Disease is the primary welfare challenge in farmed trout, with multiple bacterial, viral, and parasitic conditions causing significant suffering. Preventive approaches protect welfare better than reactive treatment.
Bacterial and viral infections in trout cause acute suffering through hemorrhagic lesions, systemic inflammation, organ damage, and behavioral disruption. Infected fish show reduced feeding, abnormal swimming behavior, and surface-seeking associated with impaired osmoregulation. Mass mortality events — where entire year classes can be lost — represent enormous quantities of suffering at farm scale. Disease prevention through vaccination, biosecurity, and water quality management is both the most effective and the most welfare-positive approach. The shift from reactive antibiotic treatment (which may treat infection but does not address the welfare cost of the disease itself) to proactive prevention fundamentally improves welfare outcomes.