Biofloc technology improves growth, innate immune responses, oxidative status, and resistance to acute stress in common carp (Cyprinus carpio) under high stocking density

This study aimed to evaluate the effects of biofloc technology (BFT) on the performance of common carp under high stocking density. Fish (8.41 ± 0.44 g) were stocked into 12 tanks (70 L) in a 2 × 2 factorial design consisting of clear water (CW) and BFT systems at two stocking densities. Fish were divided into four groups according to culture system and stocking density: CW6 (clear water, 6 kg m^-3), CW12 (clear water, 12 kg m^-3), BFT6 (biofloc, 6 kg m^-3), and BFT12 (biofloc, 12 kg m^-3). After 49 days, the fish were exposed to acute crowding stress (40 kg m^-3) for 7 days to evaluate the fish performance to additional stressors. Total ammonia nitrogen (TAN) concentration was significantly lower in BFT groups at the end of the trial. The growth of fish was significantly higher and the feed conversion ratio was lower in BFT groups. The final density of fish was highest in BFT12 group (25.41 ± 0.48 kg m^-3). The amylase and protease activities in BFT groups were significantly higher than the CW12 group. The highest serum catalase, superoxide dismutase, and glutathione peroxidase activities, and malondialdehyde content (3.07 ± 0.16 nmol mg^-1 protein) were observed in CW12 group. Serum LDL, cholesterol, triglyceride, total protein, and albumin levels were significantly higher in CW6 and BFT6 groups, followed by BFT12 group, and lowest in CW12 group. Serum globulin level was lowest in CW12 group (0.68 ± 0.06 g dL^-1). After 49 days, serum lysozyme, alternative complement (ACH₅₀) and ALP activities were significantly higher and serum cortisol and glucose levels and ALT and AST activities were lower in BFT groups than the CW12 group. After 7 days of acute stress, serum lysozyme activities were significantly lower and ACH₅₀ and ALP activities were higher in BFT groups than the CW12 group. Also, fish in CW12 group were more sensitive to the acute stressor, showing higher cortisol and lower glucose levels. Overall, biofloc had the anti-stress effect, and the culture of common carp in the biofloc system at a stocking density of 12 kg m^-3 could be recommended. BFT can also exert beneficial effects on the welfare of fish subjected to acute stress.