Appliance of Science: Do fish ever get thirsty?

Dr Naomi Lavelle answers a weekly science question.

Appliance of Science: Do fish ever get thirsty?

Dr Naomi Lavelle answers a weekly science question.

Fish live in water all their lives but does that mean that they never get thirsty or do they even drink at all?

To answer that question we need to look at where the fish live.

Fresh water fish

We all need water for our survival; we drink it to maintain certain balances within our bodies, mainly a balance between water and salt.

Fish that live in fresh water don’t drink much 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.

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 the fish’s 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 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 secreting salt through specialised cells in their gills.

If they drink does that mean they get thirsty?

So salt water fish do drink water, does that mean that they get thirsty? The answer is still no; as they live in water they probably don’t take it in 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 can 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. Their bodies switch from one process to another to cope with the changes in salt concentration. 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 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.

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