Interactive effects of culture systems (biofloc and clear water) and dietary protein levels on growth, digestive activity, mucosal immune responses, antioxidant status, and resistance against salinity stress in the Caspian roach (Rutilus caspicus) fry

This experiment aimed to evaluate effects of culture system and dietary protein level on performance of the Caspian roach fry. Fish (0.99 ± 0.06 g) were stocked into 12 tanks (water volume, 50 L) at a stocking density of 100 fish per tank. The experiment was designed with two culture systems (biofloc, BFT and clear water, CW) each with 38 and 28% dietary protein levels following a 2 × 2 factorial design. After 60 days, fifty fish were randomly captured from each tank and exposed to salinity challenge (13 g/L) for 72 h. On days 45 and 60, total ammonia nitrogen (TAN) values were significantly higher in CW treatments, then BFT38 treatment, and lower in BFT28 treatment. The final weight, weight gain, and SGR in BFT and CW38 treatments were significantly higher and FCR was lower than the CW28 treatment. The protein efficiency ratio was highest in the BFT28 treatment. Also, amylase, lipase, and protease activities in fish cultured in BFT system were significantly higher than the CW system. After salinity stress, skin mucus cortisol values were significantly lower and survival rate was higher in BFT groups compared to the CW treatments. After 72 h stress, the mucus glucose values in BFT and CW38 treatments were significantly lower than the CW28 treatment. Before stress, the mucus alkaline phosphatase (ALP) activity was lowest in the CW28 treatment. Skin mucus protease activity was significantly higher in BFT groups at before and all sampling times after salinity challenge. The skin mucus lysozyme activity and total protein value in all treatments increased at 24 h post stress, and then only returned to the pre-stress values at 48 h for the BFT38 treatment. After salinity stress, lysozyme activity in BFT and CW38 treatments was significantly lower than the CW28 treatment. After 48 h stress, the highest mucus ACH50 activity was observed in the BFT38 treatment, but it was not significantly different from the BFT28 and CW38 treatments. After salinity stress, muscle catalase and SOD activities were significantly higher and MDA content was lower in BFT treatments than the CW treatments, suggesting that biofloc can enhance the antioxidant defense in fish. Overall, biofloc system and high dietary protein level beneficially affect growth, skin mucus immune responses, and resistance against salinity challenge in fish. Also, microbial proteins contained in bioflocs as a supplemental protein source can compensate 10% reduction of protein in the Caspian roach fry diet. •Biof