You probably know that all living things need water to survive. Humans can’t go more than a week without water, though normally even less. We’re already like 60% of the stuff, and everyone drills it into your head that liquid water is one of the most basic necessities of life. At some point you’ve probably thought about our wet friends in the ocean though. How do fish get their water? Do they even need drink water in the first place?
It turns out the answer is actually both yes, and no.
Do Fish Drink? Let’s Talk Equilibrium
Whether or not your favorite fish is actively drinking does in fact depend on where they live. You can thank the process of osmosis for that. We have a pretty healthy handful of posts talking about equilibrium on this blog (see: How Long Can You Survive in Space Without a Suit?), and the same principles apply for whether or not fish have to drink.
The concentration of something dissolved in water will always try to remain equal to the environment around it. For example, fill a bag with salt and water, but only let water move in and out of the bag (this is more of a thought experiment–you’d need more than just a Hefty’s garbage back for this). Now, place that bag into a container full of water with no salt. You have created an environment where the bag has a higher concentration of salt than the water surrounding the bag. Because salt cannot exit the bag, water will enter the bag to lower the salt concentration in the bag and make the system “more equal”.
It Depends on How Salty the Fish Is
What does this mean for your fish? Well, imagine the bag we were talking about is now the cells of the fish (this is actually how cells take in water). If the fish is freshwater, it’s like the scenario we just described. Lots of water will passively enter fish cells via gills (the gills are a close vector to fish bloodstreams) and skin. This means no, freshwater fish do not drink water. But they will have to expel a lot of water via urination. If they didn’t pee all the time like that, their cells would explode from taking too much water.
If your fish is in saltwater (the water is saltier than the fish), a lot of water is going to be coming out of the fish cells, so the cells would just shrivel up if the fish didn’t do anything. In this case, our fishy friends have to actively drink water. But unlike their freshwater brethren, they don’t have to do as much urination, since water is always passively leaving their system instead of entering. Otherwise they’d get shriveled on a cellular level.
Know how salty your fishes are? Flex your freshwater fish knowledge here.