what does the fish symbolize in the novel

The fish is first and foremost a symbol of Christianity, and Hemingway has given him several Christian virtues: kindness, patience, and determination. Although hooked by Santiago, the fish act almost human. He's not in a frenzy; he doesn't pull overly hard on the hook or dive into the depths. What he does do is guide Santiago, and in doing so wins his respect.

Regardless, it isn't the fisherman who ultimately causes his death. The fish becomes a symbol of sacrifice; eaten away and attacked by his own kind. The old man gave him up to his destiny, and the fish in turn served to teach the old man a valuable lesson.