The old man’s battle with the fish is not only a battle of strength, but a battle of wills. The old man makes up for his old age with incredible endurance, willing to withstand hunger, physical pain, and isolation from the rest of the world as he battles the fish. Endurance becomes a way we connect the old man and the fish he fights, as they share a determination that, in its magnitude, separates them from other people and creatures.
Although the old man is physically inferior to the fish, he wins because he possesses a greater will.
The old man loses the fish because, in his determination to succeed, he violated the implicit rules of his profession.