LAUSR

Abstract This study evaluated the use of Gracilaria by-products in gilthead seabream diet to modulate intestinal bacterial community, physiological responses as well as skin and fillet attributes during cold storage. Fish were fed for 52 days with six experimental diets: i) a diet containing a commercial antioxidant (CTR), ii) a diet without antioxidant (NO-AOX), iii) a NO-AOX with 0.5% ethanolic extract (0.5% EE), iv) a NO-AOX with 5% ethanolic waste (5% EEW), v) a NO-AOX with 5% agar waste (5% AW), vi) a NO-AOX with 2.5% agar waste (2.5% AW). Fish growth performance was not affected by the experimental diets. Proteobacteria (63–83%), Firmicutes (4–19%) and Actinobacteria (2–4%) were the predominant phyla in the intestine of fish fed all diets. The diet with 5% AW increased the relative abundance of Firmicutes phyla as compared to the other diets. Fish fed NO-AOX, 5% EEW, 5% AW and 2.5% AW showed a lower abundance of Oxalobacteraceae than those fed CTR diet. The Clostridium genera increased in the 5% AW compared to 5% EEW. Nevertheless, no major differences were detected in the richness and diversity of the intestinal bacterial communities of gilthead seabream fed the experimental diets. Fish fed 5% AW showed higher intestinal trypsin activity than those fed 0.5% EE. The skin yellowness was higher in the skin of fish fed 2.5% AW diet compared to those fed 0.5% EE diet. The results presented provided evidences of the potential application of AW and EEW as a novel and eco-friendly dietary supplement to be included in future aquaculture feed formulations.