A fish's ability to propel itself efficiently through water is paramount to its likelihood to succeed.  But before a fish need worry about any of the complications associated with moving through water (hydrodynamic drag, turbulence, buoyancy, etc.) it must first solve the problem of locomotion.  The most common method for solving this problem is by muscle contraction and relaxation.  The force diagram below illustrates how forward propulsion is achieved

                                  Force diagram

The forward thrust force is created by movement of the caudal (tail) fin and varying amounts of the surrounding muscle (up to the entire body for fish that swim similar to eels) in an undulating motion.  The importance of this mechanism manifests itself in the fact that 80% of a fish's body is composed of muscle used for propulsion and maneuvering.                             

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