Salmon are born freshwater, migrate to the ocean, and then return to freshwater to reproduce. However, populations of several species are restricted to freshwater throughout their lives. Various species of salmon display anadromous life strategies, while others show freshwater resident life strategies.
Many fish migrate on time scales ranging from daily to annually or longer and over distances ranging from a few meters to thousands of kilometres. Fish usually migrate to feed or to reproduce, but in other cases, the reasons are unclear.