Low temperature-driven physiological responses in ballan wrasse (Labrus bergylta): Stress, inflammation, and liver health

This study investigated the impact of winter conditions on the physiology of ballan wrasse (Labrus bergylta), a widely used biological control agent in salmon farming. Fish were subjected to temperatures encompassing winter and summer conditions for short-term (8 days at 6, 10, or 14 °C) or long-term (2.5 months at 7 or 15 °C) period. Their stress, inflammatory, and hepatic responses were then examined. After short-term exposure, no significant differences were found in terms of morphology (bod weight, total length, condition factor), levels of stress markers (cortisol, glucose, and head kidney expression of stress-related genes), and inflammatory markers (circulating lipid inflammatory mediators, liver expression of cox and lox genes). However, at 6 °C, fish exhibited lower lactate levels and increased head kidney cyp11a1 expression, indicating some metabolic and physiological changes during thermal acclimation. In comparison, long-term exposure to 7 °C resulted in stunted growth, along with elevated levels of stress markers, indicating tertiary stress response. Additionally, long-term exposure to 7 °C significantly lowered the levels of lipid anti-inflammatory mediators, particularly those synthesized by COX and derived from DHA and EPA, while increasing the pro-inflammatory cytokine il-1β expression in the head kidney, suggesting a heightened inflammatory capacity at low temperatures. Moreover, ballan wrasse at 7 °C displayed a larger hepatosomatic index and enlarged hepatocytes, indicative of energy substrate accumulation and storage in the liver. Findings highlight the physiological challenges and adaptive responses of ballan wrasse to winter temperatures, providing crucial insights for optimising farming practices and enhancing species' health, welfare and performance during deployment.