In The Old Man and the Sea, why does Santiago choose to fish for marlin as opposed to squid, bait fish, tuna or shrimp?

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.