Saprolegnia

Channel catfish with saprolegniasis (American Fisheries Society)

Saprolegniasis is a common fungal disease of fish and their eggs. Many different species of disease-causing fungus are collectively referred to as Saprolegnia or “water molds”.

All species of freshwater fish and fish eggs are susceptible to saprolegniasis. Malnutrition, external parasites, physical injuries, bacterial and viral diseases, poor water quality and toxic substances all predispose fish to invasion by Saprolegnia fungi.

Infection appears as cotton-like tufts or gray-white patches on the surface of fish or their eggs. Dead fish and eggs are a fertile medium for fungal colonization, and their prompt removal can aid in controlling fungal outbreaks.