Can saltwater fish survive in freshwater? 0 72

Saltwater fish have a higher concentration of salts in their tissues than in freshwater and lower than in seawater. If a saltwater fish ends up in a river, freshwater will actively penetrate its body due to osmotic pressure.

The blood and other fluids will dilute, the cells will lose their functionality (in particular, muscles will be unable to contract) or even burst, leading to the fish's death. Freshwater species excrete excess water through their kidneys, while marine species, on the contrary, retain water and excrete salts, which exacerbates the situation.

For migratory fish, before entering rivers, the kidneys adapt from a “marine” mode of operation to a “river” mode. Some species, such as pike, are capable of living in both freshwater and slightly brackish water, changing the functioning of their kidneys.