Influence of Docosahexaenoic and Eicosapentaenoic Acid Ratio and Temperature on the Growth Performance, Fatty Acid Profile, and Liver Morphology of Dusky Grouper (Epinephelus marginatus) (Teleostei: Serranidae) Juveniles

SIMPLE SUMMARY: Aquaculture production is the fastest-growing activity in the agriculture sector worldwide. Nutritional studies are always in demand because fish feeding is one of the main points that need to be solved in modern marine aquaculture since feeding is responsible for almost 70% of the total cost of production. Nutrition is still considered a challenging part of dusky grouper aquaculture due to the high costs of fish meal and fish oil, which need to be incorporated into aquaculture diets to satisfy the nutritional requirements of marine species. We evaluated the effects of fatty acids, provided as different lipid classes in diets, to provide new insights about dusky grouper juvenile nutrition, and we also demonstrated the influence of temperature on lipid metabolism. The main contribution of our study was to show that temperature influences lipid metabolism and that vegetable oils can replace fish oil, but only if the diet is supplemented with different lipid classes, mainly with omega-3. Another important outcome from the study was to show that increasing omega-3 levels in the diet does not increase its content proportionally in animal tissues, information that is important from an economic perspective. Therefore, our results provided valuable knowledge of dusky grouper nutrition and physiology, helping to solve one of the bottlenecks in modern aquaculture. ABSTRACT: An 8-week trial was performed to evaluate the influence of docosahexaenoic (DHA) and eicosapentaenoic acid (EPA) ratios and temperature on the fatty acid (FA) metabolism and liver morphology of Epinephelus marginatus. A basal diet was manufactured, and DHA (D1:0.5%; D2:0.75%; D3:1%; and D4:1.5%) and EPA (D1:1.0%; D2:0.75%; D3:0.5%; and D4:0%) levels were added. E. marginatus were kept in twelve tanks with a lower temperature (LT, 23.17 ± 0.12 °C) and twelve with a higher temperature (HT, 28.63 ± 0.06 °C). The DHA/EPA ratio did not affect performance, regardless of the temperature, but the animals fed with the D4 diet showed better growth at HT. A higher lipid deposition and a large lipid vacuole area in the hepatocytes at HT were observed, regardless of the diet. Triacylglycerol (TG) in reflected the diet FA profile mainly in the muscle and liver, whereas the phospholipid (PL) was less influenced by the diet. The same DHA content in the TG fraction of muscle (D3 and D4) were observed at LT; however, only the DHA/EPA ratio of D4 could be differentially stored in the TG fraction of muscle (HT). Monounsaturated and polyunsaturated FA increased in the PL of the eyes at HT, whereas saturated FA was reduced in the TG and PL fractions at LT. These results evidence the importance of temperature and a balanced DHA/EPA ratio in the diet of marine fish.