Similar to Finding Nemo's "Fish are friends...," Santiago believes, "The fish is my friend too."
And he even goes one step further about the marlin: "I have never seen or heard of ... or more beautiful, or a calmer or more noble thing than you, brother."
On page 217 of The Old Man and The Sea, Santiago delivers a prayer:
Hail Mary full of Grace...pray for us sinners now and at the hour of our death. Amen." Then he added, "Blessed Virgin, pray for the death of this fish. Wonderful though he is.
Here, Hemingway is using anthropomorphism to compare the fish's suffering and death to Santiago's. After all, Santiago calls the sea "La Mar," as mother, and he calls the marlin his "brother." Santiago believes all of God's creatures are family, connected by nature symbiotically. If one suffers, the others feel it too. So, saying the prayer honors the fish's death.
After he says the prayer Hemingway writes:
With prayers said, and feeling much better, but suffering exactly as much, and perhaps a little more...
So, the prayer helps Santiago cope with the death of the marlin and his own suffering. The fish's death will end its suffering and Santiago's, as they are in a struggle to the death.