Effect of biofloc system on growth indices, body composition and digestive enzymes of juvenile common carp (Cyprinus carpio) in different stocking rates

In the present study, growth indices, body composition and digestive enzymes of juvenile common carp (Cyprinus carpio) were investigated in biofloc system with different stocking rates. So, 825 juvenile fish (17±1.2g) were randomly transferred to 15 fiberglass 200 liter tanks in lab of Khorramshahr University of Marine Science and Technology. Twenty five fish were stocked in a tank (2.12 kg/m³) with water exchange (control), and also 25 (2.12 kg/m³, D25), 50 (4.24 kg/m³, D50), 75 (6.37 kg/m³, D75) and 100 fish (8.5 kg/m³, D100) were stocked in tanks as a biofloc treatments. The photoperiod included 12 h light and 12 h darkness and the duration of the experiment was 40 days. Control and other treatments treatment were fed with a commercial diet containing 35% and 25% protein, respectively for 40 days. The fish were fed manually, three times a day (8:30, 12:00 and 15:30) based on 3% of the body weight. In the control treatment, the water was exchanged 25% every two days. Dissolve oxygen, temperature, pH and salinity parameters were not significantly different in biofloc system compared to the control (P>0.05). Biofloc volume (BF) and total suspended solids (TSS) values were higher in biofloc treatments than the control (P<0.05).  Total ammonia and nitrite levels were lower in biofloc treatments than the control (P<0.05). The nitrate levels were higher in D100 treatment than other biofloc treatments (P<0.05). Final weight, percentage of weight gain, specific growth rate and feed conversion ratio were not significantly different in D25 and D50 treatments compared to the control (P<0.05). The protein efficiency was significantly increased in B25 and D50 treatments compared to the control (P<0.05). The lowest protein content was observed in D100 treatment compared to the other treatments (P<0.05). The lipase, amylase and alkaline phosphatase activities did not show significant difference in biofloc treatments compared to the control (P<0.05). The results of this study showed that biofloc system decreased the consumption of dietary protein while reducing the pollution load of nitrite and total ammonia.