Effect Of Flow Velocity On Largemouth Bass(Micropterus Salmoides) Juveniles In Recirculating Aquaculture Systems

The recirculating aquaculture systems(RAS)has become a growing trend due to its advantages of high-yield,water-saving,environmental controllable and product quality safety.The flow velocity in the RAS has great influence on the growth of fish and water quality,and the appropriate flow velocity is also the necessary conditions for ensuring the welfare of fish.In this study,based on largemouth bass,the effect of the flow velocity on the growth,water quality and ammonia excretion regularity were explored in recirculating aquaculture systems,from the perspective of real production.The main research content and results are as follows:1.The actual aquaculture flow velocities were used as the basis for setting the flow velocities for the experimental groups.Based on stable water quality,juvenile largemouth bass with a body mass of 1.92±0.38 g and body length of 4.5±0.36 cm were cultured at three different flow velocities of high flow(HF,18 cm/s),middle flow(MF,11 cm/s,actual average flow velocity of aquaculture)and low flow(LF,4 cm/s).The results showed that different flow velocities had significant effects on the growth,digestive enzyme activities,and antioxidant 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).Although flow velocity had no significant effect on feed conversion rate(P>0.05),feed conversion rate of the 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 differences were observed between groups(P>0.05),its activity increased as the flow velocity increased.With regarding to the antioxidant and immune capacity,the MDA content in the HF group significantly decreased(P<0.05),and the Ig M content significantly increased(P<0.05).Thus,under the high flow velocity condition,fish growth,oxidative defense,immune capacity,and digestive capacity could be improved to a large extent.2.On the basis of no water change and sewage suction,the largemouth bass(8.13±0.54 g)were cultured under HF(18 cm/s),MF(11 cm/s)and LF(4 cm/s).During the experiment,the water quality was treated only with the self-cleaning capacity of the recirculating aquaculture systems to explore the water quality changes at different flow velocities.The results showed that under high flow velocity,the fluctuation range of the ammonia nitrogen significantly reduced(P<0.05),although the fluctuation range of nitrite nitrogen content didn’t significantly reduce(P>0.05),but fluctuation in nitrite nitrogen content decreased as the flow velocity increased.Therefore,increasing the flow velocity can reduced the fluctuation range of ammonia nitrogen and nitrite nitrogen content between 12 h after feeding.It can prevent ammonia nitrogen and nitrite nitrogen content from being too high to hinder the growth of fish.In terms of particulate matter,increasing the flow velocity can cause more large particles in water,which is advantageous to allow these large particles to flow through the filter to purify the water quality.The study found that the ammonia nitrogen content of low flow velocity presented an upward trend at 7-12 days,which may be more large particles at the bottom of tanks under low flow speed,releasing more ammonia nitrogen.Therefore,on the basis of saving energy consumption,increasing the flow velocity at the ammonia excretion peak of fish can decompose ammonia nitrogen and remove the particulate matter rapidly.3.The largemouth bass(6.25±0.68 g)were cultured under HF(18 cm/s),MF(11cm/s)and LF(4 cm/s)and the ammonia released were measured in 12 h after feeding.The results showed that under the high flow velocity(18 cm/s),the ammonia releasing peak of largemouth bass were 4-8 hours after feeding.And the ammonia releasing peak under the medium flow velocity(11 cm/s)and low flow velocity(4 cm/s)were 4-9 hours after feeding.The flow velocity had no significant effect on the ammonia excretion,but the ammonia excretion under the low flow velocity was minimal.Therefore,changing the flow velocity between 4-8 hours after feeding can get better water quality.