CO2 is widely used to stun or kill fish in aquaculture, but mounting evidence shows it causes aversion and distress before loss of consciousness, raising serious welfare concerns.
The widespread use of CO2 stunning represents a significant welfare deficit in aquaculture. Fish immersed in CO2-saturated water show escape behaviour, increased opercular beat rate, and other distress indicators before becoming insensible. The pain science supports the view that this method causes suffering. Transitioning to percussion or electrical stunning at harvest is technically feasible and should be prioritised.