Effects of new microalgae and macroalgae products on live prey and fish performance

Resum

Aquaculture has acquired a worldwide essential importance, growing steadily over the last years and exceeding global capture fisheries. Among the aquaculture products, algae global production is expected to increase to the highest rate in the coming decades. In fact, two algae (Saccharina japonica and Euchema spp.) are presently at the top of total production statistics. Algae are rich in valuable compounds such as polysaccharides, vitamins, minerals, proteins, phenolic compounds, sterols and other lipids including n-3 long chain polyunsaturated fatty acids (n-3 LC-PUFA). They also have high contents of pigments that exert antioxidant and anticancer activities. Thus, algae are considered good candidates to be used in human and animal nutrition because of their biochemical composition. Besides, macroalgal wracks that regularly detach from offshore beds and accumulate in coastal areas are usually removed and discharged, and consequently, new uses for this biological biomass are being currently evaluated. Live prey are essential tools in aquaculture to feed larvae of marine organisms. However, they are usually poor in some essential nutrients such as LC-PUFA, needing to be enriched prior to larval feeding. In this sense, lipid emulsions and microalgae are commonly used to enrich or feed live preys. While enrichment protocols are usually associated to high oxidative stress condition, which is increased by the addition of a lipid emulsion, the use of microalgae involves high production costs, both leading to a serious bottleneck in hatcheries. Thus, substitutes of live microalgae including pastes, dried formats, microencapsulates or flocculated microalgae are receiving increasing research attention due to its nutritional stability and antioxidant potential, since oxidative stress may be partially compensated by the antioxidant compounds present in microalgae. On the other hand, algae have been proposed as suitable alternative sources of lipids and proteins for farmed fish due to their high nutritional value and balanced composition, high production rates, and potential availability. In fact, algal inclusion has already demonstrated some beneficial effects when partially replace fish meal and fish oil in aquafeeds. In our study, new Isochrysis galbana and Navicula salinicola formats, together or not with lipid emulsions in rotifer and Artemia enrichment protocols are assayed. The lipid composition of enriched live preys, and the potential ability of products to palliate oxidative stress is studied. Our results suggest that I. galbana was better than N. salinicola to feed live preys. In particular, spray-dried was the best format to enhance n-3 LC-PUFA content and phospholipids retention, due to a higher nutrient availability in this format. In spite of increasing oxidative stress, the addition of a lipid emulsion for a short enrichment period, together with the spray-dried I. galbana, showed the best results regarding lipid composition and oxidative status protection. On the other hand, lipid characterization of seaweeds regularly present in beach casts from the Macaronesian region is assessed, in order to discuss their potential use for animal feeding and as human food. Thus, a 7% inclusion of a multispecific or two monospecific (Lobophora sp. and Dictyota sp.) macroalgal wracks in Ctenopharyngodon idella and Sparus aurata diets is assayed, and the lipid composition, oxidative status, digestive enzymes activities and plasmatic parameters of fish determined. The present study also provides evidence of the high variability of macroalgal lipid profiles. Their n-6/n-3 ratio, and their content in some benefitial lipids such as phytosterols make the algae analysed interesting from both a nutritional and health perspective.In this sense, Asparagopsis taxiformis and Dictyota dichotoma can be considered as good sources of n-3 fatty acids for human nutrition because of their EPA+DHA content. Finally, a 7% of macroalgal wrack dietary inclusion did not display any negative effect in the culture performance of C. idella and S. aurata juveniles. In fact, the multispecific wrack displayed less fat deposition in C. idella, while both multispecific and monospecific (Lobophora sp.) wracks caused some protective effect by enhancing the antioxidant capacity in the liver compared to the control. S. aurata also showed a higher capacity to mold the glutathione metabolism with the monospecific Dictyota sp. wrack feeding. Furthermore, S. aurata showed some altered patterns in the digestive enzymes activities with respect to the control, although without any detrimental effects on fish performance.