Santiago is the Christ-figure and Manolin is his disciple. On a symbolic level, the fish is a "true brother," one who gives life to the fisherman. Santiago says:
It is good that we do not have to try to kill the sun or the moon or the stars. It is enough to live on the sea and kill our true brothers.
From a biological view, Hemingway sees a connection between animals and humans--in terms of the hunt and in terms of suffering. Santiago respects the Marlin for putting up such a glorious battle, and he thanks him for giving his life. Santiago, however, does not feel the same about sharks. He calls them scavengers.
In terms of meta-fiction, the author, of course, is Santiago and the Marlin is his novel. He is trying to get the novel ashore--in tact, preserving it and his dignity. The sharks, or the critics, are those who try to undermine the author's dignity: they ravage the book out of spite.