The reader is introduced to the marlin because Santiago sees one of the projecting sticks dip in the water (B).
In The Old Man and the Sea, the main character becomes aware of the presence of the marlin when he realizes something is tugging at his hundred-fathom-deep fishing line. Because of the strength of the bite, the stick to which the line is attached is pulled into the water. Santiago immediately recognizes the fish as being a large marlin.
