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.