The reason some fish normally live in freshwater and others live in seawater is that one or the other environment provides them with opportunities that have traditionally contributed to their survival. An obvious difference between the two habitats is salt concentration. Freshwater fish maintain the physiological mechanisms that permit them to concentrate salts within their bodies in a salt-deficient environment; marine fish, on the other hand, excrete excess salts in a hypertonic environment. Fish that live in both environments retain both mechanisms.
The various species of fish found in oceans, lakes, rivers, and streams have evolved over millions of years and have adapted to their preferred environments over long periods of time. Fish are categorized according to their salinity tolerance. Fish that can tolerate only very narrow ranges of salinity (such freshwater fish as goldfish and such seawater fish as tuna) are known as stenohaline species. These fish die in waters having a salinity that differs from that in their natural environments.
Fish that can tolerate a wide range of salinity at some phase in their life-cycle are called euryhaline species. These fish, which include salmon, eels, red drum, striped bass, and flounder, can live or survive in wide ranges of salinity, varying from fresh to brackish to marine waters. A period of gradual adjustment or acclimation, though, may be needed for euryhaline fish to tolerate large changes in salinity.
Salmon spend a relatively short time in freshwater before developing the capacity to osmoregulate in seawater, where they live for the majority of their lives. Some species of salmon, like pink salmon, migrate to sea as soon as they emerge from the gravel as free-swimming juveniles. Others, such as sockeye and coho and some chinook salmon, remain in freshwater for one or two years or more before the urge to migrate downstream overcomes them, in a sequence of physiological and physical events that coincides with the development of their capacity to osmoregulate in seawater. So the different species of salmon exploit different aspects of the freshwater environment, but evidently they all enjoy better life prospects if they are spawned in a freshwater habitat and spend their adult lives in seawater.
Other related species, like trout, are physiologically less tolerant of salty water. Most have permanently adapted to life in freshwater. They have probably also lost characteristics (e.g., mating behaviors) that might enable them to lead a successful life in the marine environment. For reasons that may relate to their geographic distribution, the characteristics that once made life in seawater natural to them eventually became excess baggage and fell into disuse and disrepair.
In seawater, fish must drink salt water to replace lost fluids and then eliminate the excess salts. Their kidneys produce small volumes of fluid containing high concentrations of salt. Freshwater fish produce large volumes of dilute urine, which is low in salt. Less demand is placed on the kidneys to maintain stable concentrations of blood salts in brackish or low salinity waters.
Via: Scientific America, Why do some fish normally live in freshwater and others in saltwater? How can some fish adapt to both?