Zaroff, according to himself, is a great hunter. He admits to Rainsford that normal hunting of lions, tigers, and bears (oh my) has gotten boring.
Hunting was beginning to bore me!
So in order to combat his hunting boredom, Zaroff invented a new prey. A prey that could reason as well as he could.
Every day I hunt, and I never grow bored now, for I have a quarry with which I can match my wits."
Rainsford's bewilderment showed in his face.
"I wanted the ideal animal to hunt," explained the general. "So I said, `What are the attributes of an ideal quarry?' And the answer was, of course, `It must have courage, cunning, and, above all, it must be able to reason."
The prey that Zaroff invented is people. Zaroff now hunts human beings.
Zaroff stocks his hunting grounds with humans that wash up on his island. The island is called "Ship Trap" because it has a tendency to sink ships on its shallow reef and rocks. If Zaroff is running short on humans to hunt, he helps ships crash by turning on lights that indicate a channel toward the rocks.
"They indicate a channel," he said, "where there's none; giant rocks with razor edges crouch like a sea monster with wide-open jaws. They can crush a ship as easily as I crush this nut."
As for the sailors' feelings about the island, the reader is told right away in the story that sailors have a "curious dread" of the island. That makes sense, since word would have gotten around that ships that pass too closely to the island end up sinking. Between the rocks and Zaroff, no human ever survives the encounter with Ship Trap island. That's why sailors are wary of the place.