Effects Of Dissolved Oxygen On Growth, Respiratory And Intestinal Flora Of Juvenile Grass Carp In Industrial Recirculating Aquaculture System

Fishes exposed to different dissolved oxygen levels for long time have different susceptibility and adaptation strategies.In this study,healthy grass carps with an average weight of（214.6±18.7）g were selected for long-term culture at recirculating system in 3mg/L、5 mg/L and control group 7 mg/L[（2.57±0.33）mg/L、（4.61±0.11）mg/L and（6.50±0.48）mg/L,expressed as DO2.57,DO4.61 and DO6.50 in the following.]dissolved oxygen levels.Blood,gills,muscle,and liver were sampled after 60 d.Hematoxylin eosin staining,fluorescent quantitative PCR,enzyme activity assay,enzyme-linked immunosorbent assay and 16s r RNA high-throughput sequencing were used to study juvenile grass carp cultured at different dissolved oxygen levels and we found that:1.There were certain differences in the blood transport capacity of juvenile grass carp under different dissolved oxygen environments,and they have different respiratory strategies.At different dissolved oxygen levels,the blood transport ability of juvenile grass carp was different.Compared with the control group DO6.50,the number of red blood cells（RBC）and hemoglobin content（HGB）in the blood of juvenile grass carp in DO2.57 significantly increased（P<0.05）.The physiological adaptation of juvenile grass carp in the DO4.61 group was consistent with that in the DO2.57 group,but the degree was reduced.When the concentration of dissolved oxygen in the environment decreases,the distance between the carbon dioxide dissociation curve（CDDC）of venous blood and mixed arterial blood of juvenile grass carp becomes farther and farther,and the difference in CO₂content between venous blood and mixed arterial blood becomes larger and larger,which is caused by the difference in anaerobic respiration level.Compared with the control group DO6.50,the respiratory rate of juvenile grass carp increased,the gill segments became thinner and longer,and the mucus attached to them decreased,with a decrease of the interstitial cell clusters significantly in DO2.57.The activity of lactate dehydrogenase（LDH）and lactate content（LA）in the liver increased,while the activity of pyruvate dehydrogenase（PDH）decreased,accompanied by a significant decrease in oxygen consumption rate（P<0.05）;In DO4.61 the LDH activity,and the relative expression level of LDH-A4 increased,while the oxygen consumption rate slightly decreased.2.Different levels of dissolved oxygen affected the physiological status,growth performance,and muscle growth of grass carp juveniles.Compared with the control group DO6.50,the levels of triglycerides（TG）and alkaline phosphatase（ALP）in the serum of grass carp at DO2.57 were significantly reduced（P<0.05）;There was no significant difference in serum biochemical indexes of juvenile grass carp in DO4.61group,including cholesterol（TCHO）,glucose（GLU）,total protein（TP）,alanine transaminase（ALT）and aspartate transaminase（AST）（P>0.05）.In addition,there was no significant difference in ACTH（adrenocorticotropic hormone）and cortisol secretion between the two treatment groups and the control group（P>0.05）,while T3 was significantly reduced in the DO2.57 group（P<0.05）and FT3 secretion was significantly increased at DO4.61（P<0.05）.The above results indicate that DO2.57 treatment bring a certain degree of environmental stress to grass carp juveniles.In this experiment,the weight gain rate（WGR）and specific growth rate（SGR）of grass carp at DO2.57significantly decreased（P<0.05）,while the feed coefficient（FCR）significantly increased（P<0.05）.The difference in WG and SGR between the control group DO6.50and the DO4.61 was not significant（P>0.05）.In addition,compared with the control group,the gh gene in the brain,ghr gene in the liver,and igf1,igf1r,igf2 and igfbp genes in the muscle at DO2.57 were significantly down-regulated（P<0.05）.There was no significant difference in the relative expression levels of various genes in the brain tissue and muscles of the DO4.61,while the igf1 and igf2 genes were significantly up-regulated in liver tissue（P<0.05）.3.Different dissolved oxygen levels affect the composition of gut microbiota in juvenile grass carp.The unique number of ASVs in the DO2.57 group was only 4127,lower than DO4.61 and DO6.50（9565 and 9293）;The Chao1 index value and Observed specifications index value significantly decreased,mainly reflected in the composition and abundance of Brevinema andersonii compared to the other two groups.In addition,the similarity of gut microbiota composition between DO4.61 and control group DO6.50was high in terms of diversity,richness,and evenness.In summary,grass carp may experience environmental stress when exposed to different levels of dissolved oxygen for a long time,and exhibit different adaptation strategies for different dissolved oxygen environments,ultimately resulting in differences in growth.These results indicate that unsuitable dissolved oxygen environments in industrial recirculating aquaculture systems can affect the healthy growth of grass carp.