The Collaborative International Dictionary of English v.0.48:

Ichthyologist \Ich`thy*ol"o*gist\, n. [Cf. F. ichthyologiste.]
   One versed in, or who studies, ichthyology.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ichthyologist
    n 1: a zoologist who studies fishes

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "ichthyologist":
   animal physiologist, anthropologist, biologist, conchologist,
   ecologist, entomologist, ethologist, helminthologist,
   herpetologist, malacologist, mammalogist, ophiologist,
   ornithologist, protozoologist, taxidermist, zoo-ecologist,
   zoographer, zoologist, zoonomist, zoopathologist, zoophysicist,
   zootaxonomist