Effects of a fishmeal-replaced diet fortified by nano-selenium on body composition, digestive enzymes, and intestinal morphology in Cyprinus carpio

Background Fishmeal (FM) is a traditional source of protein and essential trace elements in aquaculture diets. However, concerns over sustainability, cost, and resource depletion have driven interest in plant-based alternatives. Methods This study conducted a 60-day trial to evaluate the dose-dependent effects of dietary nanoselenium (nano-Se) on Cyprinus carpio (initial weight: 30.63 ± 0.61 g) fed plant-based diets as a replacement for FM. Two control diets were used: one containing FM (FM-D) and one composed solely of plant ingredients (PM-D). To assess the effects of nano-Se, the PM-D was supplemented with three nano-Se levels: 1 mg/kg (PM-Se1), 2 mg/kg (PM-Se2), and 3 mg/kg (PM-Se3). Results Carp fed the FM-D and PM-Se3 diets exhibited significantly higher daily weight gain and feed efficiency than those on other diets ( P < 0.05). Plasma reduced-glutathione (GSH) levels were also higher in these groups ( P < 0.05). Whole-body composition did not differ significantly in moisture, protein, or lipid contents ( P > 0.05), though ash content was significantly higher in PM-Se3 compared to PM-D ( P < 0.05). Intestinal histology revealed increased epithelial thickness and villus height in PM-Se3, alongside the highest digestive enzyme activities (trypsin, chymotrypsin, lipase, and amylase). Economic analysis showed that PM-Se3 provided a cost-effective alternative to FM-D, yielding comparable growth performance and feed efficiency at a lower feed cost. Conclusion These findings suggest that dietary supplementation with 3 mg/kg nano-Se was associated with improved growth performance, plasma GSH, intestinal morphology, and digestive enzyme responses in fishmeal-free diets, while their economic advantage was maintained.