rainbow trout vs steelhead

rainbow trout

noun
  • A trout of the species Oncorhynchus mykiss, that has black spots and a pink streak running along the body. 

  • The fish as food. 

steelhead

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

  • The ruddy duck. 

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