Why are there freshwater fish and saltwater fish?

There are about 22,000 species of fish in the world, which are distributed in almost all saltwater or freshwater environments that have not been seriously polluted. These fish living in oceans, lakes, rivers and streams have experienced millions of years of long evolution and have become accustomed to their different living environments. Different fish have different physiological mechanisms: freshwater fish live in salt-deficient waters, so they need to accumulate salt in their bodies; while saltwater fish, on the contrary, live in a hypertonic environment, so they must excrete excess salt. Fish that can live in both freshwater and saltwater are even more amazing. They have both the physiological mechanisms of salt accumulation and salt excretion!

In fact, fish are classified according to their salt tolerance. Fish that can only survive in waters with a narrow range of salt are called stenohaline fish; freshwater fish such as goldfish and marine fish such as tuna belong to this category. Fish that can survive in waters with different salinities are called euryhaline fish, such as salmon, eels, and the eye-spotted drum found on the Atlantic coast of North America. They can migrate from freshwater areas to slightly brackish waters, and from slightly brackish waters to very salty waters - of course, if the salinity changes greatly, they need a period of adaptation.

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