Salmon undergo radical metamorphosis as they go through life stages from hatching in streams, spending most of their adult life out at sea, to returning to their places of birth to spawn. Their metamorphosis is one of the greatest individual adapatations to environmental change. Depending on its sex, by the time Coho salmon are ready to spawn, the smooth silvery fish you've seen in the fish market has changed into a red, humpbacked, and hook-jawed one.
To learn more about the life cycle of salmon:
University of Washington Research & Teaching Hatchery