After the river flows into the sea, where did the freshwater fish go?

River water carrying freshwater fish will pour into the sea. The average salinity in seawater is 35‰ so it is not drinkable without too much treatment and freshwater fish cannot survive in the ocean. So, where did the freshwater fish go or are they going to die?

Based on different habitats, humans divide fish species in the wild into two types, saltwater fish and freshwater fish.

If two types of fish exchange habitat, they will die because the body composition of freshwater fish and marine fish is different, and the habitat is also different.

There are more than 36,000 known species of fish in the world, including about 8,600 freshwater fish. Less than 10% of migratory fish migrate back and forth between freshwater and marine habitats.

There are some fish that grow in the sea but need to swim to fresh water to spawn (such as the Chinese sturgeon) called infinity migration. And vice versa, there are also fish that grow in freshwater but need to reproduce in the sea called downstream migrating, for example flower eels.

Even if the river washes freshwater fish into the sea every day, they will sense the danger and try to swim again even when faced with fast currents. Eventually, most freshwater fish will return to freshwater.

In case they do not swim back to fresh water in time and are washed out in salt water, they will die from edema.

Isabella Humphrey

Update 27 June 2023