Effects of dietary pomegranate peel supplementation on growth performance and biochemical responses of common carp, Cyprinus carpio, to chronic crowding stress

In the present study, the effects of dietary pomegranate peel (PP) at 0 (PP0; control), 5 (PP5), 10 (PP10), 15 (PP15), and 20 (PP20) g/kg were investigated in common carp, Cyprinus carpio (18.1 ± 0.42 g), under normal and stressful conditions. The fish were fed the aforementioned diets over two 4-week periods, one under normal stocking density (5 g/L; basal) and the other under high stocking density (15.5–16.3 g/L; stressed). The results showed that there were no significant differences in growth performance between the PP0 and PP5 treatments. However, PP10, PP15, and PP20 showed deteriorated growth performance, compared to PP0 treatment. The poorest growth parameters have been observed in PP15 and PP20 treatments. Plasma cortisol, glucose, alanine aminotransferase, and aspartate aminotransferase significantly increased after the stress in all treatments. PP5 and PP10 had significantly lower plasma cortisol and PP10 and PP15 had significantly lower plasma glucose and aspartate aminotransferase, compared to PP0 treatment. Increase in dietary PP levels significantly decreased alanine aminotransferase activity. The stress and increase in dietary PP levels significantly decreased the plasma T3 and T4 levels. Dietary PP significantly improved plasma lysozyme (PP5, PP10, and PP15) and complement (PP5 and PP10) activities and mitigated their decrease after the stress (the best effects in PP10), but was not beneficial in mitigating plasma total immunoglobulin decrease after the stress. Dietary PP (5–15 g/kg) significantly decreased hepatic antioxidant enzymes activities and mitigated their elevations after the stress. Hepatic malondialdehyde significantly decreased by dietary PP administration, but increased after the stress. The lowest hepatic malondialdehyde level was observed in PP10 treatment. In conclusion, dietary 5 g/kg PP is recommended as the best supplemental level that improves the fish health, without adverse effects on the fish growth performance.