Are there any species of fish that can live in saltwater and freshwater?

There are certain species of fish that can survive in both freshwater and saltwater.  Fish are actually categorized according to their salinity tolerance, which is essentially how much salt they can endure, or are accustomed to, in the water in which they live.  Fish that can tolerate only narrow ranges of salinity – like the goldfish, who can exist strictly in freshwater, or the tuna, who can exist only in saltwater – are known as stenohaline species.  These types of fish species will die if they try and exist in a body of water that has a salinity different than their own.  Fish that can tolerate a wider ranger of salinity at some point in their lives are euryhaline species.  Some fish, like salmon, eels, red drum, striped bass, and flounder, are examples of euryhaline species.  They can live or survive in waters that have a wide range of salinity, from freshwater (like certain rivers) to saltwater (such as the ocean).  Though they can survive, the fish will still needs a gradual period of acclimation to fully adjust to the new environment.  It is interesting to note that it’s been suggested that there is actually a competitive advantage to the fish when they are in their natural habitat (whether it be freshwater or saltwater).  For euryhaline species, it’s been proposed that theses types of fish are actually able to eliminate harmful parasites by moving from saltwater to freshwater and vice versa.  The varying habitats also allows for the fish to find new (or more) food and to even escape from certain predators. 

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