On page 99, consider the image of Santiago side by side with the fish. What does this represent in The Old Man and the Sea?
For one thing, this particular passage of the book shows the increasingly close, complex relationship between Santiago and the marlin he has just caught. In his fevered state of mind, the old man has come to identify with his prey, and no longer sees it as a mere creature to be killed and eaten but as a worthy opponent in a punishing battle of wills.
Santiago lives by an ancient code of honor. He sees himself as one of nature's aristocrats, who fights valiantly against the tide of fate as symbolized by the cruelty of the sea and its crashing waves. After finally having killed the marlin he now ennobles the dead fish too, and recognizes in him a kindred spirit, similarly tough and unyielding. Santiago and his vanquished prey are now side by side, both literally and figuratively, hunter and hunted united in their participation in the epic struggles of nature.
But in his growing state of delirium Santiago begins to wonder if there really is a true equality between himself and his quarry. Though dead, it seems as if it is the marlin who is bringing in Santiago rather than vice versa. The old man famously says, "A man can be destroyed but not defeated." The marlin has been destroyed but the nature it increasingly comes to symbolize remains undefeated. Its fundamental unity, its oneness of being, remains unbroken despite the best efforts of humankind to impose itself upon it.
But they were sailing together lashed side by side and the old man thought, let him bring me in if it pleases him. I am only better than him through trickery and he meant me no harm.
As Santiago heads for home with the great marlin attached to the side of his boat because it weighs nearly fifteen hundred pounds, he acknowledges the prowess of the fish, whom he has finally conquered only by means of his skill as a fisherman. Thus, they are rivals of a sort since the marlin is not made to give up what Santiago terms his "dignity" by lying in the bottom of the skiff.
In fact, earlier in the narrative, Santiago has called the fish his "brother," and after he has killed the great fish, Santiago says he must do "the slave work" of securing the marlin to his boat so that he can turn for home.