sea trout vs steelhead

sea trout

noun
  • A fish of the species Salmo trutta morpha trutta, closely related to salmon and a subspecies of brown trout. It is lighter in colour than the brown trout, and lives in salt water, returning to fresh water only to spawn. 

  • The weakfish, Cynoscion regalis 

steelhead

noun
  • The anadromous form of the rainbow trout, Oncorhynchus mykiss. 

  • The ruddy duck. 

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