There are 2 possible reasons.

The first is some sharks have to keep swimming because they need to in order to circulate blood.

The second being that sharks don't have swim bladders to keep them buoyant in the water. They rely on the oil in liver. So some start to sink (usually the bigger sharks) when they don't swim. I believe due to their weight verses the amount of oil in their bodies.

You are correct, in the first answer. Sinking to the bottom of the ocean when no longer swimming has almost nothing to do with their bladders, it is because they have to continuously swim in order to keep their blood circulating.


They do in fact rely on the oil in the liver, but when they sink, it is because it is natural for the shark body to do whatever it takes to keep the blood circulating. And there is more oil in their bodies than almost any other substance, therefore, they do not sink due to "The weight verses the amount of oil in the body."

I hope i was of help. :-)

Sharks don't have swim bladders to help them stay up in the water. To keep from sinking, they must always swim in a slightly upward direction. If they stop swimming, the weight of their bodies pulls them down to the bottom.
hope that helped :):)