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The paradox of fish: They live in the sea all the time, but why aren't they salty?

2026-08-26 23:26:11Lifestyle SHKRUAR NGA REDAKSIA VOX
The paradox of fish: They live in the sea all the time, but why aren't they
It took scientists more than a century to understand the mechanism

There are many ways to add salt to fish in the kitchen: from frying it with a few grains of salt to traditional salting, marinating, or other preparations. But an interesting question is: why do saltwater fish, which spend their entire lives in saltwater, not taste salty?

The answer lies in a biological mechanism called osmoregulation, through which the body controls the amount of water and salts in its fluids. The vast majority of fish we know, including sea bream, sardines, cod, and tuna, are teleosts.

They are able to keep their blood and body fluids much less salty than seawater. This balance is vital. An excessively high concentration of salt in the blood would impair the normal functioning of cells and organs.

For fish, as for humans, an excess of salt in the body can cause serious problems, especially in the nervous system. In the late 19th century, Belgian physiologist Léon Fredericq was among the first scientists to try to understand this phenomenon.

He analyzed the blood of marine fish and noticed that it was much less salty than seawater. The question was simple, but the answer turned out to be extremely complex: how could an animal constantly immersed in salt water live without becoming suffocated?

Part of the answer lies in the gills. Fish take in oxygen through their gills, which allow water to pass through. But seawater has a much higher concentration of salts than the body fluids of fish. Through osmosis, water tends to move out of the body.

So, paradoxically, a sea fish can become dehydrated even though it is surrounded by water. To compensate for the loss, the fish must drink seawater. In 1930, physiologist Homer Smith was able to show experimentally that sea fish do exactly that.

He added a dye to the water the fish were swimming in and then found it in their digestive system, proving that the water entered through their mouths. But another question remained: if the fish were drinking seawater, how did they get rid of the excess salt?

For years, scientists suspected the answer lay in the kidneys. But tests didn't show sufficient concentrations of sodium and chloride, the main components of salt. Attention turned to the gills.

An important step was taken in 1957, when Danish scientist Jens Christian Skou discovered the sodium-potassium pump, a protein that uses energy to control the movement of sodium and potassium across cell membranes.

It was later discovered that this pump is specifically present in the gills of fish. Subsequent studies led to a much more complex model of how the gills remove excess salts.

This mechanism, known as the Silva model, explains the main part of the osmoregulation process in many marine fish. However, not every detail has been resolved, and differences between species continue to present scientists with new questions.

A new concern is the changing oceans. Global warming is increasing the temperature of the waters, while changes in salinity and ocean acidification may affect the way fish regulate their internal balance.

Studies on Atlantic cod have shown that under such conditions, osmoregulation may require more energy. This means that a greater proportion of the fish's energy must be spent on maintaining salt balance, leaving fewer resources for growth, reproduction and other vital functions.

So the reason why sea fish is not salty is not that it cannot cope with salt. On the contrary, its entire life it fights to prevent sea salt from disrupting its internal balance.

And it is precisely this extraordinary mechanism that allows it to live in an environment where, without osmoregulation, its body would constantly lose water and fill with salt.


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