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Research article2022Peer reviewedOpen access

Assessing the effectiveness of percussive and electrical stunning in rainbow trout: Does an epileptic-like seizure imply brain failure?

Hjelmstedt, P.; Sundell, E.; Brijs, J.; Berg, C.; Sandblom, E.; Lines, J.; Axelsson, M.; Grans, A.


Both percussive and electrical stunning have been highlighted as methods that can be used to quickly render fish unconscious before being killed. However, accurately assessing unconsciousness in animals following stunning remains challenging, and thus methods for reliable interpretation and validation of different stunning methods are urgently needed. Here, we used a non-invasive technique to continuously record electroencephalograms (EEG) of rainbow trout (Oncorhynchus mykiss) enabling us to compare the effects of both percussive stunning, using a captive bolt gun, and various combinations of electrical stun parameters delivered in water. The EEG signals were assessed for the absence or presence of an epileptic-like seizure and for visually evoked responses (VERs). No epileptic-like seizures or VERs were observed after captive bolt stunning. We found that it is possible to reliably induce an epileptic-like seizure and an immediate, but short lasting, loss of VERs following a 1 s exposure to an electrical field strength of at least 2.8 V-RMS cm(-1) and current density of 0.22 A(RMS) dm(-2) in water of conductivity of similar to 1000 mu S cm(-1) using a 50 Hz AC current. However, to avoid recovery of VERs shortly after the stun, it was necessary to increase the duration of the stun application (>= 30 s), the electrical field strength (10.2 V-RMS cm(-1)) and the current density (0.84 A(RMS) dm(-2) respectively). We found no clear relationship between presence and absence of ventilation and VERs following electrical stunning in rainbow trout, highlighting that loss of ventilation may not be a good indicator of brain failure in rainbow trout. Our results clearly show that the presence of an epileptic-like seizure following an electrical stun does not guarantee a prolonged period where the fish is unresponsive to visual stimulation (i.e. absence of VERs). It was further found that VERs can return before the end of the seizure. As both presence of a seizure and absence of VERs have been used independently as indicators of unconsciousness in fish, we emphasize the necessity to carefully consider and evaluate the reliability of neurophysiological indicators of unconsciousness when validating methods to stun fish.


EEG; Welfare; Slaughter; Visually evoked response; Tonic-clonic

Published in

2022, Volume: 552, article number: 738012
Publisher: ELSEVIER