Introduction
Fish oil (FO) has been used as a major lipid source in aquafeeds due to its abundant long-chain n-3 polyunsaturated fatty acids (LC-PUFA), particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are essential for growth and fillet quality of carnivorous marine fish such as Atlantic salmon (Salmo salar). However, the increasing global demand and limited supply of marine-derived oils have raised concerns regarding the long-term sustainability and economic stability of FO. As an alternatives lipid sources, oils derived from heterotrophic microalgae such as Schizochytrium have received considerable attention. Schizochytrium oil is particularly rich in DHA and can be produced independently of marine fisheries, making it a sustainable and scalable source of n-3 LC-PUFA. Dietary lipid composition plays a critical role in regulating membrane structure, metabolic processes and overall physiological performance in fish. These physiological functions are closely associated with the ability of fish cope with environmental stress. In particular elevated water temperature has become an increasingly important challenge in Atlantic salmon aquaculture due to climate change and seasonal warming. Heat stress can impair feed intake, growth performance, oxidative balance, immune competence and physiological condition of Atlantic salmon. Membrane lipid composition and antioxidant capacity are closely associated with thermal tolerance, suggesting that diets rich in DHA and other bioactive lipids may improve resilience under high-temperature conditions. Schizochytrium oil contains high concentrations of DHA and antioxidant compounds such as carotenoids. Therefore, it may provide functional benefits beyond functioning as a conventional FO replacement. Therefore, this study investigated the effects of replacing dietary FO with a Schizochytrium based oil on growth performance, feed efficiency, lipid metabolism and heat stress resistance in Atlantic salmon.
Materials and Methods
A basal diet (S0) containing 25% fish meal and 12% FO was prepared. Five experimental diets were formulated to replace FO with Schizochytrium oil at graded levels of 20%, 40%, 60%, 80% and 100% (designated as S20, S40, S60, S80 and S100, respectively). A total of 360 Atlantic salmon (initial mean body weight: 101 ± 0.25 g) were randomly distributed into 18 tanks (450 L) in triplicate groups (20 fish per tank) and fed to apparent satiation three times daily for 6 weeks. Heat stress was induced by adjusting the ratio of cooled water and seawater, resulting in a water temperature range of 16.5–24.9 ���. At the end of the feeding trial, growth performance, feed utilization and survival were evaluated. Blood and tissue samples were collected to determine serum biochemical parameters, non-specific immune responses, antioxidant capacity, digestive enzyme activity, muscle proximate composition, fatty acid composition and hepatic gene expression related to lipid metabolism, stress, pro-inflammation and apoptosis.
Results
Dietary substitution of FO with Schizochytrium oil significantly improved growth performance in Atlantic salmon, as indicated by higher final body weight, weight gain and specific growth rate in S20, S40 and S60 groups compared to S0 group. Feed utilization was significantly enhanced, with reduced feed conversion ratio and increased protein efficiency ratio in all FO substituted groups. Survival rate was significantly higher in all FO substituted groups than in S0 group. Serum biochemical parameters were improved, as reflected by significantly lower glucose, triglycerides, aspartate aminotransferase, alanine aminotransferase and low-density lipoprotein levels in all FO substituted groups, while high density lipoprotein level was significantly higher in S60, S80 and S100 groups compared to S0 group. Immune responses were significantly enhanced, with increased myeloperoxidase, lysozyme, nitroblue tetrazolium and total immunoglobulin activities in S40, S60, S80 and S100 groups compared to S0 group. Antioxidant capacity was significantly improved, as indicated by higher superoxide dismutase, glutathione peroxidase and catalase activities, as well as increased total antioxidant capacity in FO substituted groups, while hepatic malondialdehyde level was significantly lower in FO substituted groups. Digestive enzyme activities were significantly increased, with higher trypsin activity in S40, S60, S80 and S100 groups and higher lipase activity in all FO substituted groups compared to S0 group. Muscle DHA and crude protein levels were significantly higher in substituted groups than in S0 group. Hepatic lipogenesis and lipid transport related genes (FAS, LXR, SREBP1 and SREBP2) were significantly downregulated in all FO substituted groups. Gene involved in fatty acid desaturation and elongation (FAD5, FAD6, ELOV2 and ELOV5) showed higher expression, particularly at S100 group. In contrast, lipid catabolism related genes (AOCX1, CPT1, PPARα, PPARβ and PPARγ) were significantly upregulated. Heat shock proteins (HSP60, HSP70 and HSP90), pro-inflammatory cytokines (TNF-α, IL-1β, IL-6 and Mulan) and apoptosis-related genes (BAX, Caspase3a, Caspase7, Caspase9 and P53) were significantly downregulated, indicating suppressed stress, inflammation and apoptotic responses in FO substituted groups.
Conclusion
In conclusion, replacing FO with Schizochytrium oil can improve growth performance, feed efficiency and nutrient utilization by upregulating digestive enzymes and lipid metabolism related genes. Furthermore, dietary substitution enhanced physiological condition, antioxidant capacity and immune responses, while downregulating pro-inflammatory cytokines, stress-related genes and apoptosis-associated genes. These results indicate that Schizochytrium oil can effectively improve health status and stress resilience in Atlantic salmon. Based on the weight gain and survival, optimal dietary FO replacement level is estimated to range between 53.5% and 82.4%.