The rapid growth of aquaculture has intensified focus on fish welfare at slaughter, with efforts primarily concentrated on developing, refining and implementing stunning technologies to ensure humane processing. The focus on the moment of stunning overlooks, however, an important reality: the slaughter process extends far beyond the final moments of stunning and slaughter, encompassing extensive pre-slaughter operations that can last hours or even days. In estimating Cumulative Pain associated with different stunning and slaughter methods, these prolonged pre-slaughter operations – which subject fish to sustained periods of aggression, physical trauma, fear, social stress, and exposure to challenging environmental conditions – are likely to represent a far greater burden of pain than the final moments of stunning (see an example in the figure below, of fish slaughtered off-site). A fish may experience extremely brief pain and distress at stunning if the procedure is effective, but may have endured hours or days of pain during pre-slaughter operations.
Moreover, each stunning technology shapes how these pre-slaughter operations must be conducted, from crowding and handling to transport methods, creating distinct welfare challenges. Therefore, a meaningful quantification of the welfare impact of different stunning and slaughter practices requires accounting for both the Cumulative Pain during all pre-slaughter operations and the pain experienced during the stunning/slaughter itself. Understanding all circumstances associated with each stunning procedure, from pre-slaughter operations to loss of consciousness, along with their corresponding welfare impacts, is therefore essential for meaningful welfare improvements.
Research demonstrates significant variability in the effectiveness of different stunning methods, including electrical and percussive stunning, the most widely investigated and implemented humane stunning methods for farmed fish. The efficacy of these methods is highly dependent on specific parameters and implementation conditions. For example, electrical stunning requires precise current parameters that vary by species, size, and environmental conditions, whereas percussive stunning effectiveness depends on accurate positioning and impact force. Even under controlled conditions, these methods can produce inconsistent results, with some fish showing delayed loss of consciousness or signs of incomplete stunning.
However, evaluating stunning methods based on their performance under controlled conditions fails to capture their real-world effectiveness. The successful implementation of these methods faces not only practical challenges in commercial settings – including equipment maintenance and reliability, operator training, continuous power supply requirements, high-throughput processing demands and environmental variables affecting equipment performance – but must also account for the state of the fish arriving at stunning.
Prior to stunning, fish undergo multiple pre-slaughter procedures that significantly impact their welfare, including:
These pre-slaughter stressors have a cumulative effect on fish welfare that can further compromise stunning effectiveness. Fish arriving at stunning in a stressed state from prolonged exposure to these multiple stressors show even more variable responses to stunning attempts. Indeed, both electrical and percussive methods show highly inconsistent results under commercial conditions, where pre-slaughter stressors are unavoidable.
Crucially, different stunning methods require different pre-slaughter approaches, meaning that a method’s overall welfare impact cannot be evaluated based on its stunning effectiveness alone. For example, electrical stunning systems typically require a continuous flow of fish through the stunning equipment, directly influencing how fish are handled pre-stunning. Procedures might include more intensive but shorter periods of crowding to maintain consistent throughput, and potentially pump-based transfer systems that could reduce handling stress but introduce other welfare risks such as physical injuries. Conversely, systems designed for percussive stunning might hold fish at lower densities but for longer periods to facilitate individual handling. Transfer methods, even in automated systems, might rely more on manual techniques, potentially increasing handling stress.
The welfare implications of each of these different approaches will also depend on various factors, including not only species-specific but also individual characteristics (e.g. fish size, metabolic rate) and environmental conditions. Additionally, different stunning methods may be also associated with differences in factors such as staff skill and fatigue, processing speed requirements, differences in equipment maintenance and reliability, and reliance on power supply reliability, which will affect the success of welfare interventions at slaughter.
Thus, when evaluating and implementing stunning methods, we must move beyond the focus on stunning effectiveness. A method that appears superior in terms of rapid loss of consciousness may even result in greater Cumulative Pain when accounting for its pre-slaughter operations. By quantifying Cumulative Pain across the entire process, from pre-slaughter operations through stunning, we can better identify where the greatest welfare impacts occur and develop more meaningful practices. This approach may reveal that significant welfare improvements could be achieved by modifying pre-slaughter operations, achieving large benefits even where changes in stunning technology are not possible.
An obstacle to estimating the magnitude of welfare impacts across methods and designing effective welfare interventions is the limited data of what happens to fish before they reach the stunning equipment. While stunning parameters are often documented, data on pre-slaughter conditions is more scarce, including how long fish spend in each pre-slaughter procedure and the prevalence of adverse welfare outcomes like injuries, mortalities or other types of trauma. This data gap is particularly problematic because pre-slaughter operations can vary dramatically between facilities.
More critically, we have limited understanding of how pre-slaughter conditions affect stunning effectiveness – an interaction that could explain why stunning methods that perform well in controlled trials often show inconsistent results in commercial settings. The industry’s focus on stunning technology has created an illusory sense of progress, while the potentially larger welfare impacts during pre-slaughter operations remain largely unknown.
The path forward requires increased transparency from industry about practices, particularly regarding procedure durations, handling methods, and data on adverse outcomes, throughout the entire process. This would enable identification of points where welfare is most compromised and allow for comparison between different practices.
* Implement mandatory reporting requirements for pre-slaughter procedures, including data on procedure durations and prevalence of adverse outcomes at each stage.
* Establish standardized monitoring protocols
* Develop audit systems including unannounced inspections
