How do sharks control buoyancy?

Sharks do not have swim bladders as bony fish do.  Therefore, in order to control buoyancy sharks rely on their liver, which comprises 30% of their body weight and is filled with an oil called squalene, and the fact that cartilage is about half as dense as bone.  However, sharks also need to use something called dynamic lift in order to keep themselves from sinking.  Dynamic lift is when a shark uses its pectoral fin, much as birds use wings, to maintain its depth.