In "Old Man and the Sea", Santiago knew that he must aim for a certain part of the fish with his harpoon. What was it?
Santiago is an experienced fisherman, and he knows where to spear the marlin. He says, "I musn't try for the head. I must get the heart." Metaphorically, this becomes very important also because the fish has great heart (strength and bravery) and that is where Santiago must kill him. He actually harpoons "the fish's side just behind the great chest fin." As the marlin dies, Santiago wishes "to feel him...I think I felt his heart." Again, the two are forever intertwined as brothers, just as Santiago calls the marlin earlier, bound by the marlin's death.
Santiago must aim "into the fish's side just behind the great chest fin that (rises) high in the air". If he hits that spot and drives the harpoon into the marlin's body by pushing with all his weight, he will penetrate the great fish's heart and the fish will die.
In order for Santiago to do this, the fish must be very close to the boat. It took several days of violent confrontation for the fisherman to wear the fish down and bring the line in so that the fish would finally begin to circle the boat and give Santiago a chance to deliver his death-blow.