How do Salmon breathe underwater?

Salmon breathe underwater the same way that other fish breathe: through their gills. Gills are feathery structures that are located on the sides of a fish's head. They are composed of thin filaments that are covered in tiny blood vessels. As water passes over the gills, the blood vessels absorb oxygen from the water and release carbon dioxide. This process is similar to how mammals breathe air, except that mammals use their lungs to absorb oxygen and release carbon dioxide.