What is the conflict in the story of The Old Man and the Sea?

The conflict in Hemingway's Pulitzer Prize winning novella is the external conflict of person against nature. The character Santiago is fishing for a giant marlin and struggling against the forces of nature such as the sea itself at first. Later, his conflict is is against the marlin itself as he fights to land the marlin even as the marlin almost sinks Santiago's skiff. Still later, once he has vanquished the marlin, sharks appear and his conflict turns to fighting off the sharks as the try to eat the marlin, which Santiago has been towing. All three antagonists, the sea, the marlin and the sharks are forces of nature which Santiago must overcome.