Environmental factors affecting outbreaks of winter saprolegniosis in channel catfish, Ictalurus punctatus (Rafinesque)

. Previous studies have led to the development of a laboratory model for a disease termed ‘winter saprolegniosis’, locally referred to as ‘winter kill’, occurring in channel catfish, Ictalurus punctatus Rafinesque, raised in commercial ponds in the southeastern United States. In the laboratory, the onset of disease was routinely brought about by a combination of two related factors: (1) a rapid drop in water temperature which induces immunosuppression in catfish; and (2) maintenance of low water temperatures (∼10°C), which favour the proliferation of an ubiquitous fungus of the genus Saprolegnia to produce high levels of fungal zoospores (≥5 spores ml −1 ). In this study, two commercial catfish ponds were monitored for over one year to determine if the above factors occurred in the field and could be correlated with outbreaks of winter saprolegniosis. It was noted that passages of severe cold weather fronts were able to drop pond water temperatures ≥10°C within 24h and that such decreases in water temperature were associated with immunosuppression of the catfish in the ponds. Furthermore, when Saprolegnia sp. zoospore levels were ≥5 spores ml ‐1 , the immunocompromised catfish exhibited overt signs of winter saprolegniosis. If one of the factors implicated in the induction of disease was missing, fish in the ponds remained healthy. In addition, the onset of disease in the ponds appeared independent of pH and oxygen, total ammonia nitrogen and un‐ionized ammonia levels. Collectively, these field results confirm the laboratory‐based hypothesis concerning the aetiology of winter saprolegniosis in channel catfish.