Corn husk meal as a sustainable dietary additive in biofloc-cultured koi carp (Cyprinus carpio var. koi): Integrative effects on growth performance, immune response, histology, gene expression, and ammonia-stress resilience.

This study evaluated the effects of dietary corn husk meal (CHM) on growth, digestive enzyme activity, intestinal morphology, immune- and antioxidant-related gene expression, and ammonia-stress tolerance in biofloc-cultured koi carp. A total of 300 fish (initial weight, 32.20 ± 0.03 g) were fed diets containing 0, 5, 10, 20, or 30 g/kg CHM for eight weeks. Survival remained high across treatments (> 95%), while fish fed CHM-10 showed significantly improved growth and a lower feed conversion ratio than the control (p < 0.05). Polynomial regression estimated a modeled optimum CHM inclusion level of approximately 14.4-14.9 g/kg, which was not directly tested in the feeding trial. Serum total protein was unaffected, whereas digestive enzyme activities increased at moderate CHM levels, peaked at 10 g/kg and declined at higher inclusion levels. Fish fed 10 g/kg CHM also exhibited improved intestinal morphometry, whereas higher inclusion levels produced less favorable villus characteristics. CHM supplementation at 10 g/kg significantly upregulated tlr4, tnf-α, il-8, tgf-β, il-10, gpx, sod, gst, and lyz, while nf-κB and cat were not significantly affected. During ammonia exposure, the CHM-10 and CHM-20 groups showed significantly higher survival than the control. Overall, CHM-10 was the best-performing experimentally tested inclusion level, improving growth, digestive function, intestinal morphology, selected immune- and antioxidant-related gene expression, and ammonia-stress resilience in koi carp, while the regression analysis suggested a higher modeled optimum of approximately 14.4-14.9 g/kg. These findings support the sustainable use of agricultural by-products as dietary ingredients for aquaculture.