The aquaculture sector is exploring new sustainable strategies to promote farmed fish’s welfare and health. The utilization of phytogenics, such as oregano essential oil (OEO), has emerged as a highly effective additive in the diet of freshwater fish species. Several studies have reported it as a growth promoter and modulator of gut microbiota. Nevertheless, few studies have evaluated and described its use in marine fish. The current study evaluated the effect of incorporating OEO into the diet on the growth performance and composition of gut bacterial microbiota (GBM) in juvenile M. saxatilis raised in seawater conditions.
The bioassay was carried out in a seawater flow-through system with 12 tanks of 430L and water flow of 1.50 L min-1, each one with 23 fish. The fish with an initial weight (IW) of 110 ± 1.2 g were randomly distributed into the three replica of the four experimental groups: control (0 ml kg-1), OEO75 (0.75 ml kg-1), OEO100 (1.0 ml kg-1), and OEO125 (1.25 ml kg-1). Water temperature, salinity, and dissolved oxygen were daily recorded. While TAN, nitrite, and nitrate were evaluated every three days with API kit®. Daily, experimental diets were prepared with diluted OEO at 2.5% with distilled water and applied via spraying on the EWOS® diet before apparent satiety feeding. The diet was dried at 21°C for 20 min. and maintained at 4°C. After 70 days, the data for the growth parameters were registered and samples for the GBM characterization were collected from all the groups. For the GBM, an initial sample prior the bioassay was obtained.
The final weigh (FW) of OEO75 was higher than the control and the OEO100 (p<0.05). Feed conversion ratio (FCR) of OEO75 was reduced in contrast with the other groups (p<0.05) (Tab. 1). However, these differences were related to the times in which the sampling of the different experimental groups was carried out. For GBM, differences were observed on the differential abundance of taxa between all the experimental groups and the initial sample (p<0.05). The control exhibited more abundance of taxa in comparation with the other groups (Fig. 1). Overall, these results indicate that OEO at a dose of 0.75 ml kg-1 can act as a modulator of GBM by reducing the differential abundance of some opportunist pathogenic bacteria of M. saxatilis.