This is a brilliant question to consider. Nothing can be more heart-rending than the description of Santiago vainly struggling against the inevitable destruction of his prize catch. And yet, if we consider the novel as a whole, we can see that the very first paragraph of the story attaches the stigma of defeat to his character as his sail represents "the flag of permanent of defeat." However, instead of giving in to defeat and losing the will to live, we can see that Santiago constantly strives to shun this label. He plans to sail out in order to try and catch a big fish, and then he is successful in catching a massive fish and then chooses to keep on resisting the sharks even when he knows that his struggle is in vain.
If we consider the allegorical significance of this novel, and the way that it pits man against the natural world of which he is a part, we can see the message that Hemingway is trying to convey. Santiago and the marlin are both subject to the same ultimate realities of kill or be killed. Santiago lives his life according to his own recognition that "man is not made for defeat... man can be destroyed but not defeated." Even though death is an inescapable fact of life, the best and most noble of creatures will not give in without a fight. The answer to your question is that the measure of a man is how he responds to the inevitable struggle of life. In fighting against the inevitable, a man can prove their moral character. Although Santiago is fighting a losing battle, the fact that he continues fighting bestows him with dignity and honour.