Carbon dioxide stunning of farmed fish before slaughter is widely used but causes significant aversive distress before loss of consciousness, making it one of the most welfare-problematic stunning methods in aquaculture.
CO2 stunning is problematic precisely because it causes aversive distress before achieving its purpose. The aversion response — visible in escape attempts and behavioral changes — indicates that fish perceive and react to CO2 exposure as harmful before losing consciousness. This contrasts with electrical stunning, which renders fish unconscious within milliseconds. The continued use of CO2 in many aquaculture systems reflects the lower cost and technical simplicity of the method rather than welfare considerations. Regulatory movement in Norway and increasing industry awareness is driving gradual adoption of electrical stunning, particularly for salmon. Consumer and retailer pressure for welfare-certified fish is accelerating this transition.