Aquaculture has been challenged to find alternative ingredients to develop innovative feed formulations that foster a sustainable future growth. Given the most recent trends in fish feed formulation on the use of alternative protein sources to decrease the dependency of fishmeal, it is fundamental to evaluate the implications of this new paradigm for fish health and welfare. This work intends to comprehensively review the impacts of alternative and novel dietary protein sources on fish gut microbiota and health, stress and immune responses, disease resistance, and antioxidant capacity. The research results indicate that alternative protein sources, such as terrestrial plant proteins, rendered animal by-products, insect meals, micro- and macroalgae, and single cell proteins (e.g., yeasts), may negatively impact gut microbiota and health, thus affecting immune and stress responses. Nevertheless, some of the novel protein sources, such as insects and algae meals, have functional properties and may exert an immunostimulatory activity. Further research on the effects of novel protein sources, beyond growth, is clearly needed. The information gathered here is of utmost importance, in order to develop innovative diets that guarantee the production of healthy fish with high quality standards and optimised welfare conditions, thus contributing to a sustainable growth of the aquaculture industry.

Plant extracts are known for their high content and diversity of polyphenols, which can improve fish oxidative status. A growth trial with Senegalese sole postlarvae (45 days after hatching) fed with one of four experimental diets—control (CTRL), and supplemented with curcumin (CC), green tea (GT), and grape seed (GS) extracts—was performed to assess if supplementation could improve growth performance and oxidative status. At the end of the growth trial, postlarvae were submitted to a thermal stress to assess their robustness. Sole growth was improved by CC and GS diets when compared to those fed the CTRL. CC and CTRL postlarvae presented the lowest oxidative damage (lipid peroxidation and protein carbonylation values). Stress-related biomarkers (heat shock protein 70 and glutathione-S-transferase) decreased in CC fish compared to those fed the CTRL diet, which might be due to a direct antioxidant capacity. In contrast, oxidative damage increased in GT and GS sole reared in standard conditions. However, after a thermal stress, GT and GS diets prevented the increase of protein carbonylation content and the decrease of antioxidant glutathione, depending on exposure time. Overall, dietary supplementation with natural extracts modulated oxidative status and stress response after a short/long-term exposure to temperature.

The larval stage is highly prone to stress due to the ontogenetic and metabolic alterations occurring in fish. Curcumin inclusion in diets has been shown to improve growth by modulating oxidative status, immune response, and/or feed digestibility in several fish species. The aim of the present work was to assess if dietary curcumin could promote marine fish larvae digestive maturation and improve robustness. Gilthead seabream larvae were fed a diet supplemented with curcumin at dose of 0 (CTRL), 1.5 (LOW), or 3.0 g/Kg feed for 27 days. From 4 to 24 days after hatching (DAH), no differences were observed in growth performance. At the end of the experiment (31 DAH) LOW larvae had a better condition factor than CTRL fish. Moreover, HIGH larvae showed higher trypsin and chymotrypsin activity when compared to CTRL fish. LOW and HIGH larvae were able to maintain the mitochondrial reactive oxygen species production during development, in contrast to CTRL larvae. In conclusion, curcumin supplementation seems to promote larvae digestive capacity and modulate the oxidative status during ontogeny. Furthermore, the present results provide new insights on the impacts of dietary antioxidants on marine larvae development and a possible improvement of robustness in the short and long term.

Background Oxidative stress has a direct impact on fish production, affecting both growth and health status. Plant based extracts, such as those from green tea, curcumin and grape seeds, are known for their abundant content and diversity of polyphenols. These bioactive compounds have a high antioxidant capacity making such extracts good additives to include in fish diets potentially improving the oxidative status of fish, and therefore enhancing growth and stress resistance of farmed fish.A growth trial with Senegalese sole postlarvae (45 days after hatching) fed with four experimental diets, a control (CTRL) and three supplemented with natural antioxidants, namely curcumin (CC), green tea (GT) and grape seed (GS) extracts, was performed during 25 days to check if these supplements could improve growth performance and fish oxidative status. Moreover, a thermal stress was applied at the end of the growth trial, to assess the effect of these supplemented diets in stress resistance of sole postlarvae.ResultsSole growth was improved by the dietary inclusion of CC and GS compared to CTRL. Postlarvae from CC and CTRL present the lowest values of oxidative damage (lipid peroxidation and protein carbonylation). Moreover, the fish fed CC showed a decrease in stress related biomarkers (heat shock protein 70 and glutathione-S-transferase) compare to CTRL, which might be due to direct antioxidant capacity. In contrast the supplementation of GT and GS increased the content of oxidative damage in sole reared in standard conditions. However, after a thermal stress exposure both GT and GS treatment seem to prevent the increase of protein carbonylation content and the decrease in the levels of antioxidant glutathione in sole, depending on the time of exposure. ConclusionsOverall, dietary supplementation with these natural extracts modulates oxidative status and stress response after a short/long term temperature increase. The incorporation of curcumin seems to be a safe additive to include in sole diets, enhancing growth and oxidative status. The supplementation of the studied doses of green tea and grape seed extracts in fish diets needs further research as they may act as pro-oxidant depending of the culture conditions.