Fish obtain oxygen from the water via their gills which are found at the side of the head, covered by the operculum plate. The gills are composed of finely branched filaments (which look like feathers) across which oxygen diffuses from
the water into the blood which is then pumped around the body. By actively pumping water using the mouth and gill cover (operculum), the fish ensures that water is constantly passing over the gill filaments. Except for catfish, which in addition to gills may have an air-breathing organ,
all fish require water to survive. When a fish is removed from the water the gill filaments collapse on one another and oxygen cannot diffuse across the filaments fast enough, so the fish ‘drowns’ due to a lack of oxygen.