As Moby Dick comes to represent the forces of Nature (or god), the battle between Ahab and Moby Dick is a battle between Man and Nature (or Man and God).
Specifically, Ahab rebels against the notion that his fate might be determined by any power outside of himself. He insists, in the novel, on being the sole arbiter of his fate.
These conflicts are, of course, symbolic and for this reason require interpretation. However, in the case of this novel, the symbolism is rather clear. Moby Dick is directly aligned with nature and its abstract counterpart, Nature.
In developing the theme of the individual (Ahab) versus Nature (symbolized by Moby-Dick), Melville explores the attributes of natural forces.