Stocking density in rainbow trout farms significantly affects fish welfare through competition, aggression, injury, and chronic physiological stress responses.
High stocking densities in trout farms compromise welfare through multiple mechanisms: physical crowding prevents expression of normal swimming and spatial behaviour, competition for feed resources increases aggression and fin damage, and chronically elevated cortisol from social stress suppresses immune function and impairs growth. Fin erosion and injury from conspecific aggression are observable welfare indicators that correlate with stocking density. Lower stocking density with careful feed management can achieve comparable production outcomes with substantially better welfare — the economic argument for high density is often overstated relative to the disease and mortality costs of welfare-compromised fish.