jack mackerel vs midshipman

jack mackerel

noun
  • Any of various edible fish, especially of genus Trachurus, but also genera Caranx, Acanthocybium and Euthynnus. 

midshipman

noun
  • A midshipman fish. 

  • An officer of the lowest rank in several navies; especially, a trainee officer. 

How often have the words jack mackerel and midshipman occurred in a corpus of books? (source: Google Ngram Viewer )