What Happens if You Put a Saltwater Fish in Freshwater? (The Facts)

Have you ever wondered why most marine species of fish can’t simply explore inland areas by swimming through rivers and lakes? In the same way, even the most starved freshwater fish can’t migrate into our nutrient-rich oceans for food. They are physically and chemically incapable of tolerating the significant differences in salinity levels, which influence the saturation of their cells.

It’s true, there are some exceptions, such as the Pacific salmonids and the freshwater-tolerant bull shark, which naturally migrate between saltwater and freshwater bodies throughout their life cycle. These fish are equipped with evolutionary adaptations for regulating how much water and salt are stored and excreted by their bodies. While a few months in a river or lake may not be the worst experience for them, this same scenario can spell disaster for other saltwater species.

A true saltwater fish can quickly become uncomfortably engorged with water in a freshwater environment. This unfortunate effect, a phenomenon that is best described by the natural law of diffusion and osmosis, is a major limiting factor for habitat selection. This same law explains why the skin of our fingers shrivels up after a long bath or why we tend to get thirsty after drinking salt water.

A Saltwater Fish Would Bloat in a Freshwater Environment

If you attempt to raise a saltwater fish in a body of freshwater, such as a pond or an aquarium, you’ll find that it should quickly display signs of being bloated. A large amount of water would force its way into the fish’s body, which is not equipped with an efficient means of expelling the excess fluids.

Unlike those of freshwater species, the bodies of saltwater fish are highly concentrated with salt. The difference between the salt concentration in their bodies and that of their salt-free environment would cause the water to flow through their skin, gills, and mouth, into their bodies. Based on the law of osmosis, this direction of water flow must be sustained until the salt concentration in the fish is equal to that of the surrounding water.

The rate at which the saltwater fish is able to urinate to expel the excess water simply can’t match the rate at which freshwater would be absorbed by its body. Inevitably, it would die in an attempt to regulate its internal water levels.

What Is Osmosis and Osmoregulation?

Osmosis describes the movement of liquid molecules, via a semipermeable membrane, from one liquid solution to another. When water moves in and out of cells, via the cell membrane, the concentration of salts in either the cell or its environment can change. If both solutions are isotonic, which means they have identical salt concentrations, no water exchange occurs.

If one solution is hypertonic, which means it has a higher salt concentration that the other, osmotic pressure increases and water moves through the membrane. If the solution is hypotonic, it has a lower salt concentration than the other and is more likely to lose instead of take up water.

Take, for example, a dehydrated piece of fruit that is dropped into a glass of distilled water. Over time, the “hypotonic” water should flow into the fruit, causing it to swell. The osmotic pressure should lower the salt and sugar concentration of the fruit, supposedly until it is identical to that of the water around it. Conversely, if a piece of fruit is dropped into a “hypertonic” solution with a high salt concentration (e.g. supersaturated saltwater), it would shrivel up.

Osmoregulation in Saltwater Fish vs. Freshwater Fish

In a marine environment, saltwater is hypertonic to fish, which possess a lower salt concentration in their bodies. Thus, water would naturally move through the semi-permeable membranes (i.e. gills and skin) of, say, an anemonefish or a barracuda, and out into their bathing environment.

Of course, saltwater fish can’t afford to dry out and shrivel up into raisins! To make up for the constant loss of fluids, they must constantly drink saltwater. While a considerable amount of salt becomes stored in their bloodstream, they filter out the remainder through their gills or by urinating. Both hydration and urination thus play a key role in the osmoregulation of saltwater fishes.

In a freshwater environment, the water is hypotonic to freshwater fish. It is naturally taken up by a largemouth bass or a catfish, which would have to frequently urinate as a means of expelling excess fluids. These fish don’t necessarily have to filter out salt, as it is not present in the water. Ill-equipped with adaptations for regulating internal salt levels, they are unable to survive for long periods of time in saltwater. A freshwater fish would, unfortunately, die of dehydration in a marine environment.

When placed in a hypotonic, freshwater environment, can’t saltwater fish simply urinate to expel excess fluids?  No, because the osmotic pressure between the low-concentration water and saltwater fish is too high. Freshwater would move into the fish’s saturated bloodstream at too fast a rate for it to be expelled. In theory, the saltwater fish’s cells would burst before it could expel the excess water.

The Exceptions: Euryhaline Fish

Euryhaline fish have adapted to survive in both saltwater and freshwater environments. These include migratory species like salmon and trout, which venture into rivers and streams to spawn. Able to osmoregulate across an impressive range of salinities, they have a host of physiological modifications for feeding at sea and reproducing inland.

Estuaries, which tend to have highly variable salinity levels, are inhabited by many euryhaline species of fish and aquatic invertebrates. Some of these animals are able to retain more salts to combat seasonal increases in salinity levels. When they venture further into freshwater systems, they have certain enzymes that are activated to expel excess salts, reducing the osmotic pressure between the hypotonic water and their cells.

However, it takes time for these physiological and behavioral modifications to develop each year. Even a mature salmon can’t simply be pulled out of a saltwater tank and dropped into a freshwater stream. Euryhaline fish need time to acclimate to significant changes in water parameters. Otherwise, they too may show signs of stress and bloating.