What is the role of the old man as a fisherman in the novel "The Old Man and The Sea?"

Santiago is the pivotal character in the story; it is around him that both the story line and the main themes are woven. Santiago is first pictured as an aging old man resisting and then defeated by nature. But at the end of the story he emerges as a heroic archetype as new definitions of "success" and "failure" are fomulated.

I suggest you read the essay mentioned in the reference below as it gives greater insight into Santiago's role in both plot and theme.