Introduction
Common carp (Cyprinus carpio) is one of the most widely cultured and consumed fish species in Central Europe. Ensuring humane slaughter is essential not only for fish welfare but also for maintaining fillet quality. However, many conventional slaughter methods do not consistently meet fish welfare standards. Electrical stunning (ES) has emerged as a promising humane slaughter technique, as species-specific studies indicate that it can induce an irreversible stun or a reversible state of unconsciousness. Nevertheless, if ES is applied improperly, it may compromise both fish welfare and product quality. In common carp, the effectiveness of ES depends strongly on the optimization of electrical parameters. Therefore, precise control and validation of these parameters are critical to ensure reliable induction of unconsciousness while preserving fillet quality. This study is designed to optimize electrical stunning conditions at an industrial scale, with the aim of improving both welfare outcomes and product quality in common carp processing.
Materials and Methods
The in-water electrical stunning (ES) protocol was evaluated in common carp (3–4 kg) at a commercial fish processing plant (Ryb����stv�� Chlumec nad Cidlinou, Czech Republic). Two stunning systems were used: an existing plant-operated stunner (AC, 230 V, 50 Hz, 5 A) and a newly developed ES unit capable of both AC and DC application (50 Hz, adjustable 0–320 V and 0–15 A). A total of 80 fish were randomly assigned to four treatment groups (n = 20 per group): percussion stunning as the control (T0), T1 (commercial stunner; AC, 220 V, 5 A, 7 min), T2 (AC, 320 V, 6 A, 5 min), and T3 (DC, 320 V, 6 A, 5 min).
To assess the effectiveness of stunning, immediately after stunning, 10 fish per treatment were transferred to a recovery tank to monitor the return of consciousness at 0, 2.5, 5, 7.5, and 10 minutes post-stunning. Physiological stress responses were evaluated using blood indicators (cortisol, lactate dehydrogenase, glucose, potassium). For the product quality assessment, rigor mortis progression and muscle pH were monitored over 72 hours (0, 6, 12, 24, 36, 48, 60, and 72 h). Additionally, key fillet quality parameters such as drip loss, ATP, glycogen, glucose, lactate, haemoglobin content, color, and texture were analyzed using standard protocols.
Results
Fish subjected to percussion and the newly developed DC stunning (T3) did not regain consciousness during the 10-minute post-stunning observation period, indicating a prolonged unconsciousness and effective stunning. In contrast, 50% of the fish treated with the plant stunner (T1: AC) regained consciousness within 10 minutes, while an even higher recovery rate (70%) was observed in the T2 (AC) group, suggesting less effective stunning. Cortisol and other blood parameters did not differ significantly (p > 0.05) among treatments compared to percussion, indicating a comparable acute stress response across groups. However, clear differences were observed in post-mortem muscle dynamics and fillet quality. The onset of rigor mortis occurred significantly earlier (p < 0.05) in T1 (at 12 h), followed by T2, T3, and finally T0 (percussion), suggesting accelerated metabolic exhaustion in AC-treated fish. The pH of fish muscle is also consistent with rigor progression. Drip loss was slightly higher (p>0.05) in all electrically stunned groups compared to percussion, indicating reduced water-holding capacity. Muscle glycogen content was significantly lower in T1 and T2, but higher in T0 and T3 (p < 0.05), reflecting greater pre-mortem energy depletion in AC treatments. In contrast, glucose levels were highest in T1, consistent with increased glycogen breakdown. Additionally, muscle haemoglobin content was significantly elevated in T1 compared to other treatments (p < 0.001), suggesting greater tissue damage or blood retention. Overall, these results indicate that DC (T3) stunning provides effective induction of prolonged unconsciousness comparable to percussion, while preserving better fillet quality than AC (T1 & T2) treatments.
Acknowledgment: The study was funded by operational fishery grants of the Czech Republic: CZ.08.02.01/01/23_005/0000232.