Freshwater fish cannot survive in the ocean, and rivers flow into the sea. Everyone knows that freshwater fish cannot live in salty seawater, so where do they go?
We all understand that the reason seawater has a “salty taste” is due to an average salinity of 35‰, which is why we refer to seawater as the “homeland of salt.” In contrast, the salt content in freshwater is only “thousandths of 3/3.” This difference is incomparable. Seawater is completely undrinkable without treatment, and the same goes for fish; freshwater fish cannot exist in the ocean.
The body structure of freshwater and saltwater fish differs. (Illustrative image).
There are many species of fish in nature. Based on their living environments, humans categorize them into two types: saltwater fish and freshwater fish. These two types of fish cannot interchange their habitats; if saltwater fish live in freshwater, they will die within two or three days, and the same applies to freshwater fish.
For example, humans drink treated freshwater daily. If we were to drink saltwater daily, everything we consume would taste salty, which is simply unacceptable for the human body. Fish live all day in freshwater; once they are in the sea, they will surely end up in a “pickled jar.” Do you think they can withstand that?
Because the body structures of freshwater and saltwater fish differ and their living environments are also distinct, they cannot survive in each other’s habitats. So what should fish do if they are forced to leave their natural habitats due to external influences? Is death the only option waiting for them?
Of course not.
Fish possess a survival skill: migration. When fish feel themselves being swept into the sea and the salinity around them increases, they enter a state of “migration,” often referred to as “upstream fish.” Freshwater fish tend to migrate upstream to escape dangerous conditions as soon as possible, allowing them to survive.
Fish life is incredibly diverse, with over 36,000 species known worldwide, including about 8,600 species of freshwater fish. Most fish live in either freshwater or saltwater, with less than 10% being migratory, moving back and forth between freshwater and marine environments.
Fish that grow in the sea but need to spawn in freshwater are called anadromous fish (for example, Chinese sturgeon), while those that grow in freshwater but need to spawn in the sea are referred to as catadromous fish (such as European eel).
As long as fish are alive, they will fiercely migrate and try to swim back to freshwater. (Illustrative image).
If migration occurs for feeding and spawning between rivers and lakes, it is called semi-migratory fish; these fish generally feed in lakes and spawn in rivers (such as the four major carp species). Some fish have a limited life in major rivers and their tributaries, migrating only a relatively short distance.
This means that as long as fish are alive, they will strongly migrate and strive to swim back to freshwater.
However, a small number of fish may drift into the sea; they have a strong survival ability and do not die immediately due to the salt content in seawater not being too high. If they do not swim back to freshwater in time, they will die from edema.
Even if rivers carry freshwater fish into the sea daily, they will sense danger and swim vigorously, even when faced with strong currents and unpredictable dangers; they will do their best to swim back. Ultimately, most freshwater fish will return to freshwater, so there’s no need to worry.
Additionally, there is a type of freshwater fish known as marginal freshwater fish, which can live in both brackish and saltwater. Saltwater fish occasionally enter rivers, but the total number of these fish and animals is not large, totaling around one hundred species.
