Complex sentence: There are about 400 individuals in existence in the western North Atlantic Ocean-they migrate between feeding grounds in the Gulf of Maine and their winter calving areas off Georgia and Florida, an ocean area with heavy shipping traffic.
Split into simpler sentences: There are about 400 individuals in existence in the western North Atlantic Ocean. They migrate between feeding grounds in the Gulf of Maine and their winter calving areas off Georgia and Florida, an ocean area with heavy shipping traffic.