Fish remain alive in water as they take in oxygen from their gills just as human beings take in oxygen with the help of lungs. Since the content of oxygen can reduce in the water, fish sometimes suffocate and drown in water. Small fish ponds dry up frequently giving way to fish drowning as it has so many fishes that fight for oxygen content. In underwater fights or collision, damaged gills stop fish to take most of the oxygen leading to drowning.
Low levels of oxygen in the pond force fish to go up to the surface and intake oxygen from the air. In fish, the air they take from the oxygen is stored in a sac-like organ, called air bladder. This, however, is not possible in extremely cold conditions as the upper layers are frozen and fish cannot go up to breathe.
