Ichthyology (from Greek: ἰχθυ, ikhthu, “fish”; and λόγος, logos, “knowledge”) is the branch of zoology devoted to the study of fish. This includes skeletal fish (Osteichthyes), cartilaginous fish (Chondrichthyes), and jawless fish (Agnatha). An estimated 25,000 fish species exist, comprising a majority of vertebrates. While a majority of species have probably been discovered and described, approximately 250 new species are officially described by science each year.

previous answerer is correct to a point. It would be wonderful if people here would not just cut and paste directly from wikipedia. Use your own knowledge, people-stop trying to look smart to everyone else. To expound on the answer above, Ichthyology is more closely tied to vertebrate biology than zoology (zoology studies ALL animals-be they vertebrate, invertebrate, single cellular, etc., whereas vertebrate biology includes all creatures with an internal skeleton, and more specifically, a spine). There are also various subcategories of ichthyology that are specific groups of fish (bony fish, cartilaginous fish etc.).

The study of fish.

It is the branch of zoology which includes fish.