The marlin is the giant, 18-foot fish that battles with Santiago in the middle of the ocean for three days and three nights. Although Santiago hooks the marlin on his first afternoon at sea, the marlin refuses to come to the surface and instead pulls Santiago farther and farther from land. Santiago admires the marlin's beauty and endurance, and considers it a "noble" adversary, telling the fish repeatedly that though he loves it, he must kill it. Ultimately, the marlin is presented as Santiago's worthy opponent. Struggling against such an opponent brings out the best in an individual—courage, endurance, and love. At the same time, because Santiago comes to see the marlin as an alter-ego—he identifies the marlin as male and imagines the fish is old—the marlin comes to represent Santiago himself as well. In other words, Santiago's struggle with the marlin is in fact a struggle with himself. It is not a struggle of strength but rather of endurance, and a refusal to accept defeat. Santiago's struggle with the marlin is a struggle to face and overcome his own weaknesses as much as it is a struggle to subdue the great fish. In the process, by refusing to give in to the fish or the weakness of his mind and body, Santiago transcends those weaknesses.
Both in his bed in the village and in his boat, Santiago dreams of lions on the beaches of Africa, which he saw when he was a boy on a ship that sailed and fished the coast of Africa. The lions symbolize Santiago's lost youth as well as his pride (a group of lions is called a "pride"). Santiago's love for the lions, which are fierce predators, also mirrors his relationship with the marlin, whom he loves but whose death he feels is necessary to his survival. In this way, the lions as also symbolize Santiago's affinity with nature. Now that Santiago is no longer young, and has lost his friends, family, and strength, he sees the lions only in his dreams. Santiago's dreams of the lions at the end of the novella suggest that in triumphing over the marlin, he has undergone his own rejuvenation.
Scavengers and little more than swimming appetites, the sharks are Santiago's fiercest antagonists. Although Santiago manages to kill most of them, they tear apart the marlin's body and leave Santiago devastated. While the marlin is portrayed as both an adversary and a noble companion to Santiago, the sharks are portrayed as purely vicious. The shovel-nosed sharks can also be seen as symbolizing the destructive forces of nature and of the people of Jerusalem, whose petty jealousies and rivalries led to the crucifixion of Jesus. Some have even argued that the sharks symbolize literary critics, whom Hemingway saw as "feasting" on the creations of true artists without actually creating anything themselves.
At the end of The Old Man and the Sea, the exhausted Santiago removes his mast from his skiff, and haltingly drags it up the beach to his shack by resting one end of the cross on his shoulder. The position in which Santiago carries the mast exactly mirrors the position in which Jesus Christ was forced to drag his cross on the way to his crucifixion. The mast, then, becomes a symbol for the cross, and cements the parallel that Hemingway sets up between Santiago's ordeal and Christ's.