Recirculating aquaculture systems (RAS) offer significant welfare advantages over sea cage salmon farming by eliminating sea lice and allowing environmental control, but introduce new welfare challenges from high densities and water quality management.
RAS salmon farming represents a genuine welfare improvement over sea cage production in specific domains while introducing new welfare risks that require careful management. The elimination of sea lice and associated chemical treatments is a substantial welfare gain — these parasites cause chronic skin and gill damage, and treatments themselves stress fish. Precise environmental control allows maintenance of optimal temperature, oxygen, and pH ranges that maximise physiological comfort. However, the welfare implications of extreme stocking densities in RAS are poorly understood: fish at 80 kg/m3 experience social crowding effects that have been inadequately studied relative to sea cage research. Water quality management failures in RAS are silent and rapid: CO2 accumulation causes gill hyperplasia and acid-base imbalance before behavioural signs appear, and system failures can kill entire cohorts within hours. The welfare potential of RAS is high, but realising it requires density limits informed by fish welfare science, not just production optimisation.