Buoyancy

    Since fish live in an environment in which they need to move in three dimensions, buoyancy plays a significant role in determining a fish's ability to swim efficiently.  Fish use a couple of different strategies to solve this problem.  Denser fish use their pectoral fins to create dynamic lift, similar to planes and birds. As these fish swim, their pectoral fins are positioned in such a way as to create a difference in pressure which allows the fish to maintain a certain depth.  The two major drawbacks of this method are that these fish must stay moving to stay afloat and that they are incapable of swimming backwards or hovering.  The other solution that fish employ is to have portions of their body that are actually less dense than water, allowing the fish to rise towards the surface.  The two substances used to accomplish this task are gasses and oils (lipids).  Fish that use gasses to change or maintain their buoyancy usually do so by regulating the volume of the gas within their swim bladder.  The compressibility of gas allows it to behave according to Boyle's Law, which states that pressure is inversely proportional to volume at constant temperature.  Lipids are the more common substance used to decrease a fish's density.  The incompressibility of lipids prevent them from being susceptible Boyle's Law, which allow fish greater depth variation without the worry of compensating for pressure changes.