Seasonal comparison of uniform pre-slaughter fasting practices on stress response in rainbow trout (Oncorhynchus mykiss)

Abstract

Fasting is a common practice in aquaculture, used during fish transport and before slaughter to reduce stress and metabolic activity. Research indicates that fasting for 55 to 58 degree days (°C d) effectively reduces metabolic and stress indicators in rainbow trout. Nevertheless, seasonal temperature variations will have an influence over fasting length. This study examined the effects of pre-slaughter fasting across different seasons using 495 rainbow trout (Oncorhynchus mykiss). Fasting periods of 0, 50, and 100 degree days (°C d) were tested in both summer (22 °C) and winter (8 °C). Extended fasting resulted in weight loss and lower glucose and triglyceride levels, regardless of season. In summer, fasting increased stress response markers such as plasma lactate dehydrogenase enzyme and disrupted energy metabolism with increased non-esterified fatty acid levels and lower triglyceride levels, while winter fasting showed opposite effects, with trout better adapted to cooler temperatures. Fasting in summer minimally impacted skin color but increased chroma levels, like the 100D winter group, which exhibited reduced lightness. Liver color remained consistent across summer treatments, indicating reduced liver reserves supported by lower liver glycogen levels, comparable to the 100D winter group. Antioxidant systems were more active in winter, with higher expression of key genes like superoxide dismutase (sod) and glutathione peroxidase (gpx). Meanwhile, heat stress in summer masked the full effects of fasting, highlighting the challenges of fasting during warmer months. Overall, fasting had more pronounced effects in winter, where optimal water temperatures allowed for clearer metabolic adaptations and better energy management. These findings emphasize the importance of adjusting fasting protocols to seasonal temperature variations for improved fish welfare and management