Stocking density is a primary determinant of rainbow trout welfare in freshwater aquaculture, with high densities causing chronic stress, injury, and impaired immune function.
Crowding triggers the stress response in rainbow trout, with sustained high cortisol suppressing immune function and increasing disease susceptibility. Physical fin damage from contact and aggression increases in dense populations. Oxygen depletion in poorly managed high-density tanks causes acute respiratory distress. Evidence-based stocking density guidelines, combined with oxygen supplementation and continuous monitoring, are the key welfare interventions. Behavioural indicators — reduced feeding, abnormal swimming — should be used as operational welfare measures.