Can Saltwater Fish Live in Freshwater?

Saltwater fish can't survive in freshwater because their bodies are highly concentrated in a salt solution, as compared to freshwater fish (that is too much salt for freshwater). The water would flow into their body until all their cells accumulate so much water that they bloat and die eventually.

Fish that live in saltwater will have somewhat salty water inside themselves. Put it in the freshwater, and the freshwater will, through osmosis, enter the fish, causing its cells to swell, and the fish will die.

Fish that can tolerate a wide range of salinity at some phase in their life cycle are called euryhaline Species. These fish, which include salmon, red drum, striped bass, and flounder, can live or survive in a wide range of salinity, varying from fresh to brackish to Marine waters.

There are certain species of fish that can survive in both freshwater and saltwater. The species of fish, called euryhaline fish, can tolerate and migrate in both bodies of water at or for a certain amount of time.

There are two types of euryhaline fish, anadromous and catadromous

Anadromous fishes are born in freshwater, but spend most of their lives in the ocean, only returning to freshwater to lay their eggs. Examples of these fishes include striped bass, sturgeon, smelt, and salmon.

Catadromous fish, on the other hand, live in freshwater but have to enter the sea through rivers to spawn. An example of catadromous fish is the European Eel.

Tonicity

The reason freshwater fish cannot survive in saltwater and vice-versa has a lot to do with the property of any liquid called tonicity. It is the ability of a solution to exert osmotic pressure upon a membrane.

Tonicity comes in three types: hypertonic, hypotonic, and isotonic.

A hypotonic solution has a lower concentration of solutes (solutes are the substance dissolved in a solution eg. Sugar is the solute in a Sugar solution) inside the cell than outside of it. On the other hand, a hypertonic solution has a higher concentration of solutes outside of the cell than outside of it. On the other hand, a hypertonic solution has. A higher concentration of solutes outside of the cell than inside it. 

Seawater is hypertonic to the fish living in it, which means that the salt content of the surrounding water is higher than the content inside the fish. As a result, they lose the water inside their body to the surrounding seawater due to osmosis.

On the other hand, freshwater is hypotonic to the fish that live in it, i.e the salt content in their body is higher than the salt content of the water surrounding them. Due to osmosis, therefore water continuously flows into their body ( the area of high solute concentration, salt being the solute, in this case.