External anatomy of a salmon Click to view larger image. Scientists don't know why salmon have this fin, but they share it with their relatives. Female salmon use this fin to dig their redds, or nesting sites. Fish detect pressure changes, water movement and vibrations through a series of small holes along each of their sides, called the lateral line. Salmon use their well-developed sense of smell, housed in their nostrils, to avoid predators and to find the streams of their birth when migrating home.
Scales protect the fish.
A layer of mucous that covers the scales adds further protection from disease and helps the fish slip through the water more easily.