What does Rainsford hear that causes him to be curious while aboard the yacht?

Rainsford hears what he believes to be gunshots as he reclines aboard a yacht on his way to hunt jaguars in the jungles of South America. Just as he reflects on how genuinely "dark" the night is, he hears the sound:

An abrupt sound startled him. Off to the right he heard it, and his ears, expert in such matters, could not be mistaken. Again he heard the sound, and again. Somewhere, off in the blackness, someone had fired a gun three times.

Rainsford's curiosity draws him to leap onto the rail of the ship to listen for more shots. Unfortunately, he extends too far from the boat, first losing his pipe, and then tumbling into the "blood-warm waters of the Caribbean Sea." He struggles through the waves, swimming toward the general area where he heard the gunshots. Eventually he makes it to the shore of "Ship-Trap Island." Later, he finds out that the gunshots were those of General Zaroff, who was hunting down a sailor from a recent ship wreck on what turns out to be Zaroff's private island. Because he has grown bored with hunting animals, Zaroff has turned to hunting men. The major conflict in the story involves a hunt between Zaroff, the hunter, and Rainsford, the prey.