The Collaborative International Dictionary of English v.0.48:

Ichthyology \Ich`thy*ol"o*gy\, n. [Gr. 'ichqy`s, -y`os, a fish +
   -logy: cf. F. ichthyologie.]
   The natural history of fishes; that branch of Zoology which
   relates to fishes, including their structure, classification,
   and habits.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ichthyology
    n 1: the branch of zoology that studies fishes

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "ichthyology":
   anatomy, animal physiology, anthropology, biology,
   comparative anatomy, conchology, ecology, entomology, ethology,
   helminthology, herpetology, malacology, mammalogy, ornithology,
   protozoology, taxidermy, taxonomy, zoogeography, zoography,
   zoology, zoonomy, zoopathology, zoophysics, zootaxy, zootomy