Farmed fish are a critical source of fo od worldwide, and aquaculture is expected to provide nearly two-thirds of the world’s fish supply by 2030. However, there is growing concern about the welfare of farmed fish, as many slaughter procedures have the potential to cause substantial suffering for prolonged periods of time. To address this issue, it is crucial to conduct comprehensive evaluations of potentially more humane slaughter procedures.
Humane slaughter requires that fish are stunned prior to killing (i.e. , rendered insensible ) and remain so until death without experiencing avoidable fear, anxiety, pain, suffering, or distress. To assess insensibility following stunning , behavioral indicators such as the ability to maintain equilibrium, reactions to painful stimuli, the vestibulo-occular or ‘eye roll’ reflex, and ventilatory reflexes have traditionally been used. However, it has become increasingly clear that these indicators alone are insufficient, and that neurophysiological evidence of insensibility must be obtained to ascertain the effectiveness of stunning.
A relatively robust approach for gauging the state of sensibility in fish is by assessing the presence or absence of visually evoked responses (VERs) from electroencephalographic (EEG) measurements . VERs are measurable changes in brain electrical potential in response to a visual stimulus (i.e., a flashing light) that produce a distinct waveform in EEG recordings milliseconds after the stimulus. This makes it a powerful tool for evaluating the effectiveness of stunning procedures, as the abolition of VERs has been confirmed to be an objective and unequivocal indicator of in sensibility in numerous species of mammals, birds and fish.
Thus, here we utilized a non-invasive electroencephalographic (EEG) method to monitor changes in the state of sensibility of African sharptooth catfish (Clarias gariepinus ) to assess the effectiveness of various stunning procedures (Brijs et al., 2021).
Materials and methods
A custom-made suction cup fitted with electrodes was attached externally to the head of the catfish (Fig. 1A-B). The EEG of catfish was then continuously measured in response to 150 ms light flashes at 2 Hz from a strobe-light in a dark room. EEG signals were subsequently filtered and analysed for VERs for 10 min prior to stunning (Fig. 1C).
Following the pre-stunning period, catfish were subjected to either ice chilling (i.e., immersion in an ice slurry for 30 min), electrical stunning (i.e., 2 s, 5 s or 10 s exposure to an electrical current of 1.69±0.09 A dm−2), electrical stunning and exsanguination (i.e., 10 s exposure to an electrical current of 1.69±0.09 A dm−2 followed by a throat cut), percussive stunning (i.e., a sharp cranial blow by a fish priest), or isoeugenol immersion (i.e., immersion in 10, 20, 30, 60 or 100 mg L−1 isoeugenol). EEG signals were then continuously recorded in response to light flashes to evaluate if, and how long, it took for VERs to disappear and then reappear (Fig. 1D). Behavioural indicators (i.e. , ventilatory/body movements and aversive behaviour) were also recorded throughout the stunning procedures.
Results and discussion
Based on the abolition of VERs:
Ice chilling induced insensibility between 2.6 and 7.6 min, during which catfish exhibited aversive behaviours (e.g., thrashing around and trying to escape vigorously). Once VERs were lost, they remained absent so long as catfish remained immersed in the ice slurry.
Electrical stunning induced insensibility immediately but not irreversibly. Depending on the duration of the stun, catfish regained VERs within 0.5 to 4.9 min after the completion of the electrical insult.
Electrical stunning and exsanguination induced insensibility immediately and irreversibly if fish were immersed in an ice slurry following the throat cut. However, if placed in warm water following the throat cut, fish regained sensibility prior to death.
Percussive stunning induced insensibility immediately and irreversibly when administered correctly . However, 36% of catfish regained sensibility, which is likely explained by the difficulty associated with administering an accurate manual percussive stun of sufficient force on a live and struggling catfish.
Immersion in isoeugenol at doses exceeding that recommended for euthanasia in salmonids did not induce insensibility in catfish, which indicates that this substance may not be suitable for stunning this species. However, the potential for using isoeugenol as a pre-stunning sedative for improving handleability and reducing handling stress of this species warrants further investigation.
Conclusion and future directions
This study clearly demonstrates that when singularly administered, none of the abovementioned stunning procedures could reliably induce insensibility immediately and/or irreversibly without compromising fish welfare . However, these shortcomings can be resolved by using a combination of methods, as a correctly administered electrical or percussive stun can immediately induce a state of insensibility, which can be maintained until death if fish are exsanguinated and immersed in an ice slurry directly after the stun.
To further improve the welfare of fish during slaughter, a promising future direction is to develop a portable, user-friendly, and cost-effective diagnostic tool based on our laboratory-based neurophysiological technique. This tool can be used by various stakeholders, including stunning equipment manufacturers, fish farmers, and regulators, to verify, test, and optimize stunning methods before commercial use. By implementing these measures, we can significantly enhance the welfare of fish during slaughter and ultimately achieve more sustainable and ethical practices in aquaculture.
Brijs et al., 2021 . Humane slaughter of African sharptooth catfish (Clarias gariepinus): Effects of various stunning methods on brain function . Aquaculture 531 , 735887.