Do Fish Get Thirsty: The Surprising Truth!

Do Fish Drink


Fish are always surrounded by water.

When you look at the ocean or a beautiful lake and see the blue waves all around you, you might think that the last thing a fish needs seems to be more water.

But why should they not need to drink water?

Neither humans nor animals can survive long without drinking. When you swim in salt water, you will get thirsty. Why should it be different for fish?

If you have ever asked yourself "Do Fish Drink Water?" or "Do Fish Get Thirsty?" you have come to the right place because I have an answer for you about this amazing animals!



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Do Fish Need to Drink Water? And How do Fish Drink Water? [Process Called Osmosis]


The answer is YES - at least kind of! Just like humans, fish need to take in water to survive. But not all of them drink it in the way we might imagine them too. Instead of drinking with their mouths, like humans and other animals, they mostly absorb water with their gills and skin. This process is called osmosis.


Osmosis is all about finding a balance! Let me explain.

This means that if you have a barrier between water that has a high concentration of salt on the one side, and water with a low concentration of salt on the other side, the water with the higher concentration of salt will absorb water from the other side until there is the same concentration of salt on both sides.

In fish, the two sides I am talking about are the inside of the fish and the water in which the fish is swimming. The barrier in this case is the skin and the gills of the fish. This is how a fish can both absorb and release water.

Freshwater Vs. Saltwater Fish: Which One Can Get Thirsty?

Osmosis works in a similar way for freshwater fish and saltwater fish, but there are also differences.

In freshwater fish, the insides of their bodies are much saltier than the water they swim in. So by the logic of osmosis, the not-salty water that surrounds them is absorbed by the salty water inside.

Fish also have chloride special cells in their gills that help them get just the right amounts of salt when they absorb water from their surroundings. Because like the human body, a fish body needs the correct amount of salt to be healthy, not too little but also not too much.

The opposite happens with saltwater fish. Because they are less salty on the inside than the water they swim in, the water around them actually absorbs the water from their insides! Because, as you remember, the side with the higher concentration of salt absorbs water from the side with the lower concentration of salt, not the other way round.

Because of this, saltwater fish actually have to drink a lot of water, because they are constantly losing water due to the process of osmosis. You might have seen fish swimming around with an open mouth and might have wondered what they are doing. Now you know it: they are gulping down water!

Because the water these saltwater fish drink by swallowing it is not filtered by their gills, they have to filter it inside of their body. In this case, the job of filtering out the right amount of salt is done by the kidney.

So, now you know how the bodies of freshwater fish and saltwater fish works. 

Can Fish Get Thirsty - Freshwater Vs. Saltwater

Do Fish Ever Get Thirsty?

But can fish actually get thirsty? That is a different question and one that is very difficult to answer.

I am sure you know what being thirsty feels like for a human being, especially when it is very hot or you have been doing some hard exercise. But we simply cannot say if being thirsty feels the same for a fish.

When I feel thirsty, I get up and get a glass of water - that is a conscious decision I make.

We do not know if those saltwater fish that are drinking so much water actually actively decide to drink some amounts of water or if it is just an automatic reaction of their body like breathing is for humans and animals on land.

There have been no scientific methods invented yet that are able to accurately tell if something feels the same way for a fish as it feels for a person.

Can Fish Die From Thirst? [What About Salmon?]


If you put a saltwater fish in fresh water, it will die, and the other way round.


There are certain kinds of fish though, that migrate through freshwater and saltwater and survive it, like salmon, for example.

They can do this basically by switching from drinking a lot to peeing a lot. When a salmon lives in saltwater it drinks a lot of water because it is constantly losing water through osmosis as I have explained before. If it would continue drinking all that water when it swims in freshwater environment (in lakes or rivers), it would simply absorb too much water! This is why, when coming from saltwater into freshwater, those kinds of fish switch from drinking more than they pee to peeing more than they drink. This is how works their body through osmosis! 

Most bodies of water do not switch immediately from saltwater to freshwater. There are transition areas, and salmon swim very slowly to those areas so that their bodies can get used to the demands of their new surroundings.

And one more thing - if you want to learn more about how salmon's kidneys produce large volumes of dilute urine and concentrated urine check this article (Acclimation of Osmoregulatory Function in Salmon) of The university of New Mexico.

Can Fish Get Thirsty - Salmon


As you can see, fish indeed need to drink, because fish need to take in water. While osmosis is enough for freshwater fish, saltwater fish also have to drink by swallowing water.

We know that fish cannot survive when they are not able to get enough water, but we do not know if being thirsty feels the same for a fish as it feels to us. This we will most likely not know until fish learn to talk to us or we learn to talk to fish - which will probably never happen!

This can be a bit disappointing, but we can also simply enjoy the feeling that there are still some mysteries in this world.

Even though we know so much about different types of fish, science cannot tell us everything, but this makes these animals only more amazing.

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