Immunomodulatory and growth-promoting effects of supplementing red yeast (Sporidiobolus pararoseus) in fish meal-based diets for koi carp (Cyprinus carpio var. koi) cultured in a biofloc system

The aquaculture industry is increasingly pursuing sustainable and effective alternatives to traditional fishmeal and fish oil-based feeds due to their economic and ecological drawbacks. Red yeast (Sporidiobolus pararoseus) has demonstrated potential as a functional feed additive due to its bioactive compounds, which can boost growth and immunity in fish. Here, this study investigated the effects of varying concentrations of red yeast (RY) in the diet of koi carp (Cyprinus carpio var. koi) cultured in biofloc systems. Three hundred fish (27.62 ± 0.08 g) was randomly distributed into 15 fiberglass tanks (150 L each) across five dietary treatments: 0 g/kg (control), 5 g/kg (RY-1), 10 g/kg (RY-2), 20 g/kg (RY-3), and 40 g/kg (RY-4). Each treatment was replicated 3 times, and the experiment lasted for 8 weeks. After 8 weeks of feeding trial, fish fed the 40 g/kg RY diet showed significantly higher weight gain (WG), specific growth rates (SGR), and final weight (FW). Additionally, the feed conversion ratio (FCR) was significantly lower in the 40 g/kg RY diet group (p < 0.05). There was a strong positive correlation between RY concentration levels and WG, FW, and SGR (r = 0.695, 0.699, and 0.694, respectively), while a negative correlation was observed for FCR (r =—0.505). No significant differences were observed in survival rate (SR) between the RY-supplemented groups and the control group (p > 0.05). However, skin mucus and serum lysozyme activities increased significantly in fish fed the 40 g/kg RY diet (p < 0.05), while peroxidase activity remained unchanged compared to the control group (p > 0.05). Additionally, mRNA transcript levels of both pro-inflammatory (il-1β, il-8, and tnf-α) (approx. 1.76- to 1.87-fold) and anti-inflammatory (il-10 and tgf-β) (approx. 1.35- to 2.86-fold) cytokines in the spleen and intestine were upregulated with higher dietary RY levels compared to the control group (0 g/kg). These findings suggest that dietary inclusion of RY at 40 g/kg could be beneficial as a functional feed additive for koi carp.