Species of fish that are able to live in both salt and fresh water are called euryhaline fish. Most species of fish are only able to handle one environment or the other, and this is based on their tolerance for salinity, which is how much salt their bodies can handle.
There are two types of euryhaline species, both of which are able to handle migrating back and forth between saltwater and freshwater.
Anadromous euryhaline fish are born in freshwater environments, but they spend most of their time in saltwater. Fish like salmon, smelt, shad, striped bass and sturgeon spend their lives in the sea, and they only return to freshwater to spawn. Catadromous euryhaline fish generally live in freshwater, but they go out to saltwater in order to spawn. North American eels and European eels are examples of catadromous fish.
When euryhaline fish transition from one salinity environment to the other, it takes them time to adjust. The fish undergo an acclimation period that allows them to adjust to the differences between the salinity concentrations of their bodies and their surroundings.
The word "euryhaline" comes from the Greek words "eurus," meaning "wide," and "halinos," meaning "of salt."
The opposite of euryhaline fish are stenohaline fish, which are those that can only survive within a narrow range of salinities.