Saprolegnia spp. Multi-species colonies of the freshwater fungus Saprolegnia (or 'fin rot') feed on fish waste or other dead cells but will also feed on aquatic animals (including fish, crustacea, insects and even their eggs).  Although Saprolegnia spp. have a wide temperature range, this introduced European disease prefers cooler temperatures. Nevertheless, as many New Zealand native freshwater fish become stressed at temperatures around and above 20oC, fungus is most often seen at warm water temperatures, which also favour its rapid population increase. Usually a secondary response to physical damage and/or stress, Saprolegnia looks like cotton wool as it spread across the skin of an injured host. A Saprolegnia infection is very often fatal, with a steady spread of the disease to previously healthy organs and eventually leading to death. Pathogens such as Saprolegnia are common in freshwater environments, but only lead to disease when immune systems are compromised by stress of some kind. Removal of the stress may allow for recuperation of the fishy, providing they are not overly damaged and some immune resilience remains. Having a good habitat and water quality in general serve to built resistance to disease through minimising stressors from these other environmental factors. As such, avoiding environmental degradation for existing populations is more important for the greater population than retroactive cures.