Stunning methods in aquaculture slaughter and their implications for fish welfare.

The majority of fish farmed in aquaculture systems around the world are not stunned during slaughter, exposing them to considerable pain and distress. When stunning methods are applied, there is a lack of a comprehensive understanding of the welfare outcomes, including each stunning method's reliability and accuracy. This review critically evaluates the stunning methods predominantly used in aquaculture systems in Europe, and more widely. Studies were included if they contained discernible information on fish welfare indicators, and these were synthesised and assessed against welfare factors spanning the pre-stunning, induction, and loss-of-consciousness phases. The results reveal significant variability across methods, with no single current method presenting itself without significant welfare concerns. Gas methods are recognised as highly aversive and painful for fish and may induce unconsciousness that is delayed in onset and not always sustained. In contrast, electrical and percussive methods can provide rapid unconsciousness when delivered with sufficient energy and precision, but their effectiveness often remains highly dependent on species, size, and operational parameters. A key limitation across all methods is the lack of validated species-specific indicators of consciousness that can be relied upon in high-throughput commercial settings. This gap is compounded by a shortage of research assessing the effectiveness of stunning methods under real-world, commercial conditions. To support progress and seek improved fish welfare outcomes for the millions slaughtered each year in the European Union, and the billions slaughtered globally, we propose welfare-based criteria as an evaluative framework to guide the refinement of existing methods and the development of novel approaches. These conceptual criteria highlight principal areas for improvement, further research, and technological development. Currently, there are many gaps in knowledge regarding the effectiveness of stunning methods, the specific practices used, and the welfare impact they have on fish. Addressing these gaps requires further scientific research to provide a stronger evidence base across species and production systems. Improving the welfare of farmed fish during slaughter will therefore depend on combining rigorous research with technological innovation, robust commercial validation, and effective consciousness monitoring and reporting in real-world contexts, alongside clear legislation, enforceable guidelines, and regular inspection to ensure stunning methods are correctly applied, and welfare is safeguarded.