How can a fish survive in water with the oxygen in it only 4 percent of what is in the air?
Unlike mammals with lungs, fishes have gills as the respiratory organ. Gills are a series of membranes loaded with blood capillaries, located on each side of the head of the fish. As water passes over this system of extremely fine gill membranes, the contact area for oxygen exchange increases dramatically. Aided by a highly efficient water pumping system that involves the mouth and the outer cover of the gills, called the operculum, the fish gallops copious amount of water per unit time and this makes up for the low percentage of available oxygen. This way the fish can actually extract and absorb up to 85% of available oxygen out of the water. Since water contains only ~4% of the available oxygen that air at sea level does, such a high efficiency is extremely critical for the survival of fish in the watery environment.