The method of fish slaughter is one of the most significant welfare variables in aquaculture, with large differences in humaneness between current practices.
Fish killed by asphyxiation in ice, dewatering, or CO2 exposure remain conscious and responsive for several minutes after the process begins. During this time they experience progressive hypoxia, acid burn from CO2, and cardiovascular stress. Effective percussion stunning renders fish immediately insensible before any pain can be experienced. The welfare difference between these methods, applied to tens of billions of animals annually, represents an enormous aggregate improvement opportunity.