Stocking density is among the most important welfare determinants in fish farming. High densities trigger aggression, disease, stress, and injury — but the relationship between density and welfare is more complex than a simple linear relationship.
Stocking density management in aquaculture requires species-specific knowledge and cannot be addressed through a single universal standard. For territorial species such as salmonids, high density causes chronic inter-individual aggression resulting in fin damage, injuries, and submission hierarchies that impair feeding opportunities for subordinate fish. For schooling species, optimal density may be higher than naively assumed — excessive isolation can itself be stressful. What matters most is not absolute density but the interaction between density, water quality, feeding distribution, and structural complexity. High-density environments with excellent water quality and well-distributed feeding may produce better welfare outcomes than low-density environments with poor water quality and competition at concentrated feeding points. Welfare assessment must therefore be outcome-based — measuring fin condition, injury rates, cortisol levels, and behaviour — rather than relying solely on density figures.