Aquaculture has established a crucial role in global food security and nutrition. Applied research in this field aims to improve culture conditions, aquaculture practices, and aquafeeds to improve health, growth rates, and overall fish welfare. In this regard, the discovery of functional ingredients that can be added to fish diets has been widely and successfully explored. The main objective of this work was to evaluate the impact of natural extracts containing functional ingredients in fish diets on the stress response of gilthead seabream (Sparus aurata) post-larvae, using a multidisciplinary approach.
A trial was conducted to assess the effects of functional diets, incorporating extracts of grapeseed and Ulva spp. in increasing concentrations, on the robustness of S. aurata post-larvae. During this 1-week feeding trial, fish at 80 days after hatching (DAH) were fed one of the following experimental diets: a commercial-like diet (CTRL), three diets with different amounts of grapeseed extract (NLOW, NMEDIUM, NHIGH), and three diets with different amounts of Ulva (ULOW, UMEDIUM, UHIGH). At the end of the feeding period, key performance indicators were evaluated, and fish were sampled for biometry and molecular biomarkers of stress and antioxidant response. Subsequently, the fish underwent a stress test involving air exposure and a bacterial infection challenge using a bath of Photobacterium damselae subsp. piscicida. Fish survival was evaluated throughout the challenge.
The dietary supplementation of algal extracts had no significant effect in stress biomarkers measured in fish fed diets containing grapeseed or Ulva sp. extracts before the stress, indicating that the incorporation of these ingredients had no effect on S. aurata performance under the present conditions. However, after a stress event, fish fed the ULOW diet showed a significant increase in the levels of GPX1 and GPX4 expression and enzymatic activity when compared to the CTRL diet, highlighting a potential antioxidant effect of this diet. NLOW, NMEDIUM, ULOW and UMEDIUM diets also tended to increase SOD and HSP70 expression after stress (although not significantly), suggesting a potential antioxidant effect of these diets, which was not observed in diets with higher incorporation levels of these ingredients. Furthermore, the mortality of fish after the stress event (1h) was higher in the control group, suggesting a potential protective effect of the extracts incorporated into the fish feed. This trend was not maintained in the post-infection period, with the larvae fed diets containing extracts from grapeseed and Ulva spp. presenting a higher mortality than the control. This suggests a protective effect against an acute stress event (air exposure) but not against an acute infection with P. damselae subsp. piscicida, a finding that should be further confirmed through more in-depth analysis.
Overall, these results show that the inclusion of bioactive biomasses and/or extracts in fish diets for commercial species in the early stages can be a differentiating element, potentially improving overall fish performance and resistance to stressful events, but the doses used need to be optimised.
This study had the support of the projects PACTO DE INOVAÇÃO BIOECONOMIA AZUL (Project No. C644915664-00000026) and SAUDE&AQUA II (MAR-021.1.3-FEAMPA-00018). The technical assistance of EPPO and S2AQUAcoLAB staff was highly appreciated throughout the study.