Determination of optimal degree days of fasting before slaughter in rainbow trout (Oncorhynchus mykiss)

Pre-slaughter fasting is a common practice in aquaculture to ensure emptying of the digestive tract, but little is known about how long fish should be fasted, in terms of degree days (°C days), to clear the gut without affecting their welfare or flesh quality. In this study, 180 rainbow trout (Oncorhynchus mykiss) were fasted for 3, 4, 5, 6, 7 and 9 days (17.2, 22.3, 28.6, 35.3, 41.8 and 55.3 °C days, respectively) and different morphometric, hematological and flesh quality parameters were measured. The slaughter weight, relative growth and hepato-somatic index did not decrease until after 7 days of fasting (41.8 °C days) and stomach content was similar and near zero after 4 days of fasting (22.3 °C days). Conversely, fasting time increased carcass yield. Plasma concentrations of cortisol, glucose and creatine phosphokinase enzyme were lower with increasing days of fasting. However, trout slaughtered after 5 days of fasting (28.6 °C days) had a higher flesh pH at 0 h post-mortem than 3 and 4 days fasted trout and, consequently, earlier establishment of rigor mortis. Furthermore, liver color could be useful as a stress indicator since its hue decreases after 5 days of fasting (28.6 °C days). In conclusion, a pre-slaughter fasting period from 17.2 °C days to 22.3 °C days makes possible to obtain a complete emptying of the digestive system in rainbow trout and simultaneously it can minimize the stress response with consequent better flesh quality.