Ever wondered how a stationary fish sometimes sinks to the bottom and then rises to the surfaces? The reason behind it can be attributed to two forces of Nature gravity and buoyancy. Gravity is the force that pulls objects to the ground while buoyancy pushes them up in water. Gravity depends on mass while buoyancy depends on volume. So, how does the fish remain at one place if it has to, without being affected by those forces?
The fish is endowed with an organ called the swim bladder. The swim bladder is like an empty balloon. When the fish breathes, the bladder is filled with oxygen. So, the fish expands in volume but its weight does not increase much. So, this increases its buoyancy. When the fish exhales, the bladder loses the oxygen and shrinks in volume. Thus, the decrease in volume increases the force of gravity on the fish.
When the fish has to stay at one place, it breathes in a sufficient amount of oxygen. This slight increase in volume has the same weight as that of the fish. The two opposite forces are cancelled, and thus the fish remains stationary.
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