The occurrence of eleven diseases was monitored during five years in a production grow-out-facility for channel catfish at a coal-fired steam electric generating station on the Susquehanna River, Pennsylvania. Fish were reared at high desities in raceways up to 24°C above ambient river temperatures. Thermal enhancement promoted rapid fish growth and altered the normal seasonal patterns of several disease organisms.

Aeromonas hydrophila was restricted to the warmest months. Flexibacter columnaris was present throughout all months but was most destructive during cooler temperatures. Epistylis sp. and Trichodina sp. were present year around also. The occurrence of Ichthyophthirlus multifilis coincided with extended periods with water temperatures between 20-22° Celsius and a build-up of organics in ponds. Saprolegnia was principally a secondary invader but commonly invaded catfish eggs incubated below 25.5° Celsuis. Diseases may restrict the upper limits of fish loading and can best be managed by preventative practices. Concurrent infections by several disease organisms pose the greatest management problem experienced.