Rainbow trout are among the most widely farmed salmonids globally. Their welfare depends on stocking density, water quality, and opportunities to express natural behaviors.
Farmed trout welfare is significantly affected by the density of production systems. At high stocking densities, social hierarchies cannot function normally, subordinate fish cannot escape dominant individuals, and chronic stress affects physiology and immune competence. Trout show behavioral indicators of pain including fin flicking, rubbing, and reduced activity following noxious stimuli. Lower stocking densities, appropriate water velocities for swimming, and enriched environments improve welfare indicators. Progressive certification programs are developing evidence-based density standards.