I was asked this question by my ever inquisitive seven year old. The same child who recently wanted to know about travelling at the speed of light and whether pigs dream; I’m always curious to see what his next big question is.

It’s a good question, fish live it water, they have it around them all their lives, but do they ever get that strong urge to drink, that urge we call thirst, or do they even drink at all?

The answer to these questions depends, in part, on where the fish live.

Fresh water fish

We all need water for our survival but did you ever wonder why we drink? Basically, we drink to maintain certain balances within our bodies, mainly a balance between water and salt.

Fish that live in fresh water (rivers, lakes, ponds and streams) don’t actually drink much, if any, water at all, at least, not in the way that we do. They don’t take much water in through their mouths, if they did they would risk over diluting their blood and unbalancing the salt-water balance within their bodies.

Fresh water fish have a higher concentration of salt in their blood and body tissue than in the water that surrounds them. They take small amounts of water into their bodies through their skin and their gills and then pass excess water out through urine.  The process that allows the water to pass into their bodies this way is called OSMOSIS.

What is Osmosis?

Osmosis is the process whereby molecules move from a solution of high concentration to an area of low concentration. These molecules move through a semi-permeable membrane in a passive manner (meaning that the process is spontaneous and does not require any additional input of energy) in order to equalize the concentration of the solution on either side of the membrane.

When we apply this to our fish, what it really means is that the water moves through its skin (the semi-permeable membrane) to dilute the salt in the fish’s body and create a more equal balance of salt and water between the body of the fish, and the water it lives in.

Saltwater fish

When fish live in salt water (oceans and seas) things are reversed; the concentration of salt in the water they live in is higher that the concentration of salt within their bodies. Osmosis therefore leads to water moving from their bodies to the water around them. This puts them at a constant risk of dehydration, which seems ironic as they live in water.

To compensate for this, the fish must actively drink water through their mouths. They process the water and then produce small amounts of salty urine as well as the secreting salt through specialised cells in their gills.

If they drink does that mean they get thirsty?

So salt water fish do drink water, quite a bit of it in fact, does this mean that they get thirsty? The answer is still no; as they live in water they probably don’t drink as a conscious response to seek out and drink water. Thirst is usually defined as a need or desire to drink water. It is unlikely that fish are responding to such a driving force.

What about salmon?

Some fish may live in both fresh and salt water; salmon, for example, pass from fresh water, to salty water and then back again, at different stages of their life cycle. Such fish are called euryhaline, but how do they cope with these changes? Their bodies switch from one process to another.

When salmon move from fresh water to salt water they start to drink a lot of water and reduce the amount of urine they excrete. Specialised cells within their gills then start to pump salt out of their bodies. All these changes happen over the course of several days, usually while the fish is in the intertidal zone.

When the salmon return to fresh water near the end of their life, these transitions are reversed.

Getting back to the question

So, as is often the case, the question leads to other questions and has more than one answer. But NO, fish do not get thirsty (not in the way we do anyway) and some of them don’t actually even drink water at all.

If you have anything to add let me know in the comments below. And, as always, feel free to send on your own questions, or those of your children, and I will do my best to answer here!

Images sourced from Pixabay.com