Introduction
Fish consumption is widely recognized as the healthiest way to ensure a proper intake of n3 long chain highly unsaturated fatty acids (LC-HUFA) such as EPA (C20:5n3) and DHA (C22:6n3). The benefits of these FA for human health are well known and relate with fetal, brain and vision development, cardiovascular function, cognition, or depression disorders. The European Food Safety Agency (EFSA) even recommends for European adults a daily intake of 250-500mg of EPA and DHA based on cardiovascular risk consideration. Aquaculture fish, whose global consumption has been rising every year, will increasingly play a crucial role in delivering healthy n3 LC-HUFA to the population.
In the last decades there was an important shift in aquafeed recipes towards higher inclusion levels of vegetables, with an impact on fillets n3 and n6 FA pattern and, consequently, on their nutritional value for human consumption. It is known that n3 and n6 FA are precursors of signaling molecules with opposite effects, namely anti-inflammatory and pro-inflammatory, respectively. Gilthead seabream (GSB) and European seabass (ESB) have limited or no ability to synthesize n3 HUFAs from C18 FA [such as LA (C18:2n6) or ALA (C18:3n3)] present in vegetable ingredients. Therefore, the feed must be a direct source of EPA and DHA. The strategy to enrich fish fillets in n3 FA has been the adoption of finishing diets, for 120-150 days pre-harvesting, with greater inclusion levels of fish oil (FO), to compensate for the low intake of EPA and DHA throughout the cycle and to allow for their restoration in the fillet. Switching to a diet rich in fish oil during the late growout is likely to adjust fish FA profile for better n3 FA content. However, all lipid sources consumed during the growout period will influence the final lipid composition and fat distribution in the tissues of farmed fish.
Going further on this topic, i.e., considering not only the amount of EPA and DHA in the fillet but also the ratio n3/n6, the goal of this work was to assess the best nutritional strategy for GSB and ESB, for both species to deliver more interesting and healthy fillets for the final consumer.
Material and methods
GSB and ESB batches from 2018, 2019 and 2020, raised in a Spanish industrial land production facility, were divided in two groups considering the nutritional strategy adopted in terms of lipid sources in the feed: FORO) a mixture of fish oil and rapeseed oil or FO) 100% FO (high in n3-LC HUFA). FORO diet presented 6g EPA+DHA/100g fat and a n3/n6 ratio of 0,9, whereas FO diet presented 26g EPA+DHA/100g fat and a n3/n6 ratio of 3,5. The experiment started in September 2020 and lasted for 12 months. Initial, intermediate, and final samplings were performed, each consisting of 5 individuals per group; all fish were weighted and filleted; a composite muscle sample was therefore analyzed for the fat content and for the FA profile by Mérieux NutriSciences (Portugal).
Results
The fish sampled varied in size from 94 to 771g in GSB and from 299 to 1372g in ESB. As expected, fillet fat content positively correlated with fish body weight in both species (R2>0,5). The season effect in GSB in terms of fatty acid deposition in the fillet was very clear, with the increase in fat during the summer months and its loss during the winter season. During fat loss process, EPA and DHA are preferentially oxidized comparing to n6 FA as their proportion in fat decrease. Nonetheless, EPA and DHA in total fat amount tends to be above total n6 FA. Feeding with 100% FO diets allowed GSB to go from a n3/n6 ratio of 0,9 in April to 1,4 in September, whereas the usage of FORO diet allowed to go from 0,9 to 1,2. In seabass there was a clear response of fillet lipid profile to the diet throughout the cycle, without a season effect. The intake of 100% FO diet allowed for seabass to shift to a higher proportion of EPA and DHA in the fat over n6 FA, increasing therefore the ratio n3/n6. In fact, with a FO-based diet, seabass was able to attain 1,1 of ratio n3/n6 whereas with the FORO-based diet seabass was in average around 0,9. Both GSB and ESB showed EPA and DHA content between 500 and 1500mg/100g fillet, meaning that 100g of all sampled fish were enough to meet the recommended daily intake by EFSA of these FA.
Conclusions
This experiment conducted in industrial conditions, aimed at understanding the modulation of FA in GSB and ESB muscle depending on feed lipid sources, showed that GSB is likely to improve the characteristics of its fillets towards higher EPA and DHA levels and greater n3/n6 ratio during the summer period, whereas ESB may need a continuous feeding on selected high in n3-LC HUFA FO diets to attain this goal.