fur seal vs northern fur seal

fur seal

noun
  • A marine mammal of either of the genera Arctocephalus or Callorhinus of the eared seal family Otariidae, differing from the sea lions in having dense underfur. 

northern fur seal

noun
  • A fur seal of the species Callorhinus ursinus (formerly Arctocephalus ursinus), of the northern Pacific. 

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