Review: Electrical Stunning Does Not Yet Ensure Prolonged Insensibility In Several European Finfish Species

Short Summary

• We reviewed the evidence on electrical stunning for three small-sized European farmed fish species (~800M killed annually) to help interested stakeholders navigate uncertainty around the effectiveness of this technology in achieving humane outcomes.
• For gilthead seabream and European seabass, electrical stunning is largely unproven. Further evidence is needed to demonstrate commercial stunners can deliver humane outcomes for these species even under optimal lab conditions.
• The evidence is much more promising for small-sized rainbow trout. But results suggest performance is often variable and sensitive to machine calibration. More field studies would help confirm the technology works well in real-world conditions.
• We recommend interested stakeholders help widen the evidence base, including through pushing machine manufacturers to make performance data more widely available and through facilitating field trials, ideally using innovative low-cost protocols where possible.

Executive Summary

Background

• Around 800 million rainbow trout, gilthead seabream, and European seabass are farmed and killed annually for European markets.
• Current slaughter methods for these species are widely considered inhumane, and electrical stunning might be the only viable option to stun these small-sized fish at scale.
• Recognizing this, many major retailers, producers (1 2), and certifiers (1 2) are seeking to transition towards electrical stunning for these species.
• But the scientific evidence for whether electrical stunning actually achieves humane outcomes for these species is surprisingly sparse.
• We conducted an AI-accelerated review of evidence on electrical stunning for these species to help stakeholders navigate uncertainty around the effectiveness of this technology in achieving humane outcomes.

Findings

• For gilthead seabream and European seabass, electrical stunning remains unproven.
  • We found only a handful of lab studies for each of these species and no field trials.
  • The higher-quality studies pointed to sensibility recovering within two minutes, much quicker than the 5–50 minutes that might be needed for follow-on slaughter processes to fully complete.
• The evidence for rainbow trout is more promising, though variable and imperfect.
  • We found 11 lab studies and one field trial for small-sized rainbow trout.
  • Several high-quality lab studies pointed to machine settings associated with the majority of fish appearing insensible for at least 15 minutes, as did the field trial.
  • But performance appeared highly sensitive to settings; some fish recovered fast; indicator reliability varied across studies; and there were few field trials.

Welfare Implications and Conclusions

• Major knowledge gaps remain: Electrical stunning seems very understudied for these species. We would feel more confident endorsing electrical stunning as a humane technology for these species if the evidence base used the best indicators of sensibility, and included field studies covering a wide range of machines and commercial settings.
• Currently available electrical stunners plausibly offer counterfactual welfare improvements for rainbow trout, but the case is much weaker for seabream and seabass: If properly maintained and calibrated, we speculate the best commercial stunners could improve welfare outcomes for rainbow trout on balance, even if not all fish are humanely stunned. The limited evidence makes us far less confident in the same claim for seabream and seabass.

Next Steps

• We recommend interested stakeholders help widen the evidence base for these species, including by pushing machine manufacturers to make performance data more widely available and by facilitating field trials, ideally using innovative low-cost protocols where possible.
  • For seabream and seabass, we believe the priority is finding commercially viable parameter settings that can achieve longer-lasting insensibility.
  • For rainbow trout, we believe the focus should be on field trials to validate that the technology is working as intended, and to help develop practical recommendations to support a wider roll-out (or improve existing on-farm machines, where appropriate).
• If subsequent research suggests increasing the duration of insensibility proves too difficult, exploring ways to scale faster follow-on kill methods will likely be an important priority.

Definitions

We use the term fish to refer to finfish rather than shellfish, such as crustaceans and mollusks. We use seabass as shorthand to refer to European seabass (Dicentrarchus labrax), seabream to refer to gilthead seabream (Sparus aurata), and trout to refer to rainbow trout (Oncorhynchus mykiss).

Rainbow trout are harvested at many different sizes. Unless otherwise specified, this report considers small rainbow trout, or fish of this species harvested at a weight below 1.2kg.

We distinguish between two categories of indicators used to assess whether a fish is insensible. Operational indicators are visual signs observable in commercial settings, such as loss of equilibrium, cessation of rhythmic gill movements, and absence of the eye-roll reflex. Neurological indicators are direct measures of brain activity, such as visually evoked responses (VERs) that measure whether the brain is processing external visual stimuli and are considered the gold standard for assessing insensibility. VERs are generally recorded using electroencephalogram (EEG) technology.

The term insensibility refers to a state in which a fish is unable to perceive or respond to stimuli, including pain. Note that insensibility is distinct from immobility or paralysis: a fish may appear unmoving or unresponsive under electrical current while remaining capable of experiencing pain. We use stunning to refer to the process of rendering fish insensible by some method (e.g., electrical exposure or a percussive strike to the brain), and stunned to describe fish that have been rendered insensible by such methods. We use electrical exposure to refer to the period during which electricity flows through a fish, without implying anything about whether insensibility was achieved.

Introduction

The welfare of farmed fish at the time of slaughter is a critical issue affecting billions of animals annually. Current standard slaughter methods for Mediterranean species, predominantly asphyxia in air or ice slurry, are widely considered inhumane due to the prolonged duration of suffering they induce. We estimate that around 400 million gilthead seabream and around 200 million each of European seabass and small rainbow trout were slaughtered for European consumption in 2021. There is growing momentum among advocacy groups, retailers, and producers to transition to stunning methods that render fish insensible prior to killing, a move that represents a large potential welfare improvement. Electrical stunning has emerged as a primary candidate for this transition, as it theoretically offers a scalable solution for high-volume harvests.

This shift is increasingly driven by market forces and certification requirements. Major retailers and producers have made public commitments to adopt these technologies. For instance, since 2013, Tesco has collaborated with suppliers to test innovative electrical stunning methods for seabream and seabass, and the company currently requires pre-slaughter stunning for its own-brand products, explicitly prohibiting ice slurry without stunning. Carrefour Spain has committed to sourcing electrically stunned seabream and seabass by 2027. Similarly, major producers such as Avramar and Kefalonia Fisheries are working to implement “electrical stunning in all our farms in Spain by the end of 2027” and to harvest seabream, seabass, and trout “in a manner that is more respectful to animal welfare.”

Leading aquaculture certification bodies are updating their standards to mandate these practices, encouraging industry-wide adoption. The most recent update of the Aquaculture Stewardship Council (ASC) Farm Standard will mandate that fishes be rendered insensible by percussive, electrical, or anaesthetic stunning by 2028 or earlier, depending on species. Furthermore, the new GLOBALG.A.P. Integrated Farm Assurance v6, effective from January 2025, mandates phasing out ice slurry and asphyxia where alternative technology is available and proven effective for a given species.

Despite this rapid policy and commercial alignment, the scientific validation of electrical stunning efficacy for these specific species remains complicated. While manufacturers and producers report operational benefits and quality improvements, rigorous peer-reviewed evidence confirming that these methods consistently render fish immediately and irreversibly insensible in commercial settings is scarce (J. Saraiva et al., 2024). This literature review aims to evaluate the evidence on welfare outcomes for rainbow trout, European seabass, and gilthead seabream.

Scientific Context

To evaluate whether electrical stunning can achieve humane welfare outcomes, we must understand what constitutes humane stunning and how it can be reliably measured.

Humane Stunning and Slaughter for Fish

Most authorities define a humane stunning method as requiring immediate insensibility that lasts until death. However, widely used commercial slaughter methods for the species reviewed fail to meet this standard a majority of the time. For gilthead seabream and European seabass, the most prevalent method in Europe remains asphyxia in ice slurry (sometimes referred to as ‘thermal shock’). While electrical and percussive stunning are more widely adopted for rainbow trout, asphyxia via ice or air is still utilized in some supply chains (Clemente et al., 2023; Jung-Schroers et al., 2020). These asphyxia-based methods are generally considered inhumane because they do not induce immediate insensibility.

Electrical stunning involves passing an electric current through an animal, either directly or through a stunning medium, to render them insensible. To achieve a humane stun, the current must be sufficient to cause immediate insensibility—and not merely muscle immobility—that lasts until the fish dies from the subsequent slaughter method. The time between stunning and death varies depending on the killing method employed. Commercially common follow-on killing methods include asphyxia in air or ice slurry, which can take 5–50 minutes to cause death (de la Rosa et al., 2021; Van De Vis et al., 2003). Faster kill methods include bleeding or percussive stunning, which may take only seconds to minutes. Understanding these timelines is essential when interpreting and evaluating recovery time data. Recovery before the follow-on kill process is finished could result in a fish experiencing acute suffering.

Challenges in Achieving Humane Stunning

Several factors complicate the achievement of humane electrical stunning in fish. These include: balancing long-lasting insensibility while avoiding carcass damage and commercial viability; selecting electrical parameters to handle variation in fish size and conductivity; and ensuring equipment is appropriately maintained in real-world commercial conditions.

First, producers and researchers must identify electrical parameters that achieve the right balance: the current must be strong enough to induce genuine insensibility (not just paralysis) that persists until death, yet not so strong that it damages tissue and reduces product quality in ways that affect meat saleability. Achieving insensibility that lasts until death is particularly challenging for aquatic animals. Many fish species have evolved to be tolerant of low-oxygen environmental conditions, so it can take a long time for kill methods centered around asphyxia to fully complete. The small harvest size of Mediterranean farmed fish species can make faster kill methods commercially challenging. Consumer reliance on whole-fish presentation as a quality cue can also act as a barrier to using faster-acting follow-on kill methods (e.g., decapitation or percussive stunning) that might adversely affect appearance.

Second, determining the appropriate electrical parameters is challenging because the immediacy and duration of insensibility—that is, the way a fish experiences a stun—can depend on complex interactions between factors such as size, species, skin conductivity, temperature, orientation, and water conductivity. Lab-based studies suggest that the duration of insensibility across groups of fish can vary a lot depending on how operators set the electrical parameters (e.g., amplitude, frequency, duration). Insensibility can also vary considerably within a given set of electrical parameters for individual fish. Manufacturers often provide recommendations for given species and conditions, but ultimately, stun effectiveness can be adversely affected if settings are not routinely adjusted to take into account variation in operating conditions.

Third, even when stunners and electrical parameters are identified as working well in laboratory conditions, consistently ensuring high performance in real-world commercial settings presents additional challenges. A vast majority of the studies included in this review are laboratory studies, typically with skilled or trained staff, small sample sizes, new or custom-built stunning machines, and no power limitations. Electrical stunners in real-world commercial conditions are likely to operate under a number of constraints (e.g., space and power limits on boats, maintenance levels, gaps in operator training, different handling systems, variability in fish size, and variability in environmental conditions). We believe it is reasonable to expect that welfare outcomes in real-world commercial conditions will be worse than in controlled laboratory conditions, and this gap between lab results and commercial reality raises important questions about how we should evaluate electrical stunning from a welfare perspective.

Challenges in Evaluating Welfare Outcomes

There is often confusion between an ideal humane standard (immediate insensibility that lasts until death) and a “better than the status quo” standard (counterfactual improvement). The nuance between these two standards often gets lost when discussing whether a technology is acceptable in welfare terms, sometimes leading to miscommunication between advocates and scientists. Even if electrical stunning does not meet the ideal standard of maintaining insensibility until death, it may still represent a welfare improvement over current common practices like ice slurry or air asphyxiation if it reduces the overall suffering experienced during slaughter. However, to compare welfare outcomes of different technologies, we must first be able to measure insensibility and stress levels, which have been difficult to easily quantify.

Measuring Insensibility

The scientific consensus identifies two primary categories of indicators for assessing insensibility in fish species:

Operational Indicators: Visual signs used in commercial settings, such as loss of equilibrium, cessation of rhythmic ventilation, and the absence of the vestibulo-ocular (eye-roll) reflex. These indicators are typically lost more quickly and recover more slowly than neurological indicators. Thus, while practical and low-cost, these indicators have an important limitation: fish may be paralyzed by electrical currents yet remain capable of feeling pain.

Neurological Indicators: Direct measures of brain activity, such as electroencephalograms (EEGs) and visually evoked responses (VERs), are widely considered the gold standard for validation. VERs are particularly robust because they objectively measure whether the brain is processing external visual stimuli. A complete absence of VERs is strong evidence of insensibility, whereas operational indicators may disappear while the brain remains active. However, these direct measures of brain activity are challenging to conduct in the field because of the cost of test kits and the expertise required of testing staff.

Assessing the state of insensibility for any animal is difficult; for fishes, determining insensibility is further complicated by the physiological diversity of fish species and the aquatic medium in which stunning occurs.

Measuring Stress

Minimizing stress during processing is another key component of humane slaughter. Researchers determine stress levels through primary indicators like plasma cortisol and secondary metabolic markers such as glucose and lactate, which reflect the buildup of energy reserves for escape behaviors. Tissue-specific indicators, including the rate of adenosine 5′-triphosphate (ATP) degradation and the onset of rigor mortis, can also serve as proxies for pre-slaughter stress, since acute stress accelerates these biochemical processes and also damages the product quality that is critical for the commercial viability of slaughter methods.

Stress levels are not useful as a real-time indicator of welfare during the slaughter process in commercial conditions, as stress parameters take time to collect and analyze. Furthermore, stress indicators will not be able to identify whether stress was caused by pre-slaughter handling or crowding, or the stun and slaughter process.

Methods

To better understand the scientific consensus on electrical stunning, we conducted an AI-accelerated review of peer-reviewed academic journal publications and conference proceedings. The search focused on three species: small rainbow trout, gilthead seabream, and European seabass.

We identified 19 papers in total, of which 14 papers run experiments in laboratory settings attempting to measure duration of insensibility. All of these lab papers include visual/operational indicators (such as maintaining equilibrium and eye-roll reflex), which are commonly used in commercial settings to determine sensibility. Seven of these papers also include neurological measures of insensibility (such as VERs). We followed these steps, largely in order but sometimes returning to a previous step after refining the process.

Search

We used Elicit to identify initial papers, followed by Google Scholar for forward and backward citation mapping. Manual citation search was required to identify some key papers overlooked by Elicit that we had previously identified through engagement work with advocates and scientists.

Inclusion/Exclusion Criteria

Studies were included if they involved experimental testing of electrical stunning on at least one of the three species in question, and measured a welfare indicator falling into one of three categories: operational/visual, neurological, or physiological. Non-experimental review papers were excluded.

Although studies exist that examine percussive stunning of rainbow trout (including a few studies that are part of this review due to their examination of electrical stunning), we exclude papers that only test percussive stunning. Furthermore, we do not assess the performance of percussive stunning in this report. For small-sized rainbow trout, percussive stunning is considered less commercially viable than electrical stunning, and our focus is therefore on the latter as the primary candidate technology for this size class.

Data Extraction

All papers were read by the lead author, then loaded into a large language model (LLM), NotebookLM, in full-text version to extract specific fields. We then manually validated the automated extraction and synthesized results and implications into key findings. Gemini Pro LLM provided additional extraction and validation of our manual review where needed. In all cases, AI-extracted data were reviewed, edited, and approved by the authors.

We extracted variables key to understanding the effectiveness of electrical stunning to deliver an immediate, persistent, and low-stress stun, such as:

• stunner type and manufacturer
• stunning parameters
• welfare indicators
• percentage of “successful stuns” (electrical exposure that causes immediate insensibility)
• percentage of fish insensible after some time threshold
• time to recovery

Experimental Conditions

We calculated the number of experimental conditions to develop summary statistics of the literature. To do so, we chose to define distinct experiments within studies as examining different stunning equipment (e.g., in-water electrical stunning vs. dry electrical stunning) and different welfare measures (e.g., neurological vs. observational), while experiments that vary electric current parameters are counted as a single experiment.

Results

We identified 19 papers containing 76 experiments. A summary of all 19 studies is presented below in Tables 1, 2, and 3. We emphasize insensibility in our results and the following discussion. Stress before slaughter is another important indicator of a humane slaughter (J. A. Lines et al., 2003), yet stress levels are difficult to measure in real time and may be influenced by more than just the slaughter process. Only a few studies in this review examined stress levels, and our discussion of these indicators is limited. These results are included in the full table of studies but omitted from the tables and the figure in the report.

Due to time constraints, individual study summaries were generated quickly; however, our assessment of the evidence for each species is largely consistent with the views of scientific experts we have engaged with. We thus feel confident in our high-level findings, even if there might be discrepancies or inaccuracies in any individual summary.

Tables of All Studies, by Species

Table 1: European seabass studies with summary of insensibility data

Note: For this report, we focus on optimism around the insensibility results, although we include all papers and briefly mention the findings not related to sensibility. We also describe key papers in more detail where their results might change our overall conclusions. The full table of extracted variables is available here.

Table 2: Gilthead seabream studies with summary of insensibility data

Note: See Table 1 Note.

Table 3: Rainbow trout studies with summary of insensibility data

Note: See Table 1 Note.

Plot of Insensibility Duration

For a visual understanding of the insensibility results, Figure 1 plots mean, median, or minimum duration of insensibility for each study,^[1] compared against three colored background bins that provide a sense of the potential to deliver a humane stun in commercial conditions when paired with a slow-kill method.

We chose to visualize insensibility duration because it was the quantitative welfare measure we found easiest to extract from the studies we reviewed. We also considered using the percentage of fish recovered after specific time intervals; however, reporting across studies was not standardized, and many studies did not frame results in this way.

To produce the visualization, we collapsed insensibility duration data from each study into a single data point or range. This process necessarily omits important details, especially for studies with multiple experiments, short observation periods, or complicated reporting of results.

We attempted to provide as reasonable a summary of the evidence as possible, but our approach to extracting data points was not always consistent across studies. In choosing what data points to plot, we attempted to pragmatically balance a number of different factors, including ease of extraction (given data reporting conventions in each study), the degree of weight we placed on particular sub-experiments, and an appetite to combine information both from fish that recovered quickly and those that did not recover at all during the study observation period.

For example, for Bermejo-Poza et al. (2021), we used a 21-minute data point in the plot, reflecting how > 80% of fish did not recover during the 21-minute observation period, even though a small number of fish were breathing shortly after electrical exposure. For Brijs et al. (2025), however, we plotted the reported median, maximum, and minimum recovery time among recovered fish only, recognizing a weighted mean including both recovered and unrecovered fish would fall within the max/min range. And in Lambooij (2008), we plotted data only from one experiment, due to study design concerns in the other experiments. A more detailed description of our plot choices and calculations is available in the full table of studies.

We feel reasonably confident that Figure 1 provides a visual summary of the amount of evidence we have for each species and the degree of optimism about electrical stunning inducing long-lasting insensibility. Nevertheless, we would recommend reading Tables 1, 2 ,3 in full for a more detailed summary of the key findings from each study and factors that might affect how much weight we should place on individual study results.

Figure 1: Duration of Insensibility After Electrical Stunning in Peer-reviewed Literature

Finding 1: Virtually No Studies in Commercial Settings Using Neurological Indicators

The most persuasive experimental evidence on electrical stunning effectiveness would be based on real-world commercial conditions (rather than labs) and use neurological sensibility indicators (generally, measuring brainwaves to look for response to visual stimuli).

We found no papers definitively matching both criteria. Only one paper conducted experiments on a commercial farm using realistic equipment and practices (Jung-Schroers et al., 2020); the study measured sensibility using operational indicators. Only seven papers included experiments studying neurological indicators, none of which were conducted in realistic commercial conditions (see full table of studies).

Finding 2: The Studies We Reviewed Suggest That Electrical Stunning Remains Unproven for Seabream and Seabass

Electrical stunning is likely to deliver a humane outcome if fish are rendered insensible instantaneously and remain so until death. However, we find that results in current literature fall below these standards, especially with regard to gilthead seabream and European seabass.

While results vary (see Figure 1), most studies examining gilthead seabream and European seabass found that stunned fish often recover operational indicators of sensibility earlier than the 5–50 minutes that it may take them to die via kill methods like ice slurry and air asphyxia (de la Rosa et al., 2021; Van De Vis et al., 2003). These rapid recovery times generally come from recent studies that are based on adapted versions of commercial machines at recommended settings.

The outliers that do find longer durations of insensibility tend to use commercially infeasible stunning parameters or other study-specific factors that cause us to put less weight on these seemingly promising results. For example,

• Lambooij et al. (2008) found no recovery in seabass exposed to 10 s current with EEG electrodes inserted into their brain during the current exposure, which may have affected how electricity passed through the fish. When fish were exposed to the same electrical parameters without EEG electrodes inserted into their brain, 80% recovered within 5 minutes.
• Zampacavallo et al. (2015) found no seabass recovered at 20 minutes after a 4-minute electrical exposure. It is unlikely that such a long exposure duration will be feasible in real-world commercial conditions.
• Van De Vis et al. (2003) identified stun parameters where more than half of seabream did not recover in 10 minutes, but did not comment about any impacts on meat saleability (i.e., carcass damage).

From the laboratory studies we reviewed, we do not see enough evidence to be confident that electrical stunning can achieve sufficiently long-lasting insensibility for seabream and seabass in real-world conditions, especially since the follow-on kill method (ice slurry) for these species is very slow to impart death. We would feel more confident about electrical stunning achieving humane outcomes for these species if there were lab studies demonstrating longer-lasting insensibility at commercially viable settings and field studies validating those results, or commercial validation of a follow-on kill process that can be fully completed within 1–2 minutes.

Finding 3: Electrical Stunning May Be More Promising for Rainbow Trout Than Gilthead Seabream or European Seabass

When electrical settings are calibrated appropriately and machines are well-maintained, we hypothesize electrical stunning could realistically achieve long-lasting insensibility for a large proportion of small-sized rainbow trout harvested in real-world commercial conditions.

Four recent lab-based studies show promising results of insensibility longer than 15 minutes for some stunner settings.

• Two main VER experiments showed promising neurological results. Hjelmstedt et al. (2022) identified certain settings for in-water stunners in which 100% of fish did not recover for 15 minutes, and Brijs et al. (2025) found settings for in-air stunners in which 70% did not recover after 30 minutes.
• Two studies showed promising operational results at specific parameter combinations (Bermejo-Poza et al., 2021; J. L. Saraiva et al., 2024). At these settings, over 75% of fish displayed no signs of sensibility after 20 minutes.

In their commercial field trials, Jung-Schroers et al. (2020) found that, across 10 different facilities where electronarcosis was the only stun method employed, three-quarters of fish did not appear sensible at the start of slaughter (typically 2–5 minutes after electrical exposure).

Collectively, the Bristol Langford FishLab studies give the impression that electrical stunning can result in rainbow trout appearing insensible for at least five minutes while maintaining commercial viability. Although we view these early papers primarily as demonstrating proof of concept and establishing the design and calibration of new technology, the results add to our subjective optimism around electric stunning as a humane method for rainbow trout.

While there are grounds for optimism from this evidence and insights from unpublished data gathered by researchers in the field (personal correspondence, 2026), the results are not yet perfect. Most studies point to a material proportion of fish who recover quickly. And results appear highly sensitive to machine settings that will likely affect real-world performance if machines are not well maintained and settings are not routinely adjusted to address variation in operating conditions. More research in realistic commercial settings will help to provide guidelines for which equipment achieves the best outcomes for the highest proportion of fish, and the settings that work best in different environmental conditions.

Discussion

Our results reveal important gaps in the peer-reviewed evidence for electrical stunning: few studies use neurological indicators; and gilthead seabream and European seabass plausibly recover sensibility quickly after electrical stunning, while there is greater evidence of longer-lasting insensibility for rainbow trout. These results suggest a few broad directions for future investigation.

While Unlikely to Deliver Fully Humane Outcomes, it Remains Possible that Electrical Stunners Will Still Provide Counterfactual Improvements in Fish Welfare, Especially for Rainbow Trout

Although our review suggests it is unlikely that electrical stunners will deliver fully humane outcomes for all species in commercial conditions, electrical stunning could still represent an improvement if it results in better fish welfare outcomes than current practices.

A counterfactual improvement is theoretically plausible if fish are insensible during what might be the most stressful periods of slaughter. The current most common slaughter methods—asphyxia by ice slurry or air—are very slow to impart insensibility. Even though electrically stunned fish most likely recover sensibility prior to death, the rapid onset of insensibility might mean they are insensible for critical transition periods (e.g., thermal shock of being placed in ice slurry) where stress might be highest.

This speculative scenario has not been extensively studied in the literature. However, one study on rainbow trout found that while duration of insensibility and cortisol levels were similar among fast chilling, asphyxia, and electrical stunning methods, fast chilling produced severe stress indicators, including intense mucus release, haemorrhage, and slower loss of sensibility relative to electrical stunning (J. L. Saraiva et al., 2024).

Based on our review, we currently feel more optimistic about electrical stunning offering counterfactual welfare improvements for rainbow trout, but less so for seabream and seabass. For rainbow trout, the Jung-Schroers et al. (2020) field study results support the hypothesis that a significant proportion of rainbow trout could be insensible at the time the slaughter process begins. But for seabream and seabass, the very fast recovery times observed in the Gräns et al. (2025) lab study (where parameters were set at manufacturer recommendations), make us concerned that sensibility may have recovered by the time fish are subsequently placed in ice slurry.

More Research Needed on Stunner Performance in Commercial Settings to Help Identify Ways to Improve Welfare Outcomes

Finding 1 points to important gaps in peer-reviewed studies from academic journals, most of which were conducted in laboratory settings with a limited range of machines (and some studies were conducted over 20 years ago). We expect that machine manufacturers have improved performance of their stunners, and relevant data could reside in gray literature, manufacturer documentation, or remain unpublished for commercial reasons.

Therefore, academic or collaborative studies to increase availability of this real-world performance data would help. More validation studies in many different contexts would enable informed, species-specific recommendations about improving fish welfare at slaughter.

• For seabream and seabass, we feel the most important priority is lab studies evaluating whether existing stunner machines can deliver long-lasting insensibility when calibrated at commercially realistic settings. Field studies could be useful for these species, but we see them as a less obvious priority for now, unless using innovative protocols that could yield insights faster and at lower cost than lab studies.
• For rainbow trout, more field studies evaluating the duration of insensibility in real-world conditions will be helpful, ideally using neurological indicators where possible. It would be useful to know whether the promising results observed in Jung-Schroers et al. (2020) might be specific to the type of stunning machine and/or how they are used in trout farms in Germany, or whether commercial performance looks similar across lots of settings and machine types.

Strategy Recommendations

More academic validation studies: More validation studies testing the performance of commercial stunners (using neurological measures) in the lab for gilthead seabream and European seabass, and field studies for rainbow trout.

Developing quicker and cheaper validation approaches: Interested stakeholders could look to develop quicker and cheaper ways of validating on-farm stunner performance. Advancements in video technology mean that it may be possible to develop a protocol to measure the recovery of operational indicators of sensibility remotely. Alternatively, it may be possible to train a much wider set of people to collect such data. Implementing such protocols could mean that field studies might yield practical insights faster and cheaper than lab studies.

Incentivizing disclosure of commercial performance data: There may be creative opportunities to make unpublished performance data (for example, from machine manufacturers) more widely available in a way that supports transparency and competition to improve welfare outcomes. This could be through an anonymized dataset, benchmarking exercises, or a new type of peer-reviewed publication with different acceptance criteria than traditional journals.

Incentivising innovation: Innovation prizes or market commitments could incentivize technology developers to demonstrate their best-performing stunners meet necessary welfare criteria, and also incentivize innovation to improve welfare outcomes.

Developing and testing faster follow-up kill methods: If it seems technologically infeasible to extend duration of insensibility for some species, faster follow-on kill methods (for example, bleeding or automated percussion) might offer a way to improve welfare. Such methods would need to be tested for commercial viability and consumer acceptance.

Acknowledgements

This report is a project of Rethink Priorities—a think-and-do tank dedicated to informing decisions made by high-impact organizations and funders across various cause areas. The authors are Samara Mendez and Sagar Shah. Thank you to Albin Gräns, whose insights helped motivate this work and shape our understanding of this topic. Discussions with staff at Future for Fish and participants of the 2025 Catch Welfare Platform Conference provided useful perspectives. Jeroen Brijs, Luca Melotti and Miriam Martínez provided helpful comments on an early draft. Thanks also to William McAuliffe for guidance, Shane Coburn for copyediting, and to Urszula Zarosa and Elisa Autric for publishing the report online and assisting with dissemination.
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AI Transparency Statement

AI use in data search and extraction is discussed in the Methods section.

This report was written by the authors with AI assistance at various stages of the writing process. The report’s structure and outline were developed entirely without AI input. All AI-generated content was reviewed, edited, and approved by humans.

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1. Choice of summary metric depends on the study’s reporting style. Studies that only examined stress indicators of welfare are omitted from the plot. ↑