Assessment of dietary supplementation with carvacrol or thymol containing feed additives on performance, intestinal microbiota and antioxidant status of rainbow trout (Oncorhynchus mykiss)

The manipulation and control of fish health and production by natural substances has been identified as an important area for future developments in aquaculture. A study was conducted to investigate the effect of two phytogenic feed additives, one rich in carvacrol (CARV containing 12 g/kg carvacrol) and the other rich in thymol (THYM containing 6 g/kg thymol) on growth performance, gut microbiota and antioxidant status of rainbow trout (Oncorhynchus mykiss) under commercial farming conditions. Groups of 30 sub-adult trout (113.0 g +/- 10.4) were randomly allocated into 3 different treatments with three replicates each. The control group was fed a basal diet, while the other two groups were fed diets supplemented with the two phytogenic diets at the level of 1 g/kg. The fish were fed to apparent satiation for a period of 8 weeks. Body weight, body length and feed intake were recorded weekly. The intestinal bacteria populations (total aerobes, total anaerobes, Lactobacilli spp., Enterobacter spp., coliforms, E. coli and Aeromonas spp.) were enumerated by conventional microbiological techniques using selective agar media. Antioxidant status of fish was assayed for levels of glutathione reductase, glutathione-S-transferase and malondialdehyde levels in fish fillet at day 0 and 5 after slaughter. Lysozyme, nitric oxide (NO), total complement concentrations and catalase activity were also evaluated in fish blood serum. Results showed that dietary phytogenic supplementation with both products improved (P<0.05) feed efficiency compared to control diet, although, body weight gain was unaffected by phytogenic supplementation. Total anaerobe counts were lower in phytogenic fed fish for both products compared to control; and lactobacillus loads in THYM group compared to CARV and control; however, other bacteria loads were similar among dietary treatments. Both phytogenic inclusion decreased significantly (P<0.05) malondialdehyde formation on day 5 of refrigerated storage compared to control fish. Activity of glutathione based enzymes at both time points were significantly high in both phytogenic supplemented groups compared to control (P