This page describes research into electrical on-board dry (i.e. out of water) stunning of commercially caught fish. Electrical stunning is used for the humane killing of farmed fish and other animals (by causing immediate unconsciousness that lasts until death), and can also improve meat quality. For an effective stun, the parameters of the current (voltage, duration etc) need to be experimentally determined for each species and this research investigates stunning of cod, haddock, dab and plaice.

According to the authors of the scientific paper and scientific presentation discussed below:

  1. Commercially-caught fish may remain conscious [and therefore likely to be suffering] for long periods during on-board storage. Trawl-caught cod and haddock were found to be conscious when tested after 2 hours.
  2. Fish that appear to be unconscious (not breathing, still, unresponsive to noxious stimuli, not displaying “eye roll” VOR reflex*) may still be conscious.
  3. Advantages of on-board stunning of wild-caught fish include:
    1. improved fish welfare
    2. improved fish quality
    3. fish are easier to handle during gutting/bleeding
    4. improved health and safety environment for fishermen.
  4. Cod and haddock can be humanely killed with a 52 VRMS dry stun lasting over 3 seconds and followed immediately by bleeding.
  5. Dab and plaice can be humanely killed with a longer dry stun (15 seconds) followed immediately by normal slaughter processing (bleeding and then chilling).

This research into electrical on-board stunning of fish forms part of a larger Norwegian-funded project to improve the effectiveness of fish handling systems (improving fish quality and the health and safety environment for fishermen) with more “gentle” handling of fish, electrical stunning for humane slaughter and automatic bleeding. This project is discussed on this website at: Norway researches effective and humane catch processing.

On-board stunning of cod and haddock

Effects of on-board storage and electrical stunning of wild cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) on brain and heart activity
E. Lambooija et al, 2010.
Fisheries Research. Volumes 127–128, September 2012, Pages 1–8


Cod fish in aquarium. Measurements of brain activity indicate cod remain conscious for long periods after capture and storage out of water.
Credit: Little Boffin (PeterEdin).

This study investigates survival and consciousness for cod and haddock landed aboard a trawler and subsequently stored out of water. Measurements of brain activity (EEG recordings) indicated that fish were still conscious (and therefore likely to be suffering) after two hours storage in dry bins, even though they had stopped breathing and lost the capacity to respond to noxious stimuli (e.g. scratching or pinching the tail). These findings demonstrate the huge welfare benefits of developing humane methods of stunning/killing as soon as possible after landing.

Besides improving welfare, humane slaughter methods can improve eating quality and fish that have been despached are easier and safer to handle during manual gutting than are fish that are alive, conscious and struggling:

“Preliminary experiments with on-board electro-stunning of fish on trawling boats indicated that this treatment could allow more rapid gutting and rinsing of fish. This could increase fish quality while at the same time safeguarding the health and security environment (HSE) of the fishermen.”

The second part of this study investigated use of a prototype “dry stunner” to stun trawl-caught cod and haddock before killing by bleeding. Humane slaughter requires that the electric stun causes a loss of consciousness that is both (a) immediate and (b) sufficient to last until death. In tests involving fish subjected to a short electric stun (less than 1 second), EEG recordings indicated that a 52 VRMS head-to-body stun delivered by the stunner caused an immediate loss of consciousness. In other tests, fish were subjected to longer stun (3 seconds) followed by bleeding. Subsequent EEG recordings indicated that the fish remained unconscious during the period tested, which suggests this duration may be sufficient for a humane kill.

Measurements of brain activity after landing and storage
Fish of both species were landed on the research trawl vessel and subsequently stored in dry bins. During this process, individual fish were removed at intervals of 0.5, 1, 1.5 and 2 hours, and tested for a number of behavioural responses (e.g. breathing, “eye roll” VOR reflex*, response to squeezing the tail and to electrical stimulation) and for brain and heart activity by electrodes placed to take EEG and ECG measurements. Results showed that, while the number of elicited behavioural responses decreased with time, brain and heart activity was maintained. EEG recordings showed alpha and beta waves (typically shown by conscious animals) indicating that both the cod and haddock were still conscious after two hours, even though many had lost the capacity for a behavioural response.

Prototype “dry stunner” to humanely kill landed fish
A second part of this study, was to test a prototype “dry stunner” for effective stunning. The prototype stunner was similar to, but smaller than, a dry stunner model used in commercial fish farming which allows large numbers of fish to be killed quickly and humanely as they pass on a conveyor belt. The humane killing of animals requires them to be stunned i.e. rendered unconscious immediately and for the stun to last until death. When a vertebrate animal is electrically stunned, a brief current applied head-to-body or head only induces an epileptic grand mal seizure in the brain, and it is generally accepted that during such a seizure the animal is unconscious. An epileptic grand mal seizure comprises a tonic phase (severe tension of all muscles), followed by a clonic phase (uncontrolled severe muscle contractions) followed by an exhaustion phase (muscle flaccidity).

Trawl-caught cod and haddock were individually stunned (head to body) for a period of 1 second at a voltage of 52 VRMS. EEG recordings indicated that the fish underwent an epileptic grand mal seizure (during which consciousness is lost) lasting from over 1 minute. Since the current had been applied for no more than 1 second, the prototype stunner had produced an immediate stun.

In a separate test, a sample of fish were stunned at 52 VRMS for a duration of 3 seconds and immediately cut to kill by bleeding. EEG recordings indicated that the fish underwent an epileptic grand mal seizure from which brain activity did not recover in the subsequent period tested (5 minutes). The unconsciousness invoked by the 3-second stun may therefore have been of sufficient duration to last until death to give a humane kill.

Initial findings in this study, presented at the 2011 HSA Symposium, have previously been discussed at Presentation: Electrical stunning of captured fish.

Electrical stunning of common dab and European plaice

Bracke et al, Barcelona 2013.
UFAW International Animal Welfare Science Symposium. 4th – 5th July 2013 Universitat Autònoma de Barcelona, Spain

European plaice

Flatfish take longer to die from lack of oxygen compared to other species. European plaice (pictured) and common dab can be humanely killed with a 15 second electric stun followed immediately by bleeding/gutting and chilling, according to a study.
Credit: Asbjørn Hansen.

Findings of this study, which follows on from the previous study to investigate electrical dry stunning of dab and plaice, were discussed at the UFAW International Animal Welfare Science Symposium in Barcelona, July 2013. The study found that both common dab (Limanda limanda) and European plaice (Pleuronectus platessa) can be rendered immediately unconscious from a brief head-first stun (lasting 1 second or less).

As in the previous study, EEG measurements and behavioural responses were used as indicators of consciousness. In both species, behavioural signs of consciousness followed the return of consciousness as measured by EEG recordings, suggesting that behaviourally quiescent fish may be capable of experiencing pain [this is an important finding, demonstrating (as in the previous study) that loss of consciousness in fish during processing needs to be scientifically validated rather than simply assumed on the basis that the animal is still and behaviourally unresponsive].

Flatfish, such as dab and plaice, are difficult to kill and take longer to die from lack of oxygen compared to other species (which means they suffer longer during commercial capture). When these species were stunned for 15 seconds, consciousness was lost for a period of 5 minutes, which the authors consider sufficient to last until death from normal on-board slaughter processing (bleeding/gutting and chilling).


* Vestibulo-ocular reflex (VOR) – a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement.