Effect of pre‐slaughter crowding on rainbow trout welfare and product quality

Abstract Pre‐slaughter processes impact trout production, but the optimization of pre‐harvest crowding in an industry affected by global warming has scarcely been studied. In this study, we evaluated the effect of two crowding methods on the welfare of rainbow trout ( Oncorhynchus mykiss ) across two seasons (winter and summer). Each season 660 trout were distributed into separate tanks, and two crowding methods were applied: reducing water level (WL) and applying lateral compression (LC) within the tanks. Welfare indicators included plasma metabolites and tissue color, while fillet quality was evaluated based on indicators such as muscle color, muscle pH, and rigor mortis. These parameters were evaluated to determine the impact of each crowding method on both fish welfare and fillet quality. Tests were conducted in winter (8.80 ± 1.79°C) (WI) and summer (22.0 ± 0.06°C) (SU). Cortisol, creatine phosphokinase, and lactate levels in plasma were lower using WL, indicating a reduced stress response, supported by various skin color attributes (a*, b*, C*, and h*), liver (a*, C*, and h*), and fillet (h*). Similarly, muscle acidification (pH) was lower using LC, which could lead to potential texture deterioration. Temperature was a crucial factor globally across all parameters, with better results in WI. The combination of both treatments showed, on one hand, abnormal values of cortisol, glucose, and skin color attributes (a* and C*) and fillet (24 h post‐mortem) (b* and C*) in summer LC, and on the other, better muscle pH and fillet color (b* and C*) at 0 h post‐mortem using WL in WI. Overall, stress response was lower and fillet quality was better in WL fish. This trend was similar in winter compared to summer trout, regardless of the crowding method. Despite the poorer outcomes from LC, whether due to increased swimming difficulties or worse distribution of dissolved oxygen, its use in summer is discouraged due to cumulative effects, recommending its use only in winter when unavoidable.