Santiago shows his respect of the marlin in the way he talks about the "great fish." He knows enough of the patterns of the marlin to understand why it acts as it does; he recognizes the urge to free itself and flee and respects the marlin's drive to survive, even as he explains why he is striving to successfully land the marlin.
Perhaps he is too wise to jump. He could ruin me by jumping or by a wild rush. But perhaps he has been hooked many times before and he knows that this is how he should make his fight...But what a great fish he is and what he will bring in the market if the flesh is good.
After catching the marlin and lashing it to the side of the skiff, Santiago has some time for reflection on the great battle that was now finished. "is he bringing me in or am I bringing him in?" Santiago recognized the tremendous size and power of the marlin, and respected that "I am only better than him through trickery and he meant me no harm."
Finally, Santiago demonstrates his affection for and respect of the marlin by the way in which he fights to protect his catch from the sharks. He did not want to reduce the worth of his catch when he got to the market, because he was proud of his ability as a fisherman and of his feat in landing the huge marlin by himself. But he was sorry that he was unable to protect the marlin from the sharks out of respect for the beauty and power it had demonstrated during the landing process. "I wish it were a dream and that I had never hooked him. I'm sorry about it, fish. It makes everything wrong."
he goes along with what the merlin wants to do and lets him die slowly rather than killing him