What Happens to Saltwater Fish in Fresh Water?

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...

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