Santiago has great respect for all of nature's creatures, except sharks. He refers to the fish (a marlin) as his "brother," showing not only a certain biological connection between man and beast but also a philosophical one. In the harsh waters, both are in a fight for survival.
I am a tired old man. But I have killed this fish which is my brother and now I must do the slave work.
Santiago regrets not only killing his brother but having to tie the fish to the side of the skiff. He especially laments the fish being attacked by sharks, for whom he has no respect. He drives his knife into the shark's brain. So, there is a clear division between the great game fish and the souless garbagemen of the sea.
Santiago also sees the fish as a means of economic survial. Later, he says,
I want to see him, he thought, and to touch and to feel him. He is my fortune.
The fish will bring a hefty price at the market, and it will bring him good fortune among the fishermen, who have seen Santiago as the unluckiest of fishemen.