Pre-rigor changes of rainbow trout (Oncorhynchus mykiss) as affected by stunning/slaughtering methods and rearing temperatures

Stunning and slaughtering processes can cause stress and disturbances to fish and, consequently, they may negatively affect the fish flesh quality. Currently, the electrical stunning/slaughtering (E) is a quite common procedure for stunning that requires a proper application to avoid skin burns, fillets bloodspots as well as a possible recover of the fish, before slaughtering. Recently, carbon monoxide asphyxia (CO) was applied for fish stunning/slaughtering without inducing any visible stress response on salmonids, but a confirmation is still needed. Hence, CO and E were applied to rainbow trout (Oncorhynchus mykiss) specimens reared at a water temperature of 8 °C or 12 °C. The fish (n. 400) were distributed in 4 tanks where water was at 8 (2 tanks) or 12 °C (2 tanks); for slaughtering, the fish were subdued to E (E group, treated for 30 s at 180 V) or CO (CO group, flushed directly into the tanks until the fish apparent death) treatments to have the E8, E12, CO8, and CO12 groups. Lactate, glucose and cortisol concentrations in plasma, rigor mortis evolution, fillets shape changes, ATP breakdown and Adenylate Energy Charge (AEC) levels were evaluated immediately post mortem in the muscle of 18 fish per group. CO groups showed higher glucose level than the E ones (7.46 vs. 5 mM, respectively) and even exhibited higher lactate amount than the other group (5.33 vs. 3.26 mM, respectively) as the fish raised at 12 than those raised at 8 °C (4.65 vs. 3.94 mM, respectively). Any of the treatments affected the cortisol concentrations and rigor mortis evolution. However, the E fillets had the strongest length contraction and height increase. The muscle of CO trout contained more ATP than the E one (around 2.28 vs. 1.12 µmol ATP/g). In addition, ATP concentration was higher (P<0.05) in fish raised at 12 than 8 °C (2.35 vs. 1.05 µmol/g). Concerning the AEC, the E8 group showed the lowest value while all the others did not differ among them. In conclusion, the CO method resulted to be a suitable alternative to electroshock, able to limit stress in rainbow trout. In addition, the rearing at 12 °C seemed to better preserve ATP and AEC in muscle.