 Atlantic salmon are an important species in Maine. Studying their unique biology as a migratory fish allows us all to better understand how interconnected the ecosystems of Earth are. When most of us picture migration, we probably think of the return of songbirds and shorebirds to the north every spring for the breeding season. Migratory fish such as the endangered Atlantic salmon move between fresh and saltwater habitats for the same reason to spawn their young. Most species of fish have very specific requirements for either fresh or saltwater and couldn't survive a drastic change in the salinity around them. Fish like Atlantic salmon who migrate between these two worlds have unique adaptations that allow them to do this. The word diadromous refers to fish who regularly migrate between fresh and saltwater. Diadromous fish possess the ability to tolerate changes in the chemistry of the water around them. Salt excreting structures in the gills, internal changes in how they excrete waste, and taking on water at different rates all allow these unique fish to move between habitats. The two types of diadromous fish are catadromous and anadromous. Catadromous species hatch in the ocean. They then migrate into fresh water for the majority of their lives before adults return to the ocean to spawn. Freshwater eels are a great example of this kind of migratory life cycle. Anadromous fish start their lives as eggs laid in freshwater rivers. There they develop through the first stages of their life before migrating to the ocean. Anadromous species such as salmon, alewife, shad, and herring further grow and develop into adulthood there. These species are able to take advantage of the nutrient rich ocean before eventually returning to the specific freshwater rivers where they each began their lives. Maine is home to the last wild Atlantic salmon population in the U.S. After spending the first few years of their life in the rocky safety of a free flowing river, Atlantic salmon develop into smolt. These silvery fish head to the North Atlantic in spring. There salmon from a number of Maine's rivers are joined by fish from Canada and elsewhere along western Greenland where they feed and grow more quickly than in the river. When adult salmon reverse this migration they make the difficult trek back up river to reproduce. Salmon use their sense of smell along with other clues to return to the gravel bottom stream where they hatched to continue the cycle. Because endangered Atlantic salmon migrate between the ocean and freshwater river ecosystems, conserving these fish in both of their habitats has a huge impact for many species. Where their rivers still run without major barriers to migration, salmon enrich the watershed and the habitats they move through.