Fish that live in seawater all year round can't be kept alive by transferring them to freshwater.

Fish on the water environment requirements are very high, the salinity of the water adaptability is very large, a variety of fish need to live in different salinity of the waters, which is related to the history of thousands of years and the regulation of fish physiological mechanisms, the regulation of these physiological mechanisms can only be limited to a certain degree of salinity, the salt concentration of seawater is generally 35%, the salt concentration of freshwater is only 0.3% or so, the gap is quite large. The difference is quite large.

If a marine fish is put into freshwater, its filtration and regulation mechanisms cannot immediately adjust and adapt. The cytoplasm of the marine fish will decrease in concentration and volume because of the difference in concentration, and finally it will absorb water and swell up and die. On the contrary, if freshwater fish are put into seawater, what will happen? Freshwater fish live in freshwater, the cytoplasm and the body's regulatory mechanism has formed an osmotic equilibrium with freshwater, if the freshwater fish into the salt water, then the fish body cells in the cytoplasm of the water will be released because of high salt, constantly through the gills respiration and the surface of the body for the release of, and finally is also dehydrated and died.

Freshwater salt content is low, the density is also low, freshwater fish body tissue salt concentration is higher than the living environment of the water salt content. We all know the principle of osmotic pressure, fresh water enters the fish's body and then the excess water is excreted through the kidneys. Whether it is swimming from fresh water to sea water or sea water to fresh water, the osmotic pressure between the fish's body and the environment changes when it enters from one environment to the other, and the kind of fish produced by living where fresh water meets sea water has to reproduce and grow within two different environments, and this kind of fish has to regulate the osmotic pressure between the inside and the outside of the body by regulating the osmotic pressure, and it also has to change its body surface mucus layer to adapt to the changing environment.

So, if the fish that live in seawater all year round, transfer it to freshwater, it can not survive, after a while will die, in the raising of seawater fish need to be raised with a salt concentration close to seawater.