shad vs twaite

shad

noun
  • The bluefish (Pomatomus saltatrix). 

  • 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. 

twaite

noun
  • A fish, the twaite shad, Alosa fallax 

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