These migrating fish stop eating before they enter fresh water and never eat again-they rely on stored body fats and protein for their energy. The hormones of the salmon also change as they enter the freshwater rivers and streams, causing physical alterations in the fish. The physical changes vary between species of salmon and the amount of time the fish are in fresh water. The snouts of the females elongate slightly and they swell in the abdominal area. The males of some species develop a hump near the dorsal fin, the snout grows longer, and the upper jaw elongates and becomes hooked.
Some species undergo incredible changes in colouration.
The heads of sockeye salmon, for example, turn green and their backs change to a bright red. At first, salmon are thought to use magnetic cues to begin their journey home.