Evaluation of blood orange (Citrus sinensis) juice by-products as a feed additive in juvenile black rockfish (Sebastes schlegelii): Effects on growth, enzyme activity, immunity, and disease resistance

This study evaluated the bioactive compounds present in by-products from producing blood orange ( Citrus sinensis L. Osbeck) juice (BBOJ) as dietary additives. A sequenced investigation was conducted to investigate the effects of BBOJ dietary supplementation on the growth performance, digestive enzyme activity, antioxidant capacity, immune response, and disease resistance against Vibrio harveyi in juvenile black rockfish ( Sebastes schlegelii ). A total of 630 juvenile rockfish (1.4 ± 0.01 g) were randomly assigned to seven treatments, with 30 fish per 21 rectangular tanks (50 L). The fish were fed seven different diets with graded levels of BBOJ: 0 (control, BBOJ0), 1 (BBOJ1), 2 (BBOJ2), 3 (BBOJ3), 5 (BBOJ5), 7 (BBOJ7), and 10 (BBOJ10) g/kg. At the end of the feeding trial, the BBOJ10 group demonstrated significantly higher final weight, weight gain, specific growth rate, feed efficiency, and protein retention than the BBOJ0 group. Fish fed the BBOJ10 diet exhibited a significantly higher protein efficiency ratio than those fed the BBOJ0, BBOJ2, and BBOJ3 diets. Dietary BBOJ levels significantly affected the intestinal trypsin activity. Fish fed the BBOJ10 diet showed a significantly higher antioxidant enzyme activity in the plasma than those fed the BBOJ0 diet. Moreover, fish fed the BBOJ0 diet demonstrated a significantly lower lysozyme activity than those fed the BBOJ7 and BBOJ10 diets; fish fed the BBOJ10 diet showed a significantly elevated serum immunoglobulin M level than those fed the BBOJ0 diet. The BBOJ10 group showed significantly higher disease resistance than the BBOJ0 and BBOJ5 groups. In conclusion, dietary supplementation with 10 g/kg of BBOJ enhanced the growth performance, trypsin activity, plasma antioxidant enzyme activity, serum lysozyme activity, serum immunoglobulin M level, and disease resistance against V. harveyi in juvenile black rockfish. • Supplementation of dietary BBOJ (10 g/kg) enhanced growth performance and trypsin activity in juvenile black rockfish. • Inclusion of 10 g/kg BBOJ to the diet improved antioxidant enzyme activity and immune response. • Fish fed the diet with 10 g/kg of BBOJ showed the highest survival rate against Vibrio harveyi .