shad vs twaite shad

shad

noun
  • Any one of several species of food fishes that make up the genus Alosa in the family Clupeidae, to which the herrings also belong; river herring. 

  • The bluefish (Pomatomus saltatrix). 

twaite shad

noun
  • A fish in the herring family Clupeidae, Alosa fallax, found in the eastern Atlantic Ocean and the Mediterranean Sea. 

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