Thermal performance of early stages of Sparus aurata integrating body condition, behavior and physiological responses.

Temperature has a profound impact on fish, particularly during the early life stages (ELS), when they exhibit a narrower range of thermal tolerance compared to later developmental phases. This study investigated the long-term effects of temperature on Sparus aurata ELS that were exposed to four temperature regimes for 11 weeks: 19 °C, 22 °C, 24 °C and 28 °C. Parameters evaluated included growth, Fulton's condition factor, behavior (risk-taking, activity and aggressiveness), routine metabolic rate (RMR), metabolic enzyme activity (citrate synthase and lactate dehydrogenase) and oxidative stress biomarkers (Lipid Peroxidation, Catalase and Glutathione-S-Transferase). Growth and condition were highest at 28 °C and lowest at 19 °C. While risk-taking behavior was unaffected by temperature, aggressiveness increased at 24 °C and 28 °C. Swimming activity increased from 22 °C to 28 °C, but RMR remained stable. Antioxidant enzyme activity increased in the brain and gills at 28 °C, but no signs of Lipid Peroxidation were found. Physiological biomarkers accounted for over 67% of trait variability. These findings suggest that S. aurata ELS adopt temperature-specific physiological strategies, such as upregulating metabolic enzymes at lower temperatures to sustain body condition and enhancing antioxidant defenses at higher temperatures to counteract oxidative stress. Thus, ELS can maintain physiological performance across a broad thermal range, albeit with acclimatize trade-offs.