Why Don’t Fish Sink?
Fish have adapted to life in water. They are able to move in water, of course. Their body density is slightly smaller than the water in which they swim. Because of that, they are like the water itself and they do not sink.
A fish, with its bones and scales, is heavier than the water it displaces. Under normal circumstances, it would tend to sink. But most bony fishes have a balloon-like sac inside their bodies called an “air bladder” that acts as a float to keep them from sinking.
The air bladder fills up with some of the oxygen dissolved in the fish’s blood. This organ’s membrane is full of capillaries. The function of the air bladder is helping fish balance themselves. Fish balance themselves in the water filling and emptying the air bladder.
When fish float intentionally, the air from the bladder transfers into the blood and is being released from the gills. The opposite thing happens when fish sink into water. This happens constantly.
Some fish, such as pike and catfish, gulp air at the surface of the water to fill their air bladders. However, some species of fish don’t have air bladder.
These fish always have to swim or they would sink. A shark has no air bladder to buoy it up. It must constantly be swimming in order to keep from sinking.