Pathogens
Fish are vulnerable to a variety of diseases caused by organisms (pathogens) such as bacteria, viruses, and fungi that are a normal part of their environment. These pathogens can be present at low levels, yet fish can remain healthy if their immune system is strong. Disease outbreaks occur when fish are stressed by poor diet or sudden environmental changes that weaken their immune system, allowing the pathogens to be more active. Outbreaks also occur when fish are exposed to new pathogens to which the fish has limited immunity. Like human diseases, fish diseases are infectious: Once a few fish get sick, the pathogens can spread rapidly in water, from fish to fish, causing large die-offs in the population. Only a fish disease expert can identify with certainty which pathogen is causing a disease. Diseases may affect fish in different ways and many of them have similar and overlapping symptoms. Common warning signs of a fish disease outbreak include: • Abnormal swimming: swimming in circles or upside down • Gasping, rapid gill movement; bleeding, eroded, or pale gills • Skinny body, hollow belly • Pale body colors • External cysts, sores, bloodstains; bloody or bulging eyes • Fungus, often resembling white or yellow fuzz, on body If you see fish exhibiting any of these symptoms, contact state and federal authorities immediately.