Do Fish Drink Water? Saltwater vs Freshwater Fish Explained

Why do saltwater fish drink seawater?

The answer lies in osmosis, the natural movement of water through thin, permeable surfaces from a less salty liquid toward a saltier one. Fish gills are covered in very thin tissue so that oxygen can pass into the blood easily, and water can pass through it too.

The body fluids of a typical bony fish are only about one-third as salty as seawater, which contains about 35 g of salt per liter (about 3.5%). So in the sea, water is constantly drawn out of the fish through its gills and skin. Without doing something about it, a marine fish would slowly dry out, even while surrounded by water.

To make up for this, sea fish drink steadily. The seawater they swallow passes into the gut, which absorbs the water together with much of the salt. The fish then has to get rid of that salt. Special cells in the gills, known as ionocytes or chloride cells, pump sodium and chloride out into the sea. The kidneys deal with other salts, such as magnesium and sulfate, and produce only a little urine, so as little water as possible is lost.

The American physiologist Homer Smith showed in 1930 that marine bony fish really do drink seawater, a finding that laid the foundation for much of what we know about how fish balance salt and water.

Why don’t freshwater fish drink much?

In rivers and lakes, the situation is reversed. A freshwater fish’s body fluids are far saltier than the water around it, so water flows into the fish through its gills and skin all the time.

Freshwater fish therefore drink very little, apart from the water they swallow by accident while feeding. Instead, their kidneys produce large amounts of very dilute urine to pump out the extra water. Because some precious salts are lost along the way, the gills work in reverse compared with those of a sea fish: their ionocytes take sodium, chloride and calcium in from the water. Food provides more salts.

How do salmon switch from rivers to the sea?

The Atlantic salmon has to cope with both systems in one lifetime. Young salmon spend one to four years or more in fresh water, then swim to the sea, and adults later return to their home river to spawn.

Before leaving the river, young salmon go through smoltification, a set of changes that prepare them for salt water. Their gills grow many more salt-pumping ionocytes and make more of an enzyme called sodium-potassium ATPase, which powers the pumps. They begin to drink, their kidneys adjust, and their body turns silver. Smolts often pause in the brackish water of the estuary, where river and sea mix, as these changes finish.

When adults return to fresh water years later, the process reverses. They stop drinking, their gills switch back to taking salt in, and their kidneys produce more dilute urine.

How do eels manage salt and fresh water?

The European eel makes the same switch, but in the opposite direction. Eels hatch in the Sargasso Sea in the western Atlantic, drift across the ocean as larvae and arrive on European coasts as small, see-through glass eels. Many then move into estuaries and rivers, where they may live for years before turning silver and swimming back to the sea to spawn, as told in Where do eels come from?

Eels are unusually flexible. They can live in fresh, brackish or salt water, and some spend their whole lives in estuaries or coastal lagoons. Because they cope so well with changes in saltiness, eels have long been a favorite animal for studying how fish handle salt. In 1932, scientists first described the salt-secreting cells of fish gills in eels, and eels are still used to study the hormones that control drinking.

What about sharks and rays?

Sharks, rays and their relatives solve the problem differently. Instead of keeping their blood far less salty than the sea, they fill it with urea, a waste product most animals get rid of, and a protective substance called TMAO. This makes their body fluids slightly saltier than seawater, so water seeps in slowly rather than leaking out, and they do not need to drink much. Extra salt leaves through a special rectal gland near the end of the gut.

A few sharks can even enter rivers. Bull sharks lower the amount of urea in their blood when they move into fresh water, as explained in Can bull sharks live in fresh water? Hagfish, jawless fish-like animals of the sea floor, take yet another route: they simply keep their body fluids about as salty as the sea around them.

Can any fish live in both salt and fresh water?

Most fish cannot. They are stenohaline, meaning they can survive only within a narrow range of saltiness. A goldfish dies in seawater and a typical reef fish dies in a river, because their gills and kidneys cannot reverse their work.

A smaller group, called euryhaline fish, can adjust. This includes fish that migrate between rivers and the sea, such as salmon, sea trout, shad, sturgeon and eels, as well as sticklebacks, many flounders, some tilapia and bull sharks. For these fish, drinking or not drinking is not fixed: it depends on where they are swimming.