When the fish wants to float, the swim bladder is inflated, so that the individual density of the fish is reduced. When it is smaller than water, the fish will float. On the other hand, when the fish wants to sink, it will shrink the swim bladder and increase the density to meet the sinking demand.

The swim bladder is the "lifebuoy" for fish swimming. The swim bladder can freely adjust the specific gravity of fish by inflating and deflating. In this way, when swimming, the fish can maintain a stable state and not sink or float in the water with minimal muscle activity. However, when the fish sinks to a certain depth (that is, the critical depth), the huge external pressure will make the volume of the swim bladder unable to be adjusted. At this point, its buoyancy is less than its own gravity, so it will involuntarily sink to the bottom of the water, can not float up again, and will eventually drown because it can not breathe. Although fish can still float upward by swinging their fins and tails, it won't help if they sink too deep.