This is an interesting question. From what we can infer, Santiago takes pride in being a fishermen. For him, as for the other fishermen in this little village, what shows your skill and your ability as a fishermen is the size of the marlin that you can catch. This is why Santiago is focused when he goes out on trying to make a big catch that will show both himself and others his skill and expertise as a fishermen. During the story, we are told that he catches a range of fish, but only considers using them as a source of food or bait for himself, so that he can try to catch his real target, which is a marlin. We can tell by the way that the skeleton of the marlin he catches attracts so much admiration that fishing for marlin was what serious fishermen did. Consider how the other fishermen respond to the skeleton:
Many fishermen were around the skiff looking at what was lashed beside it and one was in the water, his trousers rolled up, measuring the skeleton with a length of line.
Although Santiago was not able to save his marlin from being eaten, we can see that his reputation has been restored through catching such a massive fish, showing us that for Santiago and other fishermen like him, serious fishermen only fish for big fish such as marlin.