The Collaborative International Dictionary of English v.0.48:

muttonfish \muttonfish\ n.
   A fish (Lutjanus analis) similar to and often marketed as
   "red snapper".

   Syn: mutton snapper, Lutjanus analis.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

muttonfish
    n 1: similar to and often marketed as `red snapper'; [syn:
         mutton snapper, muttonfish, Lutjanus analis]