Rainbow trout is the most farmed salmonid in Europe. Welfare science for this species is advancing, driving improvements in stocking density, slaughter, and enrichment.
Rainbow trout show clear stress responses to overcrowding including elevated cortisol, fin damage, and immune suppression. Their sensitivity to dissolved oxygen means that brief power failures causing oxygen depletion can cause mass mortality with significant suffering. Non-lethal indicators of chronic stress including fin erosion and abnormal swimming patterns are observable welfare metrics. Slaughter welfare has improved most in certified supply chains responding to retailer pressure, but commodity production remains largely unmonitored. The species shows behavioral complexity including individual recognition and conditioned responses that support their capacity for suffering.