This document provides an overview of research on the efficacy of aquatic animal stunning, a widely supported intervention among aquatic animal welfare advocates to reduce pain during slaughter. It offers a brief introduction to electrical stunning, the methods used to assess consciousness, and academic perspectives on its effectiveness. The information presented may raise some doubts about the reliability of current stunning machines. While I still believe promoting stunning equipment is a strong approach overall, I think more research is needed, and we should reassess our views on the efficacy of interventions focused on aquatic animal stunning.

Key take-aways:

•  Actual field studies investigating the efficacy of stunning equipment are scarce. Reliable laboratory studies testing stunning equipment are more common, but it is unclear how well these results translate to practical field usage.
• Behavioral indicators, such as the Vestibulo-Ocular Reflex (VOR), which can be manually observed, appear to be unreliable measures of insensibility. Despite this, both the aquaculture industry and animal welfare organizations rely on these indicators for field tests, as seen in this example involving sea bass and sea bream.
• Experts express skepticism regarding the current use of behavioral indicators to determine whether fish are unconscious. Some believe that EEGs may be too conservative, while others are doubtful about the effectiveness of behavioral indicators, suggesting they may not work adequately.
• There is skepticism about the efficacy of current aquatic animal stunning machines. Some experts have noted that the poor performance of certain stunners is concerning.
• The current deployment of stunning machines is considered "premature" by some experts, due to insufficient evidence that they are effective in the situations where they're used.
• While these issues warrant concern, interventions aimed at encouraging the industry to adopt stunning equipment can still be highly impactful in reducing extreme levels of suffering. However, we need to approach this intervention with nuance, acknowledging the current limitations and advocating for further research and development to ensure that stunning systems are effective and reliable.

Background

Before delving into the methods of assessing whether an aquatic animal has been effectively stunned, it's important to understand the background of this process. Ensuring that fish are properly stunned is crucial, as ineffective electrical stunning can cause significant pain and paralysis without loss of consciousness.

Current Practices in Aquatic Animal Slaughter

An alarming number of aquatic animals are being slaughtered annually. Accurate numbers are not available; however, estimates suggest that between 1.1 and 2.2 trillion wild-caught fish are killed each year [1]. Numbers for farmed fish are also very uncertain. A 2023 study estimates that around 124 billion farmed fish are killed annually [2]. The midpoint estimate for farmed decapod crustaceans (such as shrimp and lobsters) is approximately 413 billion individuals [3]. Additionally, annual wild-caught shrimp slaughter is estimated at 25 trillion individuals [4]. The number of aquatic animals being farmed is growing, with projections estimating that marine aquaculture will more than double by 2050 [5].

The slaughter methods for aquatic animals, predominantly involving suffocation or ice-induced hypothermia, cause considerable suffering; only around 5 percent of farmed aquatic animals are rendered unconscious before slaughter^[1]. Unstunned slaughter can lead to animals enduring prolonged extreme pain. As an example Sea Bass and Sea Bream, which are commonly slaughtered using ice-induced hypothermia, will remain conscious for 5-40 minutes, during which they show signs of immense suffering [6]. These prevalent practices underscore a critical animal welfare concern, emphasizing the need for more humane treatment.

Stunning systems

Equipment exists that can stun aquatic animals so that they will no longer have to endure the extreme suffering of unstunned slaughter. Companies have developed stunning systems that can either render animals unconscious through electricity or by administering a blow to the head of animals (percussive stunning). 

Electrical stunning involves exposing fish to an electric current to rapidly render the fish unconscious, either temporarily or permanently. Electrical stunning comes in two forms: in-water and in-air. With in-water systems the aquatic animals are usually pumped through a tube in which a section of the water is electrified. In-air systems first dewater the animals before putting them on a conveyor belt that brings the animal in direct contact with an electrical current. 

Out of the companies that design/build electrical stunners Ace Aquatec and Optimar are the largest. For percussive stunners Baader is the only company known to have systems in the field. This document gives an overview of all known companies active in this market including more information.

Assessing consciousness

Determining whether an animal has been rendered unconscious is challenging since we cannot know exactly what is happening inside an individual's brain. However, some tests have been developed that are considered valuable indicators of consciousness. In a laboratory setting, fish can be reliably judged to be unconscious using electroencephalogram (EEG) recordings.

An EEG measures electrical brain activity in an individual. When used in combination with an evoked response, it allows researchers to analyze the brain’s capacity to respond to stimuli. One method that is often used is the visually evoked response (VER). VERs are analyzed in fish using repetitive visual stimuli (e.g., a flashing light as shown in Figure 1) and are considered to be one of the last functions to be lost before the brain of a fish becomes completely unable to process input from its environment.

EEGs and VERs are rarely used in the field due to the complexity of these measurements. Instead, behavioral indicators, such as the Vestibulo-Ocular Reflex (VOR) shown in Figure 2, are primarily used. These indicators can be manually assessed to determine whether an individual is conscious. However, interpreting these indicators requires caution, as they may not reliably reflect the true state of consciousness.

The different behavioral indicators that can be used are displayed in the table below. Each test includes a 3-point scoring system, where:

• 0 represents no response,
• 1 represents an attenuated or abnormal response or behavior,
• 2 represents a normal and clear response or behavior.

Field research into the efficacy of stunning equipment predominantly relies on behavioral indicators. EEGs require specialized equipment and are therefore mostly performed in laboratory settings. Most current research into stunning machines involves using laboratory EEG experiments to try and validate observable behavioral indicators for field use.^[2] 

Advancements are currently being made with the development of a system that allows Visually Evoked Response (VER) tests to be conducted in the field using a mobile EEG system. This innovation bridges the gap between laboratory and field testing, offering a more reliable method for assessing fish consciousness outside the lab. Although these systems represent a significant step forward, they are likely to remain more complex than using behavioral indicators due to the specialized equipment and expertise required. Nevertheless, they provide a more objective and precise assessment of consciousness, which is crucial for validating the effectiveness of stunning methods in real-world settings [9].

Efficacy of stunning

Evaluating the effectiveness of current stunning solutions for aquatic animals is crucial for improving welfare standards. While researching the feasibility of launching a company to design and build more affordable and effective stunning machines, I conducted an interview that prompted further investigation into the efficacy of existing technologies. This experience provided new insights into these interventions. The following section will present findings from a brief literature scan and expert interviews regarding the performance of current stunning systems.

Literature scan

This section is not intended to be a full literature review but is aimed at showing some of the skepticism in the field and to provide examples for specific fish species. Research on the effectiveness of stunning systems is limited, with most academic studies focusing on laboratory settings and rarely translating findings to farm environments. However, existing literature includes mentions on the reliability of behavioral indicators in assessing consciousness and offers some insights into stunning efficacy across different species.

One study from 2021 looking into African Catfish stunning for example exemplified how different species react to stunning. The physiology of Catfish makes them "comparatively resistant to all the investigated stunning methods when administered singularly" . This studies also mention that "validation of stunning methods should be based on neurophysiological indicators, as the use of behavioral indicators alone can have serious welfare implications"[10]. 

One other 2024 study, focusing on rainbow trout, found "no clear relationship between presence and absence of ventilation (a behavioral indicator) and VERS following electrical stunning. Highlighting that loss of ventilation may not be a good indicator of brain failure in Rainbow Trout". The study concluded by stressing that "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" [11]. A different study looking into CO2 stunning also showed that there was a "poor relationship between visual indicators of consciousness (in this case ventilation) and loss of sensibility, as VERs were present up to 3,5 min after ventilation was lost and up to 6,5 min after the fish lost equilibrium"[12].

In the case of common carp, one 2018 study noted that "Under field conditions, percussion (applied manually) and electrical stunning might be poor inducers of unconsciousness before slaughter, while a combination was most effective." The study also highlighted that "VER could be recorded already at 30 ± 8 s post stunning. This indicates a fast recovery of carp from electrical stunning when exposed to current densities in the range of those generated by commercially available stunning instruments for fish" [13]. A short stun duration brings the risk that animals recover before actually going through the slaughter process. 

For Salmon there are studies stating that "electrical stunning in combination with cold brine is deemed to be a humane slaughter method and that the Salmon did not recover before end of life" [14].  However, a newer study pointed out that "Even though most salmon are stunned using these methods, there are situations where the blow is not strong enough, or that the electrical stunning does not work as intended and the fish are injured on top of not being unconscious. It is unknown exactly how many fish this affects" [15].

Expert interviews

Given the limited applicable research, I conducted interviews with experts to gain insights into the efficacy of stunning systems in the field. To maintain confidentiality, I have omitted their names as I did not obtain permission to share them. These academics are esteemed in their fields, and their expertise has significantly informed the findings in this document. Despite the difficulty in obtaining precise information on the effectiveness of current systems, I have included some estimates based on these interviews. However, these should be interpreted with caution, as they may not fully represent the complexity of the situation.

Expert A

Expert A, who has extensive academic experience and has worked on studies examining the efficacy of electrical stunning, expressed strong skepticism about current stunning machines and the methods used to assess fish sensibility. Based on our conversation, I would estimate that they think that efficacy could be anywhere in a range between 40-80%.

Other useful takeaways from the meeting:

• For percussive systems it’s relatively obvious that the systems can be improved by making them more accurate. This is not the case for electrical stunning.
• There seems to be a large variability in how different individuals respond to the same stun. One fish from the same species and same farm might be stunned using specific settings whilst others are not.  Some of them revive almost immediately.
• There is no external validation of current stunning systems
• Very big steps are necessary before we can start calling the current electrical stunning machines “humane”. There are no clear directions to improve current stunning systems. They mentioned that in tests the systems often perform poorly, sometimes it was embarrassing how poorly the stunners performed.
• In a farm setting the fish are often already exhausted before being stunned due to the crowding methods or pumps. Exhaustion, whilst bad, might make it easier to stun the fish.
• There seems to be a lot of bad research out there. Many researchers are already closing in on retirement and there don’t seem to be enough people entering the field to replace them. Hypothesis for this: young people entering animal sciences often want to help animals, nobody thinks about fish and the step towards researching slaughter processes is even further away for many people.
• Stunning shrimp might be premature. Doesn't know what they should be looking for to check if it is working.
• On whether fish regain consciousness before end of life: No, that doesn't seem to be the case. They think that the VERs might not show because of cold induced by the ice slurry. It is also possible that fish remain stunned due to being exhausted before being stunned.

Expert B

Expert B is an academic with extensive experience in finfish stunning. They have participated in several working groups and have been involved in developing stunning machines for different types of fish. Expert B also appears to be skeptical about the efficacy of electrical stunning. When asked for an estimate of efficacy percentages, they did not provide one, likely due to uncertainty and a lack of supporting data.

Some takeaways from multiple meetings:

• ACE Aquatec is, like other manufacturers, too optimistic ^[3]. Behavior and visual signs provide insufficient information.
• A significant challenge is that when fish are stunned with electricity and then killed, it must be ensured that they do not regain consciousness. Using ice water is an option, but only if the cold chain isn't interrupted, and it remains to be seen whether that works in practice.
• Expert A, as well as we, have observed that if you cut the belly artery after electrical stunning for optimal bleeding, fish species can regain consciousness.
• Reliable methods that are currently used (EEG’s) work but require way too much time. In the last 25 years studies have been conducted to find stunning specifications for 20 species. There are roughly 400 finfish species that are being farmed, current methods will take way too long. 

Expert C

Expert C has worked on humane slaughter for many types of animals and is highly active in the academic field of animal welfare. They seem to be less skeptical about electrical stunning compared to other experts. Based on our conversation, I estimate that they believe stunning is roughly 90–95% effective.

Some takeaways from the meeting:

• VER checks with EEG’s are too conservative. It is likely that fish can be unconscious and still respond to VER’s. This is apparently also the case in some mammals?
• They agreed that implementation in the field is still a problem and did not know how many animals remain stunned until end of life. They did not seem concerned about this.

Expert D

Works on both invertebrate and finfish welfare including stunning. Has a lot of experience in this field. No clear answers on how well current systems work but they mentioned that even if they only worked for 70-80% of the fish that it would still be great. Not sure whether they saw this as a likely efficacy or if they were just considering worst case scenarios.

Expert D expressed that attempting to have the entire industry adopt stunning machines may be premature, as they believe there is not yet sufficient scientific evidence to support widespread implementation. They emphasized the need for more research and validation before advocating for industry-wide adoption.

Summary of expert interviews

There appears to be significant skepticism among academics regarding current stunning systems. Only Expert C seemed optimistic about the effectiveness of these systems, while the other experts expressed doubts about both the methods used to asses consciousness, the efficacy of current machines and their widespread implementation.

The experts also highlighted specific problems with the current machines, which will be discussed in the next section. Additionally, the interviews underscored a considerable talent gap in this field. Based on one interview I estimate that only around 10 to 20 people worldwide possess the skills to perform EEG measurements on fish. If animal welfare organizations plan to campaign for the adoption of stunning, they should consider that there are currently not enough qualified personnel to perform robust inspections or conduct the fundamental research necessary to accurately assess whether a fish is unconscious.

Issues in current stunning machines

Through these interviews, numerous examples have surfaced of stunning systems used in the field not working properly, either due to incorrect usage or inherent flaws in the systems themselves. These problems occur in addition to the fundamental issue of the efficacy of these systems, further impacting their effectiveness in real-world applications. The problems can be categorized into operational and technical challenges:

Operational problems

• Animals are often stacked on top of each other when passing through dry electrical stunning systems, rendering the shocks ineffective and leaving many animals unstunned.
• When animals are in direct contact with each other on the conveyor belt, the electrical current can pass through multiple animals not directly between the electrode and the belt. This leads to animals receiving multiple insufficient "pre-shocks," which fail to induce unconsciousness and likely cause additional stress and suffering.
• One expert mentioned a case in which a machine was left on during a personnel break. Consequently, animals in the stunner and collection hopper (structure that funnels the animals into the machine) were continuously electrocuted for about half an hour, resulting in prolonged suffering and significant animal welfare concerns.

Technical problems

• For in-water stunning systems, the conductivity of the water can cause issues. Stunning parameters are often developed in laboratory settings with controlled water conditions. However, in field applications, variations in water conductivity can prevent animals from receiving an adequate shock. This is a significant problem that currently lacks effective solutions.
• Variance in fish size makes it difficult to use one stunning setting that works for all of the individuals going through the machines.

Research into better stunning methods could potentially improve the efficacy and retroactively address operational issues for machines already being used in the field. However, I have reservations. While adjustments to operational procedures or stunning parameters may mitigate some problems, many issues appear to be rooted in the fundamental design of the machines themselves and cannot be improved by simply changing the settings of a machine.

Conclusion

Aquatic animal stunning is more complex than I previously thought, and there is significant uncertainty about the efficacy of current stunning systems. 

Despite the update around efficacy, I believe that stunning aquatic animals should remain a top priority for the animal welfare movement. Even in a conservative scenario—where only 70% of animals receive an effective shock and remain unconscious until death—the reduction in suffering for the majority would still be substantial, especially under ethical frameworks that put more value on extreme levels of suffering.

Acknowledgements: I would like to express my sincere gratitude to Kevin Xia, Johannes Pichler, Soemano Zeijlmans, and Aaron Boddy for their valuable feedback and insightful comments, which significantly improved the quality of this document. Any mistakes are mine. 

1. ^{^}

   Approximate number mentioned in a conference talk. I expect this number only applies to finfish and does not include crustaceans, for which I expect the number to be even lower.

2. ^{^}

   A more in-depth explanation on this topic can be found in this paper.

3. ^{^}

   This is referring to the claim on the Ace Aquatec website that their A-HSU system is 100% effective.