A Twenty Year Historical Perspective Of Ichthyophthirius multifilis Epizootics In New York State Fish Hatcheries     John H. Schachte   New York State Department of Environmental Conservation Fish Disease Control Unit, 8314 Fish Hatchery Road, Rome, NY     The protozoan parasite, Ichthyophthirius multifilis (Ich),has caused considerable fish mortalities in New York State (NYS) fish hatcheries in the last twenty years. While generally not considered to be a serious problem in cold water fish culture, three of our nine trout and salmon hatcheries, Caledonia, Salmon River and Rome, continue to experience seasonal epizootics of the parasite. Our three cool water facilities, which respectively rear muskellunge, tiger muskellunge, and walleye, are generally unaffected by the parasite. Seasonal water temperature conditions that favor development of Ich at the cold-water facilities are believed to be the primary factor. The use of surface waters subject to mid-summer temperature spikes, at least in part, seems to precipitate predictable epizootics. The incidence of infection from 1981 to 2001, seasonality, species most affected and the treatments and success in controlling the parasite will be discussed.