Why does the old man quit fighting off the shark?

He quits fighting off the sharks because there were too many, it was dark, and he had no weapon. It was over, there wasn't anything left of the marlin to eat.

But by midnight he fought and this time he knew the fight was useless. They came in a pack and he could only see the lines in the water that their fins made and their phosphorescence as they threw themselves on the fish. He clubbed at heads and heard the jaws chop and the shaking of the skiff as they took hold below. He clubbed desperately at what he could only feel and hear and he
felt something seize the club and it was gone.