Selection for stress responsiveness and slaughter stress affect flesh quality in pan-size rainbow trout, Oncorhynchus mykiss

The control of slaughter stress is of importance with regard to both fish welfare and flesh quality. Muscle characteristics and instrumentally measured quality parameters were determined in rainbow trout lines selected for high-responsiveness (HR) or low-responsiveness (LR) of plasma cortisol to an acute confinement stressor. Measurements were made in both unstressed and stressed fish (a 15 min period of confinement before slaughter) from both lines. Compared to LR fish, HR fish were smaller, had a slightly higher condition factor, lower fat-meter-values, and higher carcass yield. No difference between the lines was observed for muscle pH, both at slaughter and at 72h post-mortem (pm). Fillets from HR fish had a lower muscle dry matter content and had higher lightness (L*) value for raw fillet. Fillet redness (a*) was lower for fish from the HR line for both raw fillet at slaughter and 72h pm, and for cooked fillets. Fillet firmness was higher for fish from the HR line for raw fillet, but lower after cooking. Both white and red muscle fibers of HR fish were smaller than those in LR fish and HR fish had a thicker red muscle than LR fish. Imposition of an acute confinement stressor before slaughter induced a differential plasma cortisol response in the HR and LR fish. Pre-slaughter stress also lowered muscle initial pH, lowered red muscle mean diameter, and reduced raw fillet mechanical resistance, but increased cooked fillet firmness and had no effect on fillet color. Almost no interaction between selection line and pre-slaughter stress effects was observed showing that slaughter stress had similar consequences in both lines. Overall, the HR/LR trout model gave new insights in the comprehension of trout flesh quality and showed that the level of plasma cortisol response did not affect the impact of slaughter stress on fillet quality