In this study, the effects of olive leaf extract on growth, haematological parameters, immune system and carcass composition in common carp were examined. In this research, 180 common carp (32.26 ± 1.01 g) were randomly distributed to nine fibreglass tanks. The fish were fed a diet with 200 mg/kg olive leaf extract (group 1), 400 mg/kg olive leaf extract (group 2), and basal food (control group) for 75 days. At the end of the experiment, WG, SGR and PER values were significantly higher in the groups fed the diet containing 200 mg/kg of olive leaf extract compared with the control group. Furthermore, FCR and FER values were improved in the groups fed by diets with various levels of olive leaf extract than the control group. The contents of WBCs, lysozyme and NBT activity were significantly increased in the groups fed diets containing olive leaf extract. Cholesterol content was significantly lower in the group fed diet containing olive leaf extract compared with the control group. Also, serum triglyceride and carcass lipid contents were significantly lower in the group fed diet containing 200 mg/kg of olive leaf extract compared with the control group. Total protein, albumin, globulin and calcium contents were significantly higher in the group fed the diet containing 200 mg/kg of olive leaf extract compared with the control group. Our results demonstrated that diet containing 200 mg/kg olive leaf extract were efficient feed additives and has the potential for use in improving the growth performance and health status for this species.

Effects of different levels of Biomin® IMBO synbiotic, including Enterococcus faecium (as probiotic), and fructooligosaccharides (as prebiotic) on survival, growth performance, and digestive enzyme activities of common carp fingerlings (Cyprinus carpio) were evaluated. The experiment was carried out in four treatments (each with 3 replicates), including T1 = control with non-synbiotic diet, T2 = 0.5 g/kg synbiotic diet, T3 = 1 g/kg synbiotic diet, and T4 = 1.5 g/kg synbiotic diet. In total 300 fish with an average weight of 10 ± 1 g were distributed in 12 tanks (25 animals per 300 l) and were fed experimental diets over a period of 60 days. The results showed that synbiotic could significantly enhance growth parameters (weight gain, length gain, specific growth rate, percentage weight gain) (P < 0.05), but did not exhibit any effect on survival rate (P > 0.05) compared with the control. An assay of the digestive enzyme activities demonstrated that the trypsin and chymotrypsin activities of synbiotic groups were considerably increased than those in the control (P < 0.05), but there was no significant difference in the levels of α-amylase, lipase, or alkaline phosphatase (P > 0.05). This study indicated that different levels of synbiotic have the capability to enhance probiotic substitution, to improve digestive enzyme activity which leads to digestive system efficiency, and finally to increase growth. It seems that the studied synbiotic could serve as a good diet supplement for common carp cultures.

In recent years, the aquaculture industry has extended significantly and has moved towards intensive and super-intensive systems due to the growth in the world population and the demand to provide food from diverse sources. As a result, the importance of the need for water quality control, nutrition and disease prevention is increasingly felt in the aquaculture industry (Chelemal Dezfoulnejad & Molayemraftar, 2021;FAO, 2021). In the past decades, many researchers have studied various aspects of the administration of