The quest for a humane protocol for stunning and killing Nile tilapia (Oreochromis niloticus)

Considering the large number of individual Nile tilapia (Oreochromis niloticus) slaughtered yearly without adequate stunning, there is an urgent need for development and validation of stunning and killing methods that adhere to humane slaughter standards. In this study, we evaluated measurements of brain activity using electroencephalograms (EEG) to assess the effectiveness of percussive stunning, live chilling, electrical stunning, and a combination of electrical stunning followed by bleeding and chilling. Recordings were inspected for presence or absence of epileptic-like seizures, and for visually evoked responses (VERs). Percussive stunning using a pneumatic bolt gun resulted in both immediate and permanent loss of consciousness. This contrasts with live chilling, where it took up to 11 min for VERs to disappear, and where VERs recovered when the fish was returned to ambient water. Electrical stunning for 1 s at an intensity of similar to 8 V (RMS) cm(-1) and similar to 0.7 A(RMS) dm(-2) induced epileptic-like seizures, rendering the fish unresponsive to the light flashes for 16.3 +/- 2.4 s, demonstrating immediate albeit transient stun effects. Increasing the electrical intensity, either by prolonging the stun duration or by increasing the electrical energy, prolonged the time that VERs were lost. However, to achieve a permanent loss of VERs, similar to 14 V cm(-1) and similar to 1.1 A dm(-2) for 30 s, followed by immediate throat cutting and chilling in an ice slurry, was required. Our results clearly show that percussive bolt gun stunning is the most effective method to render Nile tilapia immediately and permanently unconscious. However, this method can be stressful for the fish, and if the fish struggle when the blow is executed, the effectiveness of the stun is at risk. Combining electrical stunning with subsequent exsanguination and chilling was also successful. Adopting a sequential approach where different procedures are combined could pave a path for successful stunning and killing without inducing unnecessary fear, stress or discomfort. For this purpose, an even more fruitful combination could be to integrate electrical and percussive stunning techniques.