in the exposition, how does the author establish a mood or frear?

The author gets right to work on the fear, even in the first line of the story,

"OFF THERE to the right--somewhere--is a large island," said Whitney." It's rather a mystery--"

The following paragraph's incorporate a moonless night, an ominous island emanating gun shots, and our protagonist falling off his yacht. This all adds to the tension and fear.