Dietary chrysin supplementation improves growth performance, immune responses, antioxidant status, and resistance against crowding stress in rainbow trout

Although several investigations have demonstrated the beneficial effects of chrysin (CH), there is limited information on its supportive effects on rainbow trout (Oncorhynchus mykiss) physiology. In this work, fish were fed with pellets containing 0, (CH0) 100 (CH100), 300 (CH300), 500 (CH500) and 700 (CH700) mg/kg chrysin for 56 days, then the specimens weighed and were exposed to 14-day crowding stress. Innate immune responses, antioxidant capacity, hematology indices, and biochemical parameters were tested before and after crowding stress. The final weight (FW), weight gain (WG), and specific growth rate (SGR) increased, and feed conversion ratio (FCR) decreased in the groups fed diets containing CH300-CH700 compared to control group. Also, the optimum dose for maximum and minimum SGR and FCR were obtained at 470 and 490 mg CH (P < 0.05), according to the polynomial regression test, respectively. There was an interaction between dietary CH and crowding stress on immune responses, antioxidant defense, hematological and serum biochemical parameters. Before stress, serum and mucus immune responses [lysozyme and alternative complement (ACH50) activity, and total immunoglobulin (Ig)] were increased in CH300 group (P < 0.05). After stress, immunological activities were improved by diets supplemented with 300-700 mg/kg of chrysin. Before stress, red blood cell (RBC) count, percentage of hematocrit (HCT), blood leukocyte (WBC) count, and monocyte (MON) count clearly increased in the CH300 group (P < 0.05). After stress, hematological parameters were enhanced in the CH-treated fish compared to the control. The lowest serum biochemical parameters (liver enzymes, and glucose) values were noticed in CH500 treatment before stress, except for cortisol. After stress, serum biochemical parameters in response to all dietary treatments significantly decreased, except for aspartate aminotransferase (AST) (P < 0.05). Before stress, antioxidant enzymes activities significantly improved in fish fed with diets containing 300 and 500 mg of CH (P < 0.05). After stress, administration of 300-700 mg CH remarkably boosted antioxidant enzymes activities (P < 0.05). MDA value decreased in response to diets containing 300-700 mg CH, before and after stress. In summary, CH300 and CH500 diets more effectively boosted growth performance and reduced the harmful effects of crowding on health status of the rainbow trout.