At two years of age, Atlantic salmon undergo smoltification, resulting in changes in certain cells of the gills and kidneys that allow survival in either fresh or salt water, and brings about a silvery body color (better camouflage in the ocean). Salmon smolts (shown here) instinctively migrate downstream to the North Atlantic Ocean, and eventually to the cold waters off Greenland. Credit: U.S. Fish and Wildlife Service, National Conservation Training Center.