Rainbow trout is one of the most important farmed fish species globally, but welfare challenges including high stocking densities, aggressive interactions, and inadequate water quality affect billions of individual fish annually.
Rainbow trout in high-density aquaculture experience welfare harms from the mismatch between their natural territorial behaviour and the crowded conditions of commercial farming. Territorial fish forced into close proximity engage in persistent aggression that causes fin damage and stress, or establish rigid hierarchies where subordinate individuals are unable to feed freely. Chronic social stress suppresses immune function and increases vulnerability to disease. Water quality parameters that are borderline adequate for fish health may still cause suboptimal welfare through mild hypoxia that limits normal behaviour. Slaughter methods vary from instantaneous percussion stunning to asphyxiation in air or ice water — the latter causing prolonged distress before death. Improving trout welfare requires addressing stocking density, aggression management, water quality, and humane slaughter simultaneously.