Dietary supplementation with Gracilaria sp.by-products modulates stress response, antioxidant and immune systems of gilthead seabream (Sparus aurata) exposed to crowding

By-products obtained from seaweed processing appear as a promissory source of bioactive compounds that could be included in aquafeed to improve the sustainability of aquaculture. Therefore, the present study aimed to evaluate the effects of dietary supplementation with by-products obtained from the seaweedGracilaria sp. on the stress response, antioxidant, and immune systems of gilthead seabream (Sparus aurata) exposed to an acute crowding event. For this, fish were fed six experimental diets: (i) a diet containing a commercial antioxidant (CTR), (ii) a diet without antioxidant addition (NO-AOX), (iii) a NO-AOX diet with 0.5% ethanolic extract (0.5% EE), (iv) a NO-AOX diet with 5% ethanolic waste (5% EEW), (v) a NO-AOX diet with 5% agar waste (5% AW), and (vi) a NO-AOX diet with 2.5% agar waste (2.5% AW) for 59 days. After this period, fish were exposed to 1 h of crowding (100 kg m(-3)) and sampled immediately after 0 and at 24 h post-crowding. Our results showed that the fish-fed AW and EEW diets showed lower plasma cortisol levels than those fed the CTR diet, suggesting an improved stress response to crowding on these groups. No differences were detected on hepatic lipid peroxidation levels between dietary groups after crowding. However, the fish fed 0.5% EE diet showed lower hepatic glutathione reductase (GR) activity than fish fed the NO-AOX diet. Additionally, glutathione peroxidase (GPx) activity was decreased in the liver of fish fed 2.5% and 5% AW diets compared with the other dietary groups. Concerning immune parameters, the plasma peroxidase activity was higher in fish fed the 2.5% AW diet than fish fed NO-AOX or 5% EEW diets, suggesting a modulatory effect of dietary AW at 2.5% in the humoral immune response in crowded fish. Results suggest that dietary supplementation with AW, particularly when supplemented at 2.5%, could be used as a dietary tool to mitigate oxidative stress when improving immune response in gilthead seabream exposed to a stressor as crowding.