Effects of restricted feeding levels and stocking densities on water quality, growth performance, body composition and mucosal innate immunity of Nile tilapia (Oreochromis niloticus) fry in a biofloc system

In aquaculture feed accounts for more than 50% of current costs. Biofloc has the potential to compensate for food reduction, so restricted feeding levels should be investigated to decrease food consumption. The present study conducted to evaluate the effect of restricted feeding levels and stocking densities on water quality, growth performance, body composition and mucosal innate immunity of Nile tilapia (Oreochromis niloticus) fry in biofloc system (BFT). A 5*2 factorial experimental design was used with five restricted feeding levels (0, 15, 30, 45 and 100) and 2 stocking densities(500 fish/m³ and 1000fish/m³) comprising 10 treatments for BFT and for each density there were clear water (CW) system as control with three replicates. Fry with initial weight of 3.2 ± 0.05 g were stocked in tank(40 L), fed three times a day at 8:00, 12:00 and 16:00. Molasses was added to BFT treatments as organic carbon source at a C/N ratio of 15. Results showed that, growth indices (WG, DGR, SGR, FCR), water quality parameters (TAN, NO₂, NO₃, settled solids and total suspended solids), as well as innate immunity (Total immunoglobulin, Lysozyme and Catalase activity) were improved in biofloc system compared to the CW system. The reduction of feeding rate resulted in significantly decreasing growth performances and immunity among biofloc groups (p < 0.05) except for the group with 15% feed reduction in stocking density of 500/m³. Stocking density had significantly (p < 0.05) reverse effect on growth indices except for the biofloc treatment 1000/m³ without feed reduction. Proximate analysis revealed that by reduction of feeding rate, protein, ash and moisture contents increased, whereas lipid contents decreased significantly (p < 0.05). Survival rate was high (94.8–100%), and no significant differences were seen among treatments (p > 0.05) except for the non-feeding biofloc treatment with stocking density of 1000/m³ (67/9%). Innate immunity was significantly decreased by reduction of feeding rates while, Liver enzyme activity showed an increasing trend by reduction in feeding rate and increasing the stocking density (p > 0.05). These results indicate that the BFT improved water quality, growth performance and immune function of Nile tilapia fry compared to CW system. Also it is possible to decrease 15% daily feeding rate in BFT system. In conclusion, the study suggests the stocking density of 1000/m³ for larviculture of tilapia in BFT to use water and equipment more efficiently.