How does the old man kill the great marlin in The Old Man and the Sea?

The old man, Santiago, has not caught a fish in 84 days, a long time.
He goes out to sea in his boat and drops his baited hooks in the water. A giant marlin nibbles at the sardines arranged around a hook until the hook catches in his mouth. At this point, Santiago has him, and he waits for the fish to tire so he can kill it. The marlin rears out of the water, and Santiago sees it is so big it is two feet longer than his boat. Santiago feels he must not let the marlin know it is stronger then he is. Although very tired, Santiago eats smaller fish and waits for the marlin to weaken.
On the fourth day, the marlin wearies, and Santiago can use his harpoon to kill the giant fish.