Baltic herring vs herring

Baltic herring

noun
  • A subspecies of Atlantic herring, (Clupea harengus membras), living in the Baltic Sea, smaller than Atlantic herring, with leaner flesh. 

herring

noun
  • Fish in the family Clupeidae. 

  • A type of small, oily fish of the genus Clupea, often used as food. 

  • Fish similar to those in genus Clupea, many of those in the order Clupeiformes. 

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