why does the appearance of the shark have such an impact on the reader?

Poor Santiago has just spent days catching his prized Marlin. He is exhausted and we see that this is not just another catch for the old Corsican. The catch is a communion with the fish; it is a validation of Santiago as a fisherman and as a man. When we see the sharks we just know they are going to eat his Marlin. Santiago has fastened his Marlin to the side of his skiff and the sharks are going to devour his crowning glory.