, if a salty shark swam into a mass of freshwater…you can see where this is going. They would diffuse their salt, take on too much fresh, and their kidneys would have to work extra-hard to filter out the excess. This is how they could become dehydrated, weak, and very sick.

The Bull Breaks Through

There are several species of shark that will frequent freshwater areas, they might take a swim up the mouth of a river for a bit, but they do not stay for long due to the water-salt dilemma. There is one salty exception to the saltwater rule: the Bull Shark. They have been found in freshwater rivers, such as the Mississippi River, The Amazon River, Lake Nicaragua, and the Zambezi River. Bull Sharks have only half of the salt concentration in their bodies and this makes it easy for them to go from salt water to fresh water easily. Of course, their kidneys still produce excess urine in the freshwater, but they are not affected with dehydration or any ill-effects when they swim up river. In fact, a Bull Shark can live up to 6 years in freshwater.

Because of its ability to hang out in rivers and lakes, it’s one of the most common sharks known to man. Unfortunately, it’s also one of the more aggressive sharks and has been known to attack, but that’s probably because we tend to share more waterways. Considering that Bull Sharks are very common in the Ganges River and thousands of people use the river on a daily basis, the number of attacks are very small by comparison.

Sharks, But Not Sharks

There are several sharks who do well in the freshwater; they spend their entire lives in lakes and rivers, breed, and die without ever touching the saltwater. In fact, some can even thrive in freshwater aquariums. The only problem with these sharks is: they aren’t really sharks. Most are members of the Cyprinid family, which also includes minnows, Barbs, Danios, and Rasboras. They were given the name of shark due to their shark-like appearances and feeding habits. They might have shark in their title, but that doesn’t make them part of the shark species. We won’t hold that against them, though, they can’t help that they were named that way! Plus, they are pretty cool, no matter how you look at them. But still, just as a catfish is not a cat, and a dogfish is not a dog, the following sharks are not really sharks.

  • Red Tail and Black Tail Sharks
  • Rainbow Sharks
  • Bala Sharks
  • The Golden Shark
  • The Silver Apollo Shark
  • The Chinese Hi Fin Shark
  • Iridescent Sharks

So, now you know all about sharks and their salty nature. You also know what sharks are real, and which ones wish they were. And the next time you want to observe a Bull Shark, skip the ocean and head to the nearest big river.