Effects of dietary oak (Quercus castaneifolia) leaf extract on growth, antioxidant, and immune characteristics and responses to crowding stress in common carp (Cyprinus carpio)

The aim of the present study was to assess the effects of dietary oak (Quercus castaneifolia) leaf extract supplementation on growth performance, antioxidant, immune and stress responses in common carp (Cyprinus carpio). Ethanolic extract of the oak leaf were prepared and tested for in vitro radical scavenging and antibacterial properties. The results showed that the extract has radical scavenging property below that of butylated hydroxytoluene (BHT) at 12.5–75 μg mL−1, but there was no significant difference between the two materials at 100 and 200 μg mL−1. Moreover, the extract had bactericidal effects against Aeromonas hydrophila, which was near 5-fold lower than tetracycline. Then, the extract was added to the fish diet at 0 (CTL), 0.5 (0.5E), 1 (1E) and 2 (2E) g kg−1 diet. The fish were fed with aforementioned diets for 60 days before subjecting to a 6-h crowding stress. The results showed that the extract had no significant effects on the fish growth performance (P > .05). Dietary extract supplementation (1E and 2E) significantly increased plasma superoxide dismutase (P = .003), catalase (P = .015), glutathione peroxidase (P < .001), reduced glutathione (P < .001), lysozyme (P < .001), complement (P = .001), and bactericidal activity (P = .020) and decreased malondialdehyde (P < .001) levels. Dietary oak leaf extract supplementation had no significant effects on basal cortisol and glucose levels, but significantly (P < .001) mitigated post-stress levels of these parameters, compared to the CTL fish. In conclusion, oak leaf extract stimulates antioxidant and immune system of common carp, without affecting the fish growth performance. Moreover, the extract was partially beneficial to reduce stress in the fish. Dietary levels of 1–2 g kg−1 oak leaf extract are recommended for common carp feed formulation.