Not unreasonably, there were multiple instances of colonization of the freshwater environment by seawater species of fish; some were more or less complete. The ability to escape an environment may have been seasonal, or periodic in some other way, or intermittent, and the ability to osmoregulate in freshwater need not have excluded the capacity to revert to a seawater mode of osmoregulation, as long as the capacity could be utilized by a substantial portion of the population, and selected for, rather than simply lost.
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.
William A. Wurts is an aquaculture specialist in Kentucky State University's cooperative extension program. He provides additional insight on fish evolution and physiology.
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 sea water 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.
It is believed that when the newly formed planet Earth cooled sufficiently, rain began to fall continuously. This rainfall filled the first oceans with freshwater. It was the constant evaporation of water from the oceans that then condensed to cause rainfall on the land masses, which in turn, caused the oceans to become salty over several billion years. As rain water washed over and through the soil, it dissolved many minerals--sodium, potassium and calcium-- and carried them back to the oceans.
Vertebrate animals (fish, birds, mammals, amphibians and reptiles) have a unique and common characteristic. The salt content of their blood is virtually identical. Vertebrate blood has a salinity of approximately 9 grams per liter (a 0.9 percent salt solution). Almost 77 percent of the salts in blood are sodium and chloride. The remainder is made up primarily of bicarbonate, potassium and calcium. Sodium, potassium and calcium salts are critical for the normal function of heart, nerve and muscle tissue.
If the salinity of ocean water is diluted to approximately one quarter of its normal concentration, it has almost the same salinity as fish blood and contains similar proportions of sodium, potassium, calcium and chloride. The similarities between the salt content of vertebrate blood and dilute seawater suggest a strong evolutionary relationship among vertebrates and with the primordial oceans.