Disease is one of the primary welfare challenges in trout aquaculture, with bacterial, viral, and parasitic infections causing significant suffering that requires proactive prevention strategies.
Disease in trout aquaculture represents welfare harm at three levels: the suffering of individual sick fish, the stress of prophylactic treatments that affect all fish in a system, and the culling of affected populations when disease cannot be controlled. Neurological diseases like whirling disease cause disorientation and behavioral changes that impair normal function for extended periods before death. Bacterial infections causing furunculosis produce visible lesions and systemic sepsis that are clearly painful. Prevention through water quality management, appropriate stocking density, vaccination, and biosecurity measures is more welfare-positive than reactive treatment, because it prevents suffering rather than attempting to reduce it after onset. Water temperature is particularly important: trout kept at optimal thermal ranges show significantly lower disease incidence than those in warm-stressed conditions.