For Santiago, the marlin becomes a test of his waning virility. He knows he isn't getting any younger, but he also knows that if he can reel in this big fish, he'll be able to postpone his imminent retirement for at least a little while longer. Santiago's not yet ready for his pipe and slippers; he still has much to do in life. He's an active man who can only feel truly alive out there on the high seas, manfully doing battle with one of nature's aristocrats.

Deep down, Santiago knows that this could well be his last hurrah, and if it is, then he'll most likely encounter a kind of spiritual death as he's forced to give up the only life he's ever really known. In that sense, then, one can say that Santiago's epic battle with the marlin is a case of "kill, or be killed," but in the spiritual sense of the term.

Santiago is an old man, and certainly not in the best shape. He ventured too far out into the sea and managed to catch a huge marlin. The marlin is strong, but it is caught with Santiago's hook digging deeper and deeper into its mouth. The marlin tows them both farther and farther out to sea. These two are stuck in a deadly battle which exemplifies the phrase "kill or be killed." If Santiago doesn't kill this marlin soon, his strength will give out and he will risk allowing the marlin to tow him farther away from shore. The longer Santiago has the marlin attached to his boat, the longer he risks being arbitrarily dragged down with his boat into the sea by the marlin. He also might not have the strength to get back to shore, or the supplies. If the marlin doesn't tire Santiago out or escape from the hook, he risks starving (the hook in his mouth prevents him from eating) and ultimately weakening, which would lead to him being killed by Santiago.