Why and how can a fish swim

Pressure and buoyancy

Fish can - apparently without much effort - stay in a certain water depth. They do this because they have a swim bladder whose volume they can change.

If the weight of the fish is equal to its buoyancy, the fish floats. According to Archimedes, the buoyancy force is equal to the weight of the displaced liquid. The weight of the displaced liquid is in turn directly proportional to the volume of the fish.

If a fish floats and pumps air into its swim bladder, its volume and thus the buoyancy increases. Since its weight stays the same, the fish will rise. As a rule, however, the active inflation of the swim bladder is not necessary at all, because by moving the fins slightly, it reaches a shallower water depth in which the water pressure is lower and thus the gas-filled swim bladder is no longer compressed so much and its volume increases automatically. When climbing more strongly, the fish even has to release gas from the air bladder so that the enlargement of the swim bladder caused by the decreasing water pressure does not cause it to climb too quickly.

If gas is withdrawn from the swim bladder in a floating fish, the buoyancy force decreases and the fish sinks. The increasing water pressure at greater depths then reduces the volume of the swim bladder even further, so that the sinking accelerates and the fish has to push gas into the swim bladder again in order not to sink too quickly.
The sinking and rising of the fish is mainly initiated by the movement of the fins and the balance is established by the pressure in the swim bladder.