Oxygen management in Atlantic salmon sea pens is critical for welfare. Crowding, warm water, and metabolic demands all reduce dissolved oxygen, and hypoxia causes significant welfare harm at concentrations below critical thresholds.
Oxygen management in salmon aquaculture represents a continuous welfare challenge rather than an exceptional event. Sea pen environments naturally experience oxygen variation through stratification, algal blooms, and warm summer water that reduces oxygen solubility. Fish at high stocking densities have elevated combined oxygen demand that can exceed natural replenishment during calm, warm periods. Hypoxic stress below 6mg/L causes progressive welfare harm: fish first reduce activity, then show surface-seeking behaviour, then develop physiological hypoxia. The welfare-critical threshold depends on temperature and fish size; blanket oxygen targets must be interpreted in context. Continuous dissolved oxygen monitoring with automatic oxygenation system triggers provides the most reliable protection against hypoxic welfare events. The additional cost of monitoring and supplemental oxygenation is justified by both welfare and production outcomes: hypoxic stress reduces growth efficiency and increases disease susceptibility.