The choice between net pen and recirculating aquaculture system (RAS) farming profoundly affects salmon welfare through disease pressure, water quality, and stocking density control.
From a welfare perspective, RAS offers significant advantages over sea-cage net pens — but only when well-managed. Net pens expose salmon to uncontrolled environmental stressors: harmful algal blooms, jellyfish influxes, temperature extremes, and sea lice that can cause severe skin damage and anaemia. These stressors are largely eliminated in RAS. However, RAS introduces its own welfare risks: equipment failures can rapidly deplete oxygen; high ammonia accumulation causes gill damage; and some evidence suggests that RAS-raised salmon experience higher chronic stress if water quality management is inadequate. The ideal welfare outcome is a well-run RAS with reliable systems, robust monitoring, and densities that allow normal swimming behaviour — but this requires capital investment that many operators resist. Hybrid systems combining RAS early life stages with net pens at sea are common but retain the sea lice problem.