Carvacrol is a monoterpene derived from plants, such as thyme and oregano, and has been shown to promote growth, improve health, and mitigate bacterial infections in various animal models. Previous results from this research group have demonstrated minimum inhibitory (MIC: 98 µg/mL) and bactericidal concentrations (MIB: 195 µg/mL) against Aeromonas hydrophila, Edwardsiella ictaluri, and E. piscicida, which are important bacterial pathogens in US catfish aquaculture. Interestingly, concentrations lower than the MIC and MIB presented the potential to inhibit biofilm formation and hemolytic activity of the aforementioned bacteria and pathogenic strains of Streptococcus iniae. Herein, the impact of dietary carvacrol on the growth and blood parameters, oxidative status, immune responses, and intestinal microbiota of channel catfish was evaluated.
Carvacrol was supplemented in experimental diets at concentrations of 0.0 (control), 0.5, 1.0, 2.0, and 4.0 g/kg of feed, and fish were fed twice daily at a rate of 4% body weight. Catfish juveniles were distributed to 25 aquaria, each containing 30 fish, and each dietary treatment had five experimental units. The aquaria operated as a recirculating aquaculture system maintained under a 12:12 light-dark cycle, and the feeding trial lasted for 84 days. At the end of the study, production performance parameters were evaluated. Three fish per tank were sampled for hematology and plasma analysis, as well as condition indices, Fulton’s condition factor, histology, and activity of antioxidant enzymes. A three fish from each tank were sampled for whole-body proximate composition (dry matter, protein, fat, and ash). Digesta samples were collected to assess prospective differences in the intestinal microbiota using 16S rRNA targeted amplicon sequencing. Significant differences were observed for weight gain, where fish offered diets supplemented with 4.0 g/kg had decreased production performance, feed intake, and increased viscerosomatic and hepatosomatic indices. No significant differences were observed in the intraperitoneal fat index, condition factor, and survival. No differences were observed in hemoglobin concentration, hematocrit, mean corpuscular volume, mean corpuscular hemoglobin, or mean corpuscular hemoglobin concentration. However, red blood cell production increased significantly with 0.5 g/kg and 4 g/kg carvacrol treatments. Results suggest low concentrations of carvacrol did not negatively impact the production performance of channel catfish fingerlings.