Effects of flow velocity on growth and physiology of juvenile largemouth bass (Micropterus salmoides) in recirculating aquaculture systems

The cultivation of largemouth bass (Micropterus salmoides) in recirculating aquaculture systems (RAS) has become a growing trend. To ensure the welfare of juvenile largemouth bass in RAS, the effects of different flow velocities on their growth and physiology were explored, considering the flow velocity in real production. Following the stable water quality, juvenile largemouth bass with a body mass of 1.92 +/- 0.38 g and body length of 4.50 +/- 0.36 cm were reared at three different flow velocities of high flow (HF, 18 cm/s, 4.00 bl/s), middle flow (11 cm/s, 2.45 bl/s, actual average flow velocity of aquaculture) and low flow (4 cm/s, 0.90 bl/s) respectively. The whole experiment lasted 66 days. Results showed that different flow velocities had significant effects on the growth, digestive enzyme activities, antioxidant capabilities and immune capabilities of juvenile largemouth bass. HF can significantly increase the cumulative food intake, hepatosomatic index, weight gain rate, specific growth rate and condition factor of largemouth bass juveniles (p < 0.05). Flow velocity had no significant effect on feed conversion rate (p > 0.05), but the mean of feed conversion rate in HF group was the lowest. Under the HF condition, the activities of trypsin and amylase increased significantly (p < 0.05). For lipase activity, although no significant differences were observed between groups (p > 0.05), the mean of lipase activity increased as the flow velocity increased. With regard to the antioxidant and immune capacity, the malondialdehyde content in HF group significantly decreased (p < 0.05), and the immunoglobulin M content significantly increased (p < 0.05). From the above, following the HF velocity condition, fish growth, oxidative defence, immune capacity and digestive capacity could be improved to a large extent. To achieve higher growth, it is recommended circulating flow velocity of no less than 18 cm/s in largemouth bass at the juvenile stage (approximately 1.9-37.5 g and 4.5-14.5 cm long).