How do elasmobranchs regulate buoyancy?

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