Biologists at Leavenworth NFH prepare to remove a tag from an adult Chinook Salmon. Service fisheries personnel track the fish returning to spawn each year, and use this information to assess the health of native salmon stocks. To the five species of Pacific salmon (chinook, chum, coho, pink, and sockeye), it is a long, strenuous, desperate race against time, with every obstacle taking its toll. Migration between fresh and salt water occurs during every season of the year, depending on latitude and genetic characteristics of the fish. Groups of fish that migrate together are called runs or stocks. Salmon spawn in virtually all types of freshwater habitat, from intertidal areas to high mountain streams. Pacific salmon may swim hundreds, even thousands, of miles to get back to the stream where they hatched.