Fish slaughter methods vary enormously in their welfare implications. Scientific evidence increasingly supports the need for effective stunning before killing to prevent pain and suffering at the end of life.
Fish slaughter welfare has received far less attention than mammalian slaughter despite the billions of fish killed in aquaculture each year. The most widely used method — ice slurry chilling — causes a prolonged period of consciousness during which fish are exposed to the nociceptive stimuli of extreme cold and oxygen depletion. Fish slaughtered this way typically take several minutes to lose consciousness during which behavioural and physiological indicators suggest distress. Electrical stunning, when correctly calibrated for species and size, can render fish immediately insensible — but calibration matters greatly, and under-stimulation produces immobilisation without unconsciousness. Carbon dioxide stunning is aversive and precedes unconsciousness with a period of distress. Best welfare practice is species-specific electrical or percussion stunning followed immediately by killing. Some certifications (e.g. RSPCA Assured in the UK) now require electrical stunning for farmed salmon, representing a genuine welfare improvement.