Use of different carbon sources for the biofloc system during the grow-out culture of common carp (Cyprinus carpio L.) fingerlings

A 10weeks feeding trial was carried out to evaluate the effects of different carbon sources (sugar beet molasses: SBM+BFT, sugar: S+BFT, corn starch: CS+BFT) on water quality, growth performance, digestive enzymes activity, biofloc and fish microbial community and composition in biofloc based system for common carp fingerlings. Three hundred healthy fingerlings (22.5±0.2g) were randomly assigned to 12 rectangle tanks (70L) at a density of 8.02kgm−3. Fish in BFT treatments were fed at 75% of the daily feeding rate (DFI) of control. Results showed a significant difference in water quality parameters among different culture systems; CS+BFT had the lowest amount of total ammonia nitrogen (TAN) at the end of culture period. No difference was observed between the BFT treatments in case of nitrite-N and nitrate N after 10weeks and total suspended solids (TSS) also experienced a decline during the experiment. The fish yield was the highest in CS+BFT, which also had the lowest significant feed conversion ratio (P<0.05). This was coincidental with the highest activity for total protease, lipase and amylase in BFT treatments when compared with the control (P<0.05). A significantly higher body protein and lipid content and lower carbohydrate was seen in CS+BFT treatment (P<0.05). Different carbon sources did not affect on protein and carbohydrate content of flocs but CS+BFT significantly caused an increase in lipid and ash content in microbial flocs (P<0.05).The fingerlings essential fatty acids affected by different carbon sources and the lowest values were found in BFT treatments which differed significantly with control (P<0.05). Microbial community was analyzed by PCR-DGGE as well as conventional method and results indicated the changes in both bacterial intensity and diversity among the different culture systems. Among the different carbon sources used, corn starch has increased the content of total culturable bacteria and lactobacillus spp. in tanks and fish hindgut. Overall, this study suggests that microbial flocs formed in corn starch based biofloc can improve common carp growth performance and tanks water quality under zero water exchange and hence ensures sustainability.