Introduction
Opuntia plants and their by-products have been gaining research interest due to their contents of natural bioactive compounds, such as ascorbic acid, betalains , phenols, tannins, and flavonoids, terpenes α-amyrin, β-amyrin , oleanolic acid and cycloartenol. Despite the attributes of prickly pear fruit (Opuntia ficus-indica ), limited information is available on its use as a feed additive in aquaculture. This experiment was conducted to evaluate the effects of prickly pear fruit (O. ficus-indica ) peel (PFP ) on growth performance, feed utilization, digestive enzymes activity, antioxidant activity, immune response and salinity tolerance of Nile tilapia (Oreochromis niloticus ) juveniles .
Materials and methods
PFP was incorporated into f our iso-nitrogenous (280 g kg-1 protein) and iso-energetic (18.62 MJ/Kg DM) diets at 0, 1, 2 and 4 g kg-1 diet. The diets were fed to O . niloticus juveniles (9.69 ± 0.2 g) for 75 days.
Results
The growth rates, feed utilization efficiency, body protein, digestive enzymes activities, liver function enzymes, immunological responses , and antioxidant status were all improved with increasing supplemental PFP up to 1 g kg-1 diet, followed by significant retardation with further increase in dietary PFP levels (Tables 1 and 2). However, the quadratic regression analyses of the results revealed that the maximum performance and the optimal fish health status occurred at about 2 g kg-1 of dietary PFP (Fig. 1) . Moreover, salinity challenge showed that PFP supplementation at 1 g kg -1 diet significantly decreased stress indicators in fish. These results suggest that dry PFP can play a significant role in feed digestion and absorption and enhancing fish performance and innate immune response. About 2 g kg-1 diet would be sufficient for the optimum performance and health status of juvenile Nile tilapia.