Stocking density in fish farming is one of the most significant welfare variables, with crowding causing chronic stress, aggression, injury, and disease susceptibility at high densities.
Fish crowding is a welfare issue where commercial incentives and welfare requirements are in direct tension; higher density means more fish per unit of production infrastructure, reducing cost per kilogram. Current regulatory maximum densities in most countries are set at levels that may still involve chronic stress in many species. The challenge is that fish conceal stress responses until welfare compromise is severe, making casual observation inadequate for density welfare assessment. Physiological indicators including cortisol, immune markers, and stress protein expression provide more sensitive measures. The economic incentive to maximise density must be counterbalanced by recognition that chronic crowding stress reduces feed conversion efficiency, increases disease susceptibility, and increases mortality, meaning that welfare-optimal densities may also be economically rational at the production system level.