Supplementation of carrot pomace extract as natural carotenoids in diet of Caspian trout ( <i>Salmo caspius</i> ): impacts on growth performance, hematological, immunological, and antioxidant status

Utilisation of natural carotenoids from plant waste in fish nutrition is gaining appropriate attention due to their sustainability, cost-efficiency, and potential health benefits. This study investigated the effects of natural carotenoids extracted from carrot pomace on the growth, haematological, immunological and antioxidant status of Caspian trout (Salmo caspius). Carotenoids were extracted from carrot pomace and supplemented to the Caspian trout diet at different concentrations of 0, 50, 100, 150, 200 and 250 mg kg−1. Despite no significant impact on growth performance and feed utilisation (p > 0.05), red blood cells, haemoglobin concentration and haematocrit showed a significant linear correlation with an increase in dietary carotenoids up to 200 mg kg−1 diet and levelled off thereafter. An increase in dietary carotenoids was also observed a linear decreasing trend in WBC and lymphocyte percentage. Additionally, serum lysozyme and alternative complement activities had significant quadratic correlation with dietary carotenoids to the highest values of 51.00 ± 3.61 µg mL−1 and 139.0 ± 1.03 U mL−1 in fish fed a diet supplemented with 100 mg kg−1 carotenoids (p < 0.05), suggesting its beneficial effects for disease resistance in the Caspian trout. The liver concentration of catalase and superoxide dismutase had a significantly linear and quadratic increasing trend by supplementing the carrot pomace-derived carotenoids (p < 0.05), while the malondialdehyde content had a significantly linear and quadratic decreasing trend by increasing dietary carotenoids supplementation. Based on the serum lysozyme activity as well as the liver activity of catalase, glutathione peroxidase and malondialdehyde concentration, the optimum dietary level of carotenoids extracted from carrot pomace for Caspian trout was estimated to be in a range of 130.2–157.3 mg kg−1 diet.