Abstract
Sustainable strategies, such as oregano essential oil (OEO), are being assessed to improve farmed fish's health and performance. Several studies in freshwater species describe the beneficial effects of OEO as a dietary supplement. Nonetheless, information about its use in marine fish is scarce. Hereby, this study consisted of four experimental groups: a control and three levels of OEO dietary inclusion (OEO75 (0.75 mL/kg), OEO100 (1.0 mL/kg), and OEO125 (1.25 mL/kg)) with 23 fish of 110 ± 1.2 g per tank (n = 276) in a seawater flow-through system. After 70 days, data for growth parameters and samples for gut microbiota were taken. The final weight of OEO75 was higher (242.3 ± 24.2 g), and its feed conversion rate (0.91 ± 0.0) was reduced. However, these differences could be related to the sampling interval between the groups. Bioinformatic analysis of OEO groups revealed a reduction in Proteobacteria (Vibrio, Flavobacteria, and Vibrionales order) abundance and an increase in Euryarchaeota phyla in a dose-dependent manner. The predicted functions shifted from nutritional (OEO75) to replication, proliferation, and bacterial defense mechanisms (OEO100 and OEO125). These results show that adding OEO did not improve growth performance, but it reshaped the gut microbiota by reducing the abundance of dominant bacteria and modifying their metabolic pathways.