Do fish drink water?

Is there anything more unpleasant than swallowing a mouthful of water when you’re swimming in the sea in the summer? You come out of the water coughing, with the salt stinging the back of your throat.

But then, how do fish that live in the ocean all the time manage? Do they drink? Do they get thirsty? Can they become dehydrated from taking in seawater?

To stay healthy, fish — whether they live in fresh water or salt water — must maintain a very precise balance between water and salt in their bodies.

In the ocean, bony fish have a lower salt concentration in their bodies than the water around them: their environment therefore has a higher salt concentration than they do. Conversely, in freshwater fish, the body has a higher salt concentration than the water in the lakes and rivers in which they live.

A matter of osmosis

This difference in salt concentration causes a phenomenon known as osmosis. This is a natural process that occurs when two aqueous solutions, with different salt concentrations, are separated by a permeable membrane. In this case, water moves spontaneously from the solution with the lower salt concentration to the one with the higher concentration, in order to restore equilibrium by diluting the more concentrated solution.

Seawater fish, whose bodies are less salty than the water around them, are therefore more prone to dehydration than freshwater fish, as they lose water continuously. They must therefore drink large amounts of seawater and excrete the excess salt through their gills. They urinate infrequently, but their urine is highly concentrated to prevent water loss.
In freshwater fish, the opposite happens: their bodies are saltier than the water, so they absorb water through their skin – again, this is a process of osmosis – and must therefore excrete more water to prevent themselves from swelling up. They drink much less but urinate much more, in order to get rid of the excess water.

Migratory fish, such as salmon and eels, spend part of their lives in freshwater and the rest in seawater. To meet this challenge, they have developed the ability to constantly adapt to their environment. Their bodies are capable of altering their internal ‘settings’ — a process known as osmoregulation — in order to maintain the balance between water and salt, regardless of the environment in which they live.
In mangrove forests, fish are also subject to significant fluctuations in salinity and possess a high degree of osmoregulatory flexibility.

And what about rays and sharks?

Another strategy among sharks

In the case of rays and sharks, which are cartilaginous fish, the strategy is different. The salt concentration in elasmobranchs is slightly higher than that of seawater. They therefore do not lose water continuously, as bony fish do, but absorb small amounts of it by osmosis through their gills.

Furthermore, they have the distinctive ability to accumulate urea in their tissues, which keeps their salt levels equivalent to those of the ocean. They also have a salt gland that eliminates excess salt.