Physiological Responses of European Seabass (Dicentrarchus labrax) Juveniles to Long-Term Feeding with Increasing Levels of EPA and DHA: Effects of Thermal Challenge

Fish oil has traditionally been the primary source of n-3 long-chain polyunsaturated FA (n-3 LC-PUFA) in aquafeeds but is increasingly being replaced by alternative lipid sources. However, marine carnivorous fish require dietary n-3 LC-PUFA, particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which these alternative sources lack. This study assessed the long-term impact of graded levels of dietary EPA+DHA from microalgal oil on European seabass growth, feed utilization, immunological and oxidative status, and muscle FA profile. Six isoproteic (47% protein) and isolipidic (17% lipid) diets containing 0.56%, 0.66%, 0.71%, 0.90%, 1.11%, and 1.27% EPA+DHA were fed to fish (107 g initial weight) for 180 days at 23 °C, followed by a 7-day thermal challenge at 30 °C. At the end of the feeding trial, growth performance, feed utilization, organosomatic indices, blood and innate immune parameters, and oxidative stress were unaffected. Muscle FA composition mirrored that of the diets, and linear correlations were observed between muscle and dietary n-3 LC-PUFA levels. In the thermal challenge, no mortalities occurred, but hematological parameters, immune responses, and oxidative stress were intensified. Muscle FA composition remained unchanged, indicating that a dietary EPA+DHA level of 0.56% appears sufficient to meet physiological needs during an acute temperature challenge.