Social behaviour in farmed salmon is strongly influenced by stocking density. Understanding how crowding drives aggression and stress guides welfare-improving management practices.
The social dynamics of farmed salmon in high-density systems create welfare problems that are less visible than individual injury or disease but equally significant in their cumulative impact on population welfare. Dominant individuals that control feed access and space impose chronic physiological stress on subordinate fish through persistent competition and avoidance behaviour. This chronic social stress suppresses immune function and growth in subordinate fish while creating a population-level welfare disparity that is difficult to detect through conventional observation. Regular grading that maintains similar-sized fish in tanks reduces competitive imbalance but introduces the welfare costs of handling and crowding during the grading process. Optimising stocking density to reduce social stress is one of the most effective welfare interventions available to salmon farmers.