Live-chilling and crowding stress before slaughter of Atlantic salmon ( Salmo salar)

Effects of crowding stress at high fish density and/or live-chilling in cold sea water (1°C for 1 h) before slaughter were studied in Atlantic salmon ( Salmo salar). The experiment was conducted as a 2×2 factorial design of four groups, each with 50 fish. The groups were (1) untreated before slaughter, (2) crowded, (3) live-chilled, and (4) both crowded and live-chilled before slaughter. Both crowding and live-chilling before slaughter resulted in significantly increased levels of cortisol, lactate and osmolality in blood plasma indicating significant pre-slaughter stress. Plasma glucose levels increased about 20% in the live-chilled and the live-chilled/crowded group, and by 70% in the crowding group, indicating that live-chilling decreases stress as measured by plasma glucose. Both crowding and live-chilling before slaughter significantly affected rigor mortis, pH and muscle glycogen indicating that muscle metabolic changes occurred post-slaughter. The two live-chilled groups showed a substantial delay in time to onset of rigor mortis, and had a lower maximum rigor score than untreated and crowded groups. In contrast, crowding led to an earlier onset and resolution of rigor. White muscle analyses at slaughter revealed a significant 65% decrease in glycogen due to crowding, but no significant effect of live-chilling. The crowded groups had higher muscle pH after 5 and 14 days ice-storage compared to the un-crowded groups. All gaping scores were low, but marginally higher due to live-chilling after 5 days ice-storage. Analyses of fillet quality, as measured by texture after 5 days ice storage, showed that the force at breakpoint tended to increase with crowding, and the compression gradient increased significantly due to live-chilling; whereas these differences were not apparent after 14 days ice storage. The 24-h crowding stress led to a firmer texture. This firmness is associated with DFD-meat (Dark, Firm and Dry), being characterized by high ultimate pH due to low initial glycogen. The results indicate that the live-chilling method can prevent some of the negative effects on fillet quality caused by crowding stress.