Slaughter method has a major impact on rainbow trout welfare. Electrical stunning, percussion, and CO2 methods have different welfare profiles. This guide covers the evidence.
Slaughter method selection has more impact on rainbow trout welfare than almost any other single management decision. Live chilling in ice slurry — where conscious fish are immersed in near-freezing water — causes a period of several minutes of distress before loss of consciousness. EEG studies confirm that chilling-immersed trout remain conscious for up to 9 minutes. This represents a significant welfare harm that affects millions of fish annually.
Electrical stunning, when properly calibrated and applied, renders fish immediately insensible — EEG shows immediate loss of brain activity. This is the welfare-optimal commercial slaughter method. Percussive stunning by a precise blow to the head can be equally instantaneous when applied correctly, making it suitable for smaller-scale and artisanal operations. Both methods should be followed immediately by gill-cut to prevent recovery before death.
CO2 narcosis before slaughter is used in some systems as a compromise between live chilling and electrical stunning. However, CO2 dissolved in water forms carbonic acid that causes aversion behavior before loss of consciousness — fish swim away from the CO2-rich water and show avoidance behaviors. This behavioral evidence suggests the CO2 transition to unconsciousness is not welfare-neutral. Electrical stunning remains the welfare-optimal choice where practicable.