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Abstract Ensuring fish welfare during stunning and slaughter is essential, as these procedures represent critical points in the production cycle that can significantly impact animal welfare. Cardiac activity serves as a key physiological indicator to assess stress responses and welfare in fish. Prolonged cardiac activity may reflect delayed death, which is relevant for evaluating welfare during the slaughtering process. In the present study, the effects of different slaughtering combinations on cardiac activity and internal temperature were evaluated in European seabass ( Dicentrarchus labrax ). To continuously monitor heart rate, biologgers were surgically implanted in adult seabass. Fish were subjected to different pre‐slaughter treatments (ice slurry immersion or electrical stunning) followed by various slaughter methods (ice slurry, ikejime , or anesthetic overdose). The selected stunning and slaughter combinations represent the most used techniques in commercial aquaculture (ice treatments) and new alternatives that are currently under evaluation (electric stunning) or used as control treatments ( ikejime and anesthetic overdose). Results indicated that slaughter in ice slurry resulted in the longest duration of cardiac activity, whereas ikejime and anesthetic overdose led to the fastest cessation, particularly when preceded by stunning. Electrical stunning significantly reduced the time to cardiac activity cessation compared with ice slurry immersion alone. The welfare concerns related to ice slurry slaughter are based on the prolonged cardiac activity observed, which may indicate physiological stress and delayed time to death. Based on cardiac activity and from an ethical and welfare perspective, electrical stunning followed by ikejime appeared to be the most effective method, ensuring rapid cessation of cardiac activity. However, both electrical stunning and ikejime require further neural validation to confirm that they reliably induce immediate and sustained loss of consciousness. While this combined method shows promise, its economic feasibility and practicality in commercial settings must be improved. At present, electrical stunning followed by ice slurry slaughter represents a more viable alternative, although it also requires neural validation. Future research should refine electrical stunning parameters, incorporate neurophysiological and biochemical indicators, and explore innovations to enhance welfare standards in seabass aquaculture.