Effect of microalgae in Gilthead seabream (Sparus aurata) diets

The world`s fish stocks are fully or overexplored, so the investment in aquaculture was inevitable. The witnessed rise of global human population, associated with the economic crisis experienced, lead to the decline of the principal ingredient resources utilized in aquafeeds: fishmeal and fish oil. This lead to the pursuit of alternative ingredients, such as plant feedstuffs that, although not ideal for carnivorous fish species cultivated, are already used to replace a large percentage of marine derived feedstuffs in aquafeeds. Nevertheless, the need to find new alternative nutrient sources to formulate aquafeeds that allow the development of a sustainable and profitable aquaculture while maintaining fish health is still a reality. Microalgae present themselves as good alternative ingredients to provide macro nutrients but are also interesting sources of bioactive compounds. Anti-inflammatory, antioxidant, antitumoral are some of the effects ascribed to microalgae bioactive molecules. Microalgae biomass is not yet produced in quantities suitable to be used as bulk ingredients in aquafeeds but can be supplemented to fish diets as functional ingredients capable of improving fish health and welfare. This study aimed at screening some of the effects of dietary microalgae in gilthead seabream (Sparus aurata), by evaluating fish growth, feed utilization, intestinal morphology and function as well as microbiota. The microalgae chosen to test as a functional ingredient on Gilthead seabream diets was Nannochloropsis sp. , due to its availability and stage of production technology development. To do so a 37 days short-term trial was performed in a thermoregulated seawater system with 60g Sparus aurata juveniles. A control diet (45% C.P., and 18 % C.L.) with low fish meal (20%) inclusion was formulated along with three experimental diets produced with similar composition to control diet but supplemented with Nannochloropsis sp. : 0.5% (Nanno 0.5), 0.75% (Nanno 0.75) and 1.5% (Nanno 1.5). Although not statistically significant, an increase in fish daily growth index and final body weight was verified with the inclusion of the microalgae in the diets. No significant differences were found in the histomorphological evaluation. Digestive function was also statistically similar among experimental groups. In general, the Control iii FCUP Effect of microalgae in gilthead seabream (Sparus aurata) diets group showed a tendency towards higher digestive enzyme activity, immediately followed by Nanno 0.75. Microbiota was modulated by microalgae supplementation, with Nanno 0.75 showing higher indices for richness, diversity and OTUs. Bacterial communities for each dietary treatment, demonstrated that fish fed the Nanno 0.5 and Nanno 0.75 diet were more closely related (similarity higher than 90%), while those from Nanno 1.5 and control diet diverge more (similarity bellow 90%). In fish offered the Nanno 1.5 diet the simper similarity was higher and the replicates clustered more, revealing a homogeneous response from bacterial communities to the dietary treatment. Nannochloropsis supplementation to a low FM diet for sea bream did not significantly improved the parameters analyzed and had no deleterious effects either. The trend towards improved growth performance should be evaluated in a growth trial and microbiota modulation should be further investigated to understand if it could it be advantageous for the animals. Nevertheless, the results achieved in this study are promising, supporting the use of microalgae as a functional ingredient for marine carnivorous fish aquafeeds.