The Collaborative International Dictionary of English v.0.48:

Butterfish \But"ter*fish`\, n. (Zool.)
   A name given to several different fishes, in allusion to
   their slippery coating of mucus, as the Stromateus
   triacanthus of the Atlantic coast, the Epinephelus
   punctatus of the southern coast, the rock eel, and the
   kelpfish of New Zealand.
   [1913 Webster]