Carbon dioxide stunning is widely used in fish processing but raises significant welfare concerns because CO2 exposure is aversive before loss of consciousness, making alternative methods preferable for most species.
CO2 stunning of fish imposes a period of aversive experience before unconsciousness that is increasingly difficult to justify given available alternatives. Fish research using preference paradigms consistently shows that fish will sacrifice food rewards to avoid CO2-rich water — behavioural evidence that the CO2 exposure is genuinely aversive rather than simply pharmacologically disorienting. The agitation, escape attempts, and opercular movements observed during CO2 induction reflect a period of distress that may last a minute or more before insensibility occurs. This welfare cost can be eliminated by electrical stunning, which induces immediate insensibility without an aversive induction phase. The continued use of CO2 in fish processing reflects cost and infrastructure factors rather than a welfare assessment that concludes CO2 is acceptable. As fish welfare science advances and the evidence for fish sentience becomes increasingly accepted, the case for eliminating CO2 stunning where electrical alternatives exist becomes stronger. Consumer and retailer pressure for higher-welfare fish processing standards creates the market incentive for industry investment in electrical stunning equipment.