fishing ground vs trolling

fishing ground

noun
  • An area of water used for fishing (usually a sea area). 

trolling

noun
  • A fishing technique in which one or more baited fishing lines are dragged by a moving boat. Not to be confused with trawling, which drags a net. 

  • The act of one who trolls (in various senses). 

How often have the words fishing ground and trolling occurred in a corpus of books? (source: Google Ngram Viewer )