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What Happens to Saltwater Fish in Fresh Water?

Why do some fish thrive in salt water and others in fresh? And what happens if you, say, drop a saltwater shark into a fresh water swimming pool? Here's the science!

By
Sabrina Stierwalt, PhD
6-minute read
Episode #390

Some of my best episodes, in my humble opinion, are inspired by the questions that my daughter asks me. To make it through another heatwave here in southern California, we were borrowing a neighbor’s pool when she suddenly became very concerned about sharks. I assured her there were no sharks in the pool, but she kept re-confirming with me: still no sharks in the pool? 

How fast would fresh water take out a shark?

I asked her how she thought a shark would get in the pool and she matter-of-factly pointed out that someone could drop one in via helicopter. I tried to insist that would be a very expensive operation with little to no benefit, but she wasn’t convinced. So I tried another approach: sharks are saltwater fish. (Well, most of them.) They don’t survive in freshwater pools. But how fast, she wanted to know, would the fresh water take out the shark? Fast enough to keep her safe? 

Of course, I told my child that indeed a shark dropped via helicopter into a random pool in southern California would not survive long enough to enjoy her as a snack. But I wondered, how long would it really take? 

Why do we need water?

Overall our bodies are around 60% water and that varies by organ. Our brains contain are 73% water, our lungs are 83% water, and even our bones contain 31% water. If we lose just 4% of our water, we experience dehydration. A 15% loss could be fatal. We need water to fill our cells so that they maintain their shape, which in turn supports important biochemistry that goes on inside our cells. Water also forms the membranes around cells that keep important molecules in and harmful molecules out. So even though we could survive a month without food, we would only last about three days without water

Even though we could survive a month without food, we would only last about three days without water.

And we can’t drink saltwater because it ultimately dehydrates us. Our bodies need to use more water to flush out the extra salt than we gain from drinking the salt water in the first place. 

What happens to saltwater cells in fresh water?

A process called osmosis keeps the concentration of salt in fish cells at a comfortable level. Osmosis occurs when a solution on one side of a membrane (like a cell wall) is more highly concentrated than the solution on the other side. So if there are more solutes (that’s whatever is dissolved in the water: in this, case ions, salts, and other nutrients) on one side of the cell’s outer membrane, the solvent (that’s the water these solutes are dissolved in) passes from the area of low concentration to the area of high concentration. Adding more water dilutes the side with the higher concentration to try to balance things out. The membranes that surround the cells are semipermeable, meaning the water moves in and out of the cell easily but the solute (the salt) does not. 

Osmoconformers and osmoregulators

Some fish, called osmoconformers, keep their body fluids at the same concentration level as the surrounding water so they don’t require an ongoing inflow or outflow of water. Other fish, called osmoregulators, have different concentrations inside their bodies versus outside them, but they actively work to counter balance the results of osmosis. 

For example, freshwater fish, like goldfish, are saltier than their surroundings. Osmosis causes the surrounding water to move into their bodies through their gills and skin. They urinate frequently to get rid of it and don’t drink because they don’t need to. They’ve got enough water coming in thanks to osmosis. They can also excrete any excess salt ions they may take in via their gills and mouth. This balancing act that maintains the right salt level in their cells is called osmoregulation.  

Saltwater fish, like tuna, are less salty than water around them. So for them, osmosis causes the water to move out of their bodies. That means saltwater fish actually have to drink to stay hydrated. They direct some of the water that comes in through their mouths to their digestive track, and not just back out of their gills the way freshwater fish do. But drinking saltwater, of course, adds salt to their bodies, so they also have special cells in their gills to help remove salt ions. 

Other animals that spend time in and around the ocean have their own tricks for getting rid of excess salt. Otters have very concentrated urine, albatrosses have special cells in their bills that excrete salts, and turtles cry salty tears. 

Otters have very concentrated urine, albatrosses have special cells in their bills that excrete salts, and turtles cry salty tears.

Osmosis experiments to try at home

Here are two experiments you can do at home to see osmosis at work. First, dunk a raisin in fresh water. You should see it bloat and swell up. The raisin has a higher concentration of solute (in this case the solute is sugar), so the water flows into the raisin.

Now, dunk the raisin in salt water. It shrivels up! That’s because the salty water has a higher concentration of solute than the raisin. The water passes out of the raisin in an attempt to create a balance.

You can also observe osmosis using potatoes. Cut up a potato. Fill two cups with water, and add salt to one of the cups. Let them sit in the water overnight. In the morning, you should find that the saltwater potatoes are crunchy—they've lost their water via osmosis. The freshwater slices should be softer because they've absorbed some of the surrounding water.

So what happens when you put a saltwater fish in fresh water? 

Sharks do not rely on osmosis. They are osmoconformers, meaning they keep their bodily fluids at the same concentration as the surrounding water. Those concentrations, however, are made up of different ions. In particular, sharks use the urea their bodies naturally produce. Our bodies also produce urea, through the metabolism of proteins, and we excrete it in our urine. Sharks use this ion, which is normally a waste product, and instead store it so that their cells have similar concentration levels to the saltwater around them.

Water starts to flow out of the fish but the fish doesn’t have the right coping mechanisms for taking in water to replace it.

If we were to put a freshwater fish in salt water (or a saltwater fish in fresh water), they would fare similarly to our raisins and potatoes. The freshwater fish in salt water is now less salty than its surroundings. Water starts to flow out of the fish but the fish doesn’t have the right coping mechanisms for taking in water to replace it. The cells will shrivel up. A saltwater fish in fresh water is now saltier than its surroundings. The surrounding water flows into their cells and they begin to swell and bloat, possibly rupturing. 

How much salt is too much?

Freshwater has a concentration of < 0.1% salt by weight. Ocean water, on the other hand, has a salt concentration of about 3.5% by weight. Euryhaline fish can live in both fresh and salt water and usually have salt concentrations somewhere in the middle. Salmon, for example, are born in freshwater but live out most of their lives in the ocean. They return to freshwater only to spawn and keep their solute concentrations around 1%. North American eels do the opposite, spending most of their lives in fresh water after being born in salt water. Even fish that are totally transferable between fresh and salt water still need time to acclimate as they move from one to another, much like we humans need to give our bodies time to adjust to the lower oxygen levels at higher altitudes. 

How long can a saltwater fish survive in fresh water?

Now to my daughter’s question—how long can a saltwater fish survive in fresh water?

Those who have ever maintained a saltwater aquarium know that a freshwater “dip” can be used when saltwater fish develop a parasite called “ich.” The parasite can’t adapt and its cells quickly erupt in fresh water while the fish suffering the parasite can wait it out. Perusing aquarium chat boards tells me that the recommended time for such a dip is anywhere from 30 seconds to 10 minutes. That’s really fast! 

It’s possible that a shark dropped into a swimming pool could survive in fresh water long enough to snack on a small child.

On the other end of the spectrum, bull sharks are known to travel in rivers like the Mississippi and the Amazon. They are considered advanced osmoregulators and have been seen as far as 60 miles upstream in waters with salinity as low as 2.1%. 

So, unfortunately, my daughter is right—it’s possible that a shark dropped into a swimming pool could survive in fresh water long enough to snack on a small child. It looks like I’ll have to return to convincing her that no one would ever really want to do such a thing.

Please no one tell her about the plot to Shark Night 3D.

About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.