How Do Fish Breathe?
Like humans and other animals, fish need to take in oxygen. But they get it in a different way--through their gills, which are located on either side of the fish, just behind the mouth. When fish open their mouths, water comes in; when they close their mouths the water is pumped over the gills. (This is how it works for most fish; sharks and some other species don't have as good a pumping system and so they need to keep swimming to force water over the gills.)
The gills have surfaces with many tiny blood vessels--capillaries--and when the water passes over these surfaces, the oxygen in the water passes into the blood of the fish (as it does in people's lungs). The gills are delicate structures, held up by the water. When a fish is taken out of water, the gills collapse and the fish suffocates.
Ciencia: Preguntas y respuestas: ¿Cómo respiran los peces? (Spanish Version)