Aquaculture Europe 2023

September 18 - 21, 2023


Add To Calendar 20/09/2023 11:30:0020/09/2023 11:45:00Europe/ViennaAquaculture Europe 2023PROSPECTS AND PITFALLS OF USING BRAIN FUNCTION TO ASSESS THE EFFECTIVENESS OF STUNNING IN FISHStrauss 3The European Aquaculture Societywebmaster@aquaeas.orgfalseDD/MM/YYYYaaVZHLXMfzTRLzDrHmAi181982


Albin Gräns

Department of Animal Environment and Health, Swedish University of Agricultural Sciences.




Countries within the European Union (EU) often take pride in its comparatively stringent and comprehensive animal welfare legislations ( In the EU, all animals should be protected from any unnecessary pain, suffering or injury (COUNCIL DIRECTIVE 98/58/EC). In addition, during killing and slaughter, all animals must be stunned before the method of killing is applied, and the animal must not regain consciousness before it dies (Article 4, EC No. 1099/2009). When the European Commission reviewed the welfare of its five key aquaculture species during slaughter in 2017, they identified serious shortcomings when it came to safeguarding the welfare during slaughter. For most species and in most countries, the methods used for stunning did not comply with humane killing. The report also highlighted that it essential to neurologically investigate (e.g. using electroencephalograms, EEGs) the humaneness of the stunning method in order to verify whether the fish immediately loses consciousness or not instead of relying solely on the visual verification of consciousness.

Material and methods

Recently, we have developed, validated, and implemented a non-invasive technique to measure and interpret EEG in fish (Bowman et al. 2019, Fig 1). With our technique, three silver chloride disc electrodes are fitted to a custom-made silicone suction cup. The electrodes are connected to three shielded wires that are threaded through the silicon cup and connected to an animal bioamplifier that fed the signal to a PowerLab system. A thin layer conductive EEG paste is applied in to each of the electrodes to ensure good contact between the skin of the fish and the electrodes. During EEG measurements, the cup is centered above the approximate location of the left optic lobe and secured using suction generated by the peristaltic pump. Using this technique, a fish can be judged to be unconscious and insensible if the EEG shows abolition of evoked electrical activity in the brain, e.g. visual evoked responses (VERs) and/or changes that are incompatible with consciousness (i.e. grand mal epilepsy, frequency shifts and/or prolonged quiescent periods on the EEG).


Using this technique, we have successfully carried out a series of studies and identified several serious welfare hazards with the various stunning and slaughter procedures that are currently used (Bowman et al. 2019, 2020, Brijs et al. 2021 & Hjelmstedt et al. 2022). Our studies show that the technique can be used to investigate the effects of various stunning/killing methods, including electrical stunning, CO2-narcosis, percussive stunning and chemical narcosis, on brain function (Bowman et al. 2019, 2020, Brijs et al. 2021 & Hjelmstedt et al. 2022). Furthermore our findings strongly support that it is critical to apply neurological investigations when the humaneness of the slaughter method is investigated rather than relying on visual verifications alone, as the latter is unreliable and underestimates the time it takes for a fish to become unconscious. In addition, our results indicated that some neurophysiological indicators of unconsciousness that traditionally have been used to validate electrical stunning procedures in birds and mammals might not be appropriate for fish. For example, 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). Instead, the fish can regain responsiveness within seconds after the seizure ends (Hjelmstedt et al. 2022)


These findings have severe welfare implications since many stunning methods thought to render fish unconscious may only cause paralysis, which means that fish are still sensible when exsanguinated and eviscerated. In addition, the loss of sensibility can be a gradual process, during which it is unclear when the fish is no longer susceptible to anthropogenic stress and, therefore, not capable to experience pain, distress, and anxiety. In such situations, it is difficult, even for a trained expert, to make an objective and evidence-based interpretation of an animal’s level of consciousness. Collectively, these results highlights the necessity to carefully consider and evaluate how neurophysiological indicators of unconsciousness should be used to validating different stunning methods and to safeguard the welfare of farmed fish during slaughter.


Bowman J, Hjelmstedt P & Gräns A (2019). Non-invasive recording of brain function in rainbow trout: evaluations of the effects of MS-222 anaesthetisa induction. Aquaculture Research, 50(11), 3420-3428

Bowman J, van Nuland N, Hjelmstedt P, Berg C & Gräns A (2020). Evaluation of the reliability of indicators of consciousness during CO2 stunning of rainbow trout and the effects of temperature. Aquaculture Research, 5194-5202

Brijs j, Sundell E, Hjelmstedt P, Berg C, Sencic I, Sandblom E, Axelsson M, Lines J, Bouwsema J, Ellis M, Saxer A & Gräns A (2021). Humane slaughter of African sharptooth catfish (Clarias gariepinus): effects of various stunning methods on brain function. Aquaculture, 531 735887

Hjelmstedt P, Sundell E, Brijs J, Berg C, Sandblom E, Lines J, Axelsson M, Gräns A (2022). Assessing the effectiveness of percussive and electrical stunning in rainbow trout: Does an epileptic-like seizure imply brain failure? Aquaculture, 552, 738012