What gives a fish its buoyancy?
May fish have a little air bag inside them called a swim bladder they add or remove air from this sack so that they achieve neutral buoyancy in the water and thus do not have to exert energy to prevent themselves floating or sinking. If you bring a fish from great depth to the surface very quickly, the fish will not have time to remove air from the swim bladder and it will explode, killing the fish.When divers collect fish at depth, to keep them alive, they have to stop and let air out of the swim bladders (with a needle) on the way up.