Unlike land animals, most farmed fish are not stunned before slaughter, resulting in prolonged consciousness during killing and significant welfare suffering.
Fish killed without prior stunning experience the full duration of the killing process while conscious. Ice immersion causes a slow loss of consciousness taking up to 30 minutes. CO2 immersion is aversive and may cause pain before unconsciousness. Asphyxiation in air can take many minutes. The scale of farmed fish slaughter - over 70 billion annually - makes this one of the largest single welfare issues in agriculture. Effective stunning technologies exist but are not mandated.