Growth Performance, Liver and Intestinal Histology, and Immune Response in Gilthead Seabream (Sparus aurata) Juveniles Fed with Diets Supplemented with Niaouli (Melaleuca viridiflora) Essential Oil

Abstract This study investigated the growth performance, liver and intestinal histopathology, and immune response in gilthead seabream (Sparus aurata) juveniles fed diets supplemented with Niaouli (Melaleuca viridiflora) essential oil. The study was conducted in two phases. In the first phase, a feeding trial was carried out to monitor the effects of the Niaouli essential oil-supplemented diet on fish growth, while in the second phase, the impact on blood parameters and liver-intestinal histopathology was observed in seabream groups fed with the oil-supplemented diet and control groups fed only with pellet feed after administration of a Vibrio anguillarum vaccine. The Niaouli oil was added to the feed at four different levels (0, 1, 1.5, and 3%). The study was replicated three times. Six hundred seabream juveniles, with an initial weight of 11.770 ± 0.006g, were randomly stocked in 12 tanks (50 fish per tank) and fed for 105 days. At the end of the study, feed conversion ratio (FCR), weight gain (WG), and specific growth rate (SGR) were found significantly higher in the group supplemented with 1% Niaouli oil compared to other groups. No pathological changes were observed in the intestinal histology. Similarly, no pathological disorders were detected in the livers of the control groups subjected to feeding and vaccination. However, varying degrees of hepatocyte degeneration from mild to severe were observed in all groups treated with the Niaouli essential oil additive, and the severity of degeneration was determined to be dose dependent. Most liver epithelial cells in the group supplemented with 1% Niaouli oil group appeared close to normal, but some hepatocytes had a granular appearance in their cytoplasm. It was also determined that there was less hydropic degeneration in the group supplemented with 1% Niaouli oil compared to other groups. In the second phase of the study, when vaccinated and unvaccinated groups were compared and assessed in terms of feeding parameters, the vaccine was observed to decrease the feed evaluation rate, increase live weight gain, and enhance the specific growth rate. Hematological values also indicated that the vaccine increased the red blood cell count, stimulated red blood cell production in the bone marrow, and induced erythrocytosis. In conclusion, further research is recommended, involving trials on different fish species with various oil ratios and vaccination timings, to update the literature.