The salt content of freshwater fish is equal to that of fresh water, and the salt content of fresh water is lower than that of sea water. If you put fish in fresh water into sea water, because the osmotic pressure of sea water is higher than the osmotic pressure of fresh water fish, the water of the body of fresh water fish (inside cells) will flow into seawater and the fish will eventually dehydrate and die.
The first is the question of salt content. Different marine fish adapt to different environments. Although the Earth's oceans are connected, the salt content of sea water in different areas is also different. In addition, the salt content of the salt water lake is also relatively large. If the salt content of the two places is not much different, then it is possible that marine fish survive in the saltwater lake. Necessary environmental conditionsto the survival of marine fish involve not only salt content, but also physical and chemical environmental issues such as acid and alkali, microbial content and dissolved oxygen content of the aquatic environment. If these parameters of the two environments are not significantly different, marine fish might be able to adapt to saltwater lakes. If there is a big difference between the two in some parameters, the saltwater fish will not be able to survive in the saltwater lake. Of course, there is one particular exception on earth: Lake Baikal, on the Asian continent. Lake Baikal is a freshwater lake, but many marine species live there. Many scientists are unaware of the origin of these sea creatures.
In a broad sense, fish that can live in fresh water with a salinity of three thousandths can be called freshwater fish. In the narrow sense, it is pgoslings that have only part of their biological history, such as the "juvenile stage" or the "adult stage", or that must spend their entire lives in fresh water. There are approximately 26,000 known species of fish in the world and approximately 8,600 species of freshwater fish. There are nearly 3,000 species of fish in China, including more than 1,000 species of freshwater fish. There are approximately 26,000 known species of fish worldwide, making up the largest category of vertebrates, accounting for approximately 48.1% of the total number of vertebrates. Most of them live in the ocean and there are around 8,600 species of freshwater fish.
Strictly speaking, river fish should be called freshwater fish. Common fish include carp, crucian carp, grass carp, eel, etc. ; marine fish include hairtail, tuna, large yellow croaker, small yellow croaker, etc. . The expertsline that whether they are freshwater fish or marine fish, their nutritional components are approximately the same and their overall nutritional value is very high. However, marine fish contain more nutrients than river fish and their nutritional value is slightly better.