I'm not certain which part of the story you are referring to, but if you're talking about the first time he sees the fish, he sees that it is a huge marlin, probably the biggest he has ever seen. His advantage is that he is an experienced fisherman who is very determined and strong-willed, as evidenced by the fact that he continued to fish regularly, even though he hadn't caught anything in a long time. He would not give up even though everyone advised him to. The fish's advantage is its huge size and strength. It is also apparently in its prime and Santiago is old and past his prime.