A fish disease which may lead to death is a state of imbalance between the fish and its environment. Fish seldom suffer from severe disease outbreaks under natural conditions. However, when fish are crowded and reared under unnatural conditions, the potential exists for a serious disease outbreak. Intensively raised fish are stressed by fluctuating water temperatures, changes in water quality, overcrowding, handling, and transport. Fish can handle these stresses up to a point, but when they can no longer adjust, they may succumb to disease and eventually die.
Prevention of disease is a primary goal in aquaculture.
Ideally, the fish facility should be free of pathogens, but this is not always the case. If the fish are bought, the buyer should obtain healthy fish from a reputable dealer. Trout and salmon are usually required to be purchased from a certified disease-free hatchery if stocked in waters that drain into lake Michigan. Fish must not be handled too often or too roughly because this will result in stress which may trigger diseases. Preventive methods can be incorporated to keep the amount of stress encountered by the fish to a minimum.