Crowding in fish aquaculture causes chronic stress responses that affect immune function, behaviour, and welfare; optimal stocking density is species-specific and needs to be determined for each production context.
Stocking density management in fish farming requires balancing economic pressure to maximise biomass per unit volume against the welfare costs of crowding stress. Research consistently shows that fish held above optimal stocking density show elevated stress hormones, suppressed immunity, increased aggression, and reduced feeding efficiency. The optimal density is species-specific: trout tolerate different densities from tilapia or salmon. Behavioural monitoring — observation of surfacing, abnormal aggregation, and reduced feeding — provides an accessible indicator of crowding stress that farmers can use without expensive physiological testing. The welfare and production cases for appropriate stocking density are aligned.