what do you think is the essential theme of Old Man and the Sea?

There are many themes at play in this story. I, however, think the main theme is the honour in struggle, defeat, and death. All of these are manifested through Santiago. He is a broken old man who is determined to catch one more great fish. The struggle is symbolic of his life. He strives to retain that one bit of youth that remains buried in his being. His voyage is one of empathy with the fish he loves so much (the great marlin is his brother) that he must kill. Santiago also is a Christ figure of sorts. His hands are cut and his body is broken in his final communion with the elements. Santiago feels humbled in the Marlin's presence. When the Marlin is eaten by the sharks, Santiago's defeat is of anguish not so much for himself but for the majestic fish that should be eaten in such a way.