why did the old man say," yes.yes"?

The old man said this because a marlin was tugging at his lines.

Just then, watching his lines, he saw one of the projecting green sticks dip sharply.

“Yes,” he said. “Yes,” and shipped his oars without bumping the boat. He reached out for the line and held it softly between the thumb and forefinger of his right hand. He felt no strain nor weight and he held the line lightly. Then it came again. This time it was a tentative pull, not solid nor heavy, and he knew exactly what it was. One hundred fathoms down a marlin was eating the sardines that covered the point and the shank of the hook where the hand-forged hook projected from the head of the small tuna.