Ichthyology
Ichthyology is the scientific study of fish. The term Ichthyology is derived from the Greek words ichthyus, meaning fish, and logos, meaning knowledge. It includes hagfish and lampreys (jawless fish), cartilagenous fish (such as sharks, skates and rays) (Chondrichthyes) and bony fishes (Osteichthyes).
A person who studies fish is called an ichthyologist.