| Recirculating aquaculture system(RAS)is a highly efficient farming model that enables intensive farming of small water bodies at high density.Compared with the traditional aquaculture model,it has the advantages of saving a large amount of land and water resources,recycling of aquaculture wastewater and reducing sewage discharge.In recent years,the aquaculture industry has gradually moved from crude farming to intensive farming.Under the intensive farming model,the farming density increases and the impact of environmental stress on the farmed objects intensifies.Water flow is an important abiotic factor in the aquaculture environment,driving water exchange and serving to purify the water body.However,high water flow velocities can cause stress to fish,reducing their feeding rate,reducing growth performance and disrupting normal physiological metabolism.It is therefore necessary to study the range of water flow velocities tolerated by fish species and to regulate the water flow velocity in culture according to their tolerated flow velocities in order to achieve the desired benefits.Plectropomus leopardus is a high quality,nutritious and economic fish that is farmed in Hainan,Guangdong and Fujian,and has become a major species in grouper culture.In this study,we studied the short-term tolerance and long-term adaptation of coral trout under water flow velocity stress in RAS.According to the changes of behavior,blood biochemistry,growth performance,digestive ability,antioxidant level and immune ability,with a view to providing a theoretical basis for coral trout in RAS.The experimental results of this paper are as follows:1.The tail wagging frequency and blood biochemistry were significantly affected by water flow velocity stress.There was a linear positive correlation between tail wagging frequency and water flow velocity,and the tail wagging frequency in 2 bl/s water flow velocity group and 3 bl/s water flow velocity group was significantly higher than that in control group(P<0.05).Blood glucose,lactic acid,and cortisol concentrations increase with water flow velocity.The cortisol concentration in 2 bl/s water flow velocity group was significantly increased compared with the control group(P<0.05),and the blood glucose,lactic acid and cortisol levels in 3 bl/s water flow velocity group were significantly higher than those in the control group(P<0.05).The water flow velocity of more than 2 bl/s had significant effects on the behavior and blood biochemistry of the experimental fish,increased the extra energy consumption,accelerated the fatigue of the fish,and intensified the stress level.2.The growth performance and digestive capacity of coral trout have a significant impact under long-term current tolerance.At 2.5 bl/s water flow velocity,the weight gain rate and specific growth rate of coral trout were significantly lower than those of the control group(P<0.05),the feed coefficient was significantly higher than that of the control group(P<0.05),and the growth performance was significantly decreased.The digestive enzyme activity in the intestine of coral trout is closely related to the flow velocity,and the lipase activity has no significant difference between the flow velocity groups(P>0.05).The 2 bl/s and 2.5 bl/s water flow velocity groups are significantly lower than the control group and 1 bl/s water flow velocity group(P<0.05),and the high water flow velocity has an inhibitory effect on the digestive enzyme activity.According to the intestinal section,the villus length of the midgut and hindgut of the experimental fish in the 1 bl/s water flow rate group was the longest,and the muscularis thickness was significantly higher than that of the control group(P<0.05).The high flow velocity has affected the intestinal structure of coral trout,and the number of goblet cells in the whole intestine of the 2 bl/s and 2.5 bl/s water flow velocity groups has decreased to a certain extent,accompanied by problems such as goblet cell fragmentation,separation of columnar epithelial cells from the mucosal layer,which has affected the digestion and absorption ability of the intestine.3.The blood biochemistry,antioxidant level and immune ability of coral trout have a significant impact on long-term resistance to flow.The blood glucose concentration in the 2 bl/s and 2.5 bl/s water flow velocity group were significantly lower than that in the control group(P<0.05).The blood glucose concentration was lower than the normal level,consuming additional energy storage materials for energy supply,which was not conducive to long-term growth.A certain water flow velocity can increase the activity of immune enzymes,and the activity of immunoglobulin G at 1 bl/s water flow velocity was significantly higher than that of the control(P<0.05),and the activity of immunoglobulin G at 2 bl/s flow velocity also had a certain degree of increase.However,when the water flow velocity reached 2.5 bl/s,the activity of immunoglobulin G and the concentration of lysozyme were significantly lower than those in the 1 bl/s water flow velocity group(P<0.05),which was not significantly different from those in the control group.At this time,the immune ability of the body decreased.The total antioxidant capacity,superoxide dismutase activity and glutathione concentration of the liver in the 2.5 bl/s water flow velocity group were significantly lower than those in the control group(P<0.05).There was no significant difference in the concentration of malondialdehyde and catalase activity among all groups(P>0.05).It showed that the low water flow velocity had no significant effect on the antioxidant capacity of the fish(P>0.05),and the stress caused by the high flow velocity(2.5 bl/s)inhibited the antioxidant capacity of the coral trout.In conclusion,the water flow velocity has certain effects on the behavior,growth performance,blood biochemistry,blood immunity,intestinal digestive ability and liver antioxidant capacity of recirculated coral trout.In the short term,stress over 2 bl/s water flow velocity significantly increased tail wagging frequency,blood glucose concentration and cortisol levels,and increased the stress level of fish body.Long-term culture,2.5 bl/s water flow velocity will reduce the growth performance,reduce the body’s immunity,reduce intestinal digestive ability,and reduce the antioxidant capacity of liver.Similarly,2 bl/s water flow velocity can also affect the physiological homeostasis of blood and reduce the digestive ability of intestinal tract.In an open culture environment,the coral trout was able to withstand 2 bl/s water flow velocity shock for a short period of time.However,from the perspective of recirculating aquaculture production and pollution discharge,it is recommended to control the water flow velocity in the aquaculture tank within 1 bl/s. |
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