Abstract
A 56-day feeding trial was implemented to evaluate the effects of promoted biofloc on the water quality, growth performance, digestive enzymes activity, immune-related genes and antioxidant status of golden crucian carp in negligible exchange culture tanks. Four handred fifty healthy golden crucian carp juveniles (5.03 ± 0.22 g) were randomly assigned among 15 tanks. Five C:N ratios in triplicate tanks were tried: C/N = 5.48:1 with a commercial diet (Control), C/N = 10:1, C/N = 15:1, C/N = 20:1 and C/N = 25:1. Anhydrous glucose (99.97 %) was added to the commercial diet to maintain elevated carbon ratio in the three treatments. Uninterrupted 24 -h aeration was supplied during the test by using an air-stone connected to an air pump to develop the biofloc in the water column. The results demonstrated that BFT significantly reduced ammonia nitrogen (TAN), nitrite (NO2−-N), total phosphorus (TP) concentration and water transparency (Secchi depth) in C/N 15–25 than control (p < 0.05), wheras the chemical oxygen demand (COD) and nitrate nitrogen (NO3−-N) concentration gradually increased over time. Apart from, weight gain (WG), specific growth rate (SGR), and protein efficiency ratio (PER) were considerately elevated in C/N 20 and C/N 25 than control (p < 0.05).This was coincidental with the higher activities for protease, lipase and amylase in liver. Significant increases in lysozyme (LSZ), acid phosphatase (ACP) and alkaline phosphatase (AKP) activities of golden crucian carp were found in the C/N 20 group than control (p < 0.05), and regulating the expression of immune-related genes including IL-1β, TNF-α and HSP70 after a 56-day feeding trial. Comparing the antioxidant capacity of golden crucian carp from the five experimental groups, the activity of superoxide dismutase (SOD), total antioxidant capacity (T-AOC), catalase (CAT) activity and glutathione peroxidase (GSH-PX) activity of juveniles in the C/N 20 and C/N 25 groups were significant higher (p < 0.05), and the content of malondialdehyde (MDA) was much lower (p < 0.05) than in the control under this experiment condition. Overall, this findings suggest that BFT not only can enhance golden crucian carp growth performance, elevate digestive enzymes activity, and strengthen antioxidant status but also upgrade immune response.
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