Readers may attribute various meanings to the great fish Santiago catches and then looses in The Old Man and the Sea.
The fish could be considered a symbol of eternal optimism, the human spirit triumphing over age and declining abilities. Even after a long stretch of bad fishing, Santiago was still hopeful when he departed. He was confident of his skill as a fisherman and didn't hesitate to sail far from land and the usual fishing areas to seek a change in his luck.
The fish could also symbolize the challenges and hardships in life. The marathon struggle between fish and fisherman and the loosing battle to protect the captured fish from the sharks called into play every bit of Santiago's experience and skill and knowledge of the sea and its ways. But even when he gave his all, Santiago did not succeed in bringing the large fish to shore.
In another sense, the fish symbolizes everything Santiago values in the world. He considers the fish to be both friend and foe, something to be caught and a separate and very special being to be loved and respected.
Then he was sorry for the great fish that had nothing to eat and his determination to kill him never relaxed in his sorrow for him. How many people will he feed, he thought. But are they worthy to eat him? No, of course not. There is no one worthy of eating him from the manner of his behaviour and his great dignity.