Salmon is a general term for three kinds of fish: salmon, trout and cod. In addition, Arctic grey trout, Atlantic herring, North American herring and Arctic cold trout also belong to Salmonidae. Scientists have proved through the study of paleontological fossils that salmon existed on this earth more than 100 million years ago. During Devonian, boneless fish gradually evolved into later bony fish through evolution. So far, scientists have found eleven series of sixty-six kinds of salmon from the structural characteristics of fish through taxonomy. All cold-water fishes belonging to the family Salmonidae lay eggs and hatch in fresh water.