Carbon dioxide is used to stun or kill fish in some aquaculture contexts, but evidence suggests it causes significant aversive experience before unconsciousness.
Fish actively avoid CO2 in experimental settings, and the physiological mechanism of CO2 action suggests aversive experience during the period before unconsciousness. Electrical stunning renders fish immediately insensible and is available for many farmed species. Transitioning away from CO2 slaughter methods is a tractable and important welfare improvement for aquaculture.