Abstract
Common slaughter procedures for African sharptooth catfish (Clarias gariepinus) include asphyxiation, ice chilling and exsanguination, which may all cause substantial suffering over prolonged periods of time before death. Therefore, comprehensive evaluations of potentially more humane slaughter procedures for this species are urgently needed. Here, we use a non-invasive electroencephalographic (EEG) method to assess the state of sensibility in African sharptooth catfish in response to various stunning methods (e.g. ice chilling, electrical stunning, electrical stunning followed by exsanguination, percussive stunning, and immersion in isoeugenol). Based on the abolition of visually evoked responses (VERs) on the EEG, ice slurry immersion induced insensibility between 2.6 and 7.6 min, during which catfish exhibited aversive behaviours. Once VERs were lost, they remained absent so long as catfish remained immersed in the ice slurry. Electrical stunning (i.e. exposure to ~1.7 A dm−2 at a water conductivity of ~997 μS cm−1) induced insensibility immediately but not irreversibly. Depending on the duration of the stun (i.e. from 1 to 10 s), catfish either regained VERs immediately or within 4.9 min after the completion of the electrical insult. However, when a 10 s electrical stun was immediately followed by exsanguination and immersion in an ice slurry, the duration of insensibility was sufficient to humanely kill catfish. When administered correctly, manual percussive stunning with a fish priest induced insensibility immediately and irreversibly. However, 36% of catfish regained VERs, which is likely explained by the difficulty associated with administering an accurate manual percussive stun of sufficient force on a live and struggling catfish. Catfish appeared to be sedated following immersion in isoeugenol (i.e. catfish were calm and easy to handle), yet VERs remained present at doses exceeding that recommended for euthanasia in salmonids, which indicates that this substance may not be suitable for stunning catfish. However, the potential for using isoeugenol as a pre-stunning sedative for improving handleability and reducing handling stress of this species warrants further investigation. In conclusion, this study clearly demonstrates that when singularly administered, none of the abovementioned stunning methods could reliably induce insensibility immediately and/or irreversibly without welfare implications. Yet, our findings indicate that these shortcomings can be resolved by using a combination of methods. This could include an electrical or percussive stun to immediately induce insensibility that should be immediately followed by exsanguination and immersion in an ice slurry to maintain insensibility until death.
Highlights
For humane slaughter, fish should be immediately rendered in sensible prior to killing and remain so until death without avoidable fear, anxiety, pain, suffering and distress (EFSA, 2004; OIE, 2019).if insensibility is gradually induced, it should be ensured that fish do not experience the abovementioned negative states during the induction phase (EFSA, 2004; Lines and Spence, 2012; OIE, 2019; Robb and Kestin, 2002; van de Vis et al, 2003)
One minute prior to dis appearing, the latency of visually evoked responses (VERs) had increased by 103 ± 37 ms (t5 = 2.813, P = 0.037) while the amplitude of VERs had decreased by 66 ± 8% (t5 = 4.126, P = 0.009) when compared to VERs prior to ice chilling
It took 5.8 ± 0.8 min for the VERs of catfish to disappear in response to ice chilling, but once lost, VERs remained absent throughout the rest of the immersion period in the ice slurry
Summary
Fish should be immediately rendered in sensible prior to killing and remain so until death without avoidable fear, anxiety, pain, suffering and distress (EFSA, 2004; OIE, 2019).if insensibility is gradually induced, it should be ensured that fish do not experience the abovementioned negative states during the induction phase (EFSA, 2004; Lines and Spence, 2012; OIE, 2019; Robb and Kestin, 2002; van de Vis et al, 2003). To establish whether different commercial stunning methods can be considered humane, a Aquaculture 531 (2021) 735887 range of behavioural indicators have been implemented to evaluate the degree of sensibility in fish (EFSA, 2004; Kestin et al, 2002; Lines and Spence, 2012; van de Vis et al, 2003). These behavioural measures include coordinated swimming and escape behaviours, ability to maintain equilibrium, reactions to painful stimuli, the vestibulo-occular or ‘eye roll’ reflex, and ventilatory reflexes (EFSA, 2004; Kestin et al, 2002). It is necessary to obtain neurophysiological or neurochemical evidence of insensibility to ascertain the impact of various commercial slaughter procedures (EFSA, 2004)
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