Fish have an aquatic respiratory system that isn't the same as those of land animals. Fish can breathe under water which marine mammals, such as whales, can't do as they have to make frequent trips to the surface to take a breath. Fish have gills, which are their breathing organ. The gills are feathery and thin sheets of membrane that contain air vessels. Oxygen passes through the gills for fish to breathe.
Less Energy Used
Fish are cold-blooded, which means they take on the temperature of their surroundings. Fish use less energy than warm-blooded creatures. Warm-blooded animals must convert their food into energy to adjust their body temperature. As cold-blooded animals, fish do not need as much oxygen either. Fish use their gills to exchange oxygen and carbon dioxide.
Hot Water Versus Cold Water
Fish are sensitive to temperature because they are cold-blooded and can't keep a constant body temperature like humans and other mammals do. Most fish species can't survive in water that is too cold or too hot. When the water is too cold their metabolism becomes sluggish and they slow down. When the water warms up, their metabolism speeds up and they eat and digest food faster and grow faster. They also have more energy to reproduce. However, as the water warms, the fish can experience an oxygen squeeze. This means they need more oxygen to support their faster metabolism. This may not be possible, though, because warm water tends to be oxygen poor.
Breathing Slows Down
Fish don't have to breathe as fast in cold water because their metabolism has slowed down and simply doesn't require as much oxygen. Furthermore, cold water contains more dissolved oxygen than warm water so they don't have to work as hard to extract the oxygen. A decrease in water temperature automatically means a decrease in oxygen.
Oxygen From Water
Fish breathe the dissolved oxygen from the water using their gills. If there is not enough dissolved oxygen the fish can suffocate. A reduction in the oxygen content in the water can occur with a drop in temperature, the presence of contaminants, or stagnation of the current due to blockages or other obstructions.