Evaluation of oregano essential oil as a growth promoter and resistance stimulator against Ichthyophthirius multifiliis (Protozoa, Ciliophora) in silver catfish juveniles, Rhamdia sp. (Siluriformes, Heptapteridae)

Agroecology-based aquaculture emerges as a particularly interesting production alternative for rural family farmers. In such systems, it is desirable to use means for enhancing the zootechnical performance and improving fish immune responses from natural products. Thus, the present study aimed to evaluate the effect of organic diet supplementation with oregano essential oil, Origanum vulgare, on zootechnical performance, hematological parameters, and resistance to the ectoparasite Ichthyophthirius multifiliis (ich) in silver catfish juveniles, Rhamdia sp. A completely randomized design with five treatments (concentrations 0, 0.5, 1.0, 1.5, and 2.0 g oregano essential oil kg-1 diet) and four replicates was adopted. Groups of 20 silver catfish juveniles (initial weight 12.5 ± 0.5g) were stocked in 20 polyethylene tanks (54 L usable volume) connected to a water recirculation system with individual aeration. The diets were prepared with certified organic ingredients and offered for 89 days in two meals a day. At the end of the experimental period, survival, zootechnical performance, hematological parameters, and fish body composition were evaluated. After the final biometry, an experimental challenge with ich infection was performed. Dietary concentrations of oregano essential oil had no effects on growth, hematology, and body composition of silver catfish juveniles (p > 0.05). Neither was there any influence on the level of infection and accumulated mortality of the fish under challenge with ich. Supplementation with oregano essential oil did not prove to be an effective dietary additive in improving the performance or increasing resistance against ich in silver catfish juveniles. However, in view of the importance of the topic, further studies should be carried out to increase knowledge about the effect of this and other natural additives in silver catfish, including at other stages of development.