Carbon dioxide immersion is widely used to stun fish before slaughter but is aversive and stressful; better alternatives including electrical and percussive stunning are available.
The widespread use of CO2 immersion in fish slaughter represents a significant gap between available welfare science and commercial practice. Research clearly shows that fish find CO2 immersion aversive — they actively attempt to escape the high-CO2 water, show rapid opercular movement, and take several minutes to lose consciousness. During this time they experience the equivalent of chemical suffocation. Percussion stunning, when applied correctly to the head, renders fish immediately insensible. Electrical stunning in-water can achieve insensibility in under a second. Both alternatives are available for commercial use; the barrier to adoption is cost and familiarity rather than technical feasibility.