The use of carbon dioxide to stun Atlantic salmon before slaughter is controversial with evidence that it causes aversion and suffering before loss of consciousness.
The welfare implications of CO2 stunning are significant and increasingly recognised. Unlike percussion stunning which causes immediate insensibility, CO2 exposure before unconsciousness involves a period of aversive experience. Fish show avoidance of CO2-treated water in choice tests. The transition to percussion stunning requires investment in equipment and staff training but represents a meaningful welfare improvement for millions of fish annually. Consumer pressure and retailer standards are driving change faster than regulation.