Rainbow trout are the most widely farmed salmonid species globally; emerging welfare science is transforming standards around stocking density, water quality, and slaughter.
Rainbow trout show robust evidence of pain and stress responses. High stocking densities create competition for space and oxygen, drive fin-nipping, and suppress immune function. Poor slaughter — including asphyxiation in ice — causes prolonged consciousness and suffering. Water quality deterioration, particularly elevated carbon dioxide from respiration in crowded pens, causes respiratory distress. Welfare-conscious producers now monitor individual fish behavior using cameras and AI, enabling earlier detection of stress events.