| Atlantic salmon（Salmo salar）is an important factory cultural species.Basing on different cultural species,optimal cultural conditions are the key points to determine the outcome of the factory culture.Eventhough the cultural time of Atlantic salmon has been several decades,however the optimal cultural conditions for its factory cultural model remains enigmatic.In this study,by setting different levels of dissolved oxygen and density,the physiological response,immune response and growth performance of Atlantic salmon to different densities and dissolved oxygen in the water environment were discussed,and the relationship between dissolved oxygen in the water environment were analyzed.The results revealed that the hypothalamic-pituitary-thyroid-liver axis participates in the state of Atlantic salmon density stress and the role of growth immunity and stress and the mechanism of density restriction.The appropriate density and dissolved oxygen level for the growth of Atlantic salmon in the recircling aquaculture system（RAS）were evaluated and screened.The main intestinal microflora of the Atlantic salmon farmed by RAS were discussed and analyzed,and the bacterial flora that induced Atlantic salmon to produce a series of immune responses was established.The appropriate density and dissolved oxygen level provide basic information and basis for deeply understanding of the interaction mechanism between fish and the environment under intensive aquaculture,and for the standardization of fish breeding model in recirculating water.The results and innovations of this study are as follows:1.The effects of rearing density on the survival,growth and feeding of Atlantic salmonUnder the same dissolved oxygen（pure oxygen increasing oxygen）and different breeding densities were lowdensity group（6 kg/m3 and 9 kg/m3）,medium density group（15 kg/m3）and high density group（21 kg/m3and 24 kg/m3）,during 220 days period of culture,the final mortality rate of each group was 9.20%,9.16%,17.78%,22.68%,30.15%,the breeding density increased,and the mortality rate also increased.Atlantic salmon with an initial breeding size of 1 kg-1.5 kg and a breeding density of15 kg/m3-21.5 kg/m3 were in the rapid growth period,with the highest weight gain rate and specific growth rate.When the breeding density reached about 30 kg/m3,the specific growth rate began to decrease and the mortality rate increased.The analysis of the correlation between the specific growth rate（SGR）and the breeding density and body weight showed that the SGR and the density were y=-5E-05x3+0.0023x2-0.027x+0.7616（R2=0.6707）.The breeding density reached 25 kg/m3,SGR begin to decline;weight and density showed a correlation between y=-0.1824x2+0.7713x-0.0545（R2=0.7971）.2.Physiological and immune responses of Atlantic salmon under different cultural densitiesIn the analysis of the impact of breeding density on Atlantic salmon stress and immune related indicators,we found that when the rearing density reach 29.81kg/m3-33.81 kg/m3,the crowding stress of the breeding environment caused changes in related physiological indicators,and blood growth hormone（GH）and thyroxine（T3）both decreased with the increase in density.The cortisol content of the high-density group was significantly higher than that of the medium and low-density groups on 40 days.Until 130d,the differentiation between the groups was reduced.The blood aspartate aminotransferase（GOT）activity of each treatment group increased with the time of the experiment and the breeding density.The high-density group（21 kg/m3 and 24 kg/m3）blood GOT activity increased significantly,which revealed the physiological response and immune response law of Atlantic salmon under the action of density stress.3.The effects of dissolved oxygen on the survival,feeding and growth of Atlantic salmonUnder the conditions of RAS,the dissolved oxygen stress experiments were carried out by setting up 5 different gradient dissolved oxygen saturation groups:60%,80%,100%,140%and 200%.Through a 60-day breeding experiment,the results of the experiment on the effect of dissolved oxygen on the growth of Atlantic salmon show that:60%dissolved oxygen saturation has the highest mortality rate,followed by 200%dissolved oxygen saturation.The lowest dissolved oxygen saturation group（60%）and the highest group（200%）had significantly lowered weight gain rates,specific growth rates,feeding rates and feed coefficients than the moderately dissolved oxygen groups.The feed coefficients of 60%,80%,100%,140%and 200%dissolved oxygen saturation groups were 1.87,1.04,1.20,1.28 and 1.41,respectively,and the specific growth rates was 0.22,0.45,0.58,0.3 and 0.28,respectively,which showed both too low and too high dissolved oxygen would inhibit growth of Atlantic salmon.Experiments have shown that extremely high and extremely low dissolved oxygen content had an adverse effect on the nutritional status of Atlantic salmon.Atlantic salmon has a better growth performance when the dissolved oxygen saturation was 80%-100%.4.The effects of dissolved oxygen on the blood physiology,stress and antioxidant system of Atlantic SalmonBy analyzing the influence of dissolved oxygen on the stress and antioxidant indexes of Atlantic salmon,it was found that the blood glucose level decreased with the increase of dissolved oxygen level,and the blood glucose level of the high dissolved oxygen group was significantly lower than that of the low dissolved oxygen group（P<0.05）;The hemoglobin content decreased with the increase of dissolved oxygen,and the high dissolved oxygen group was significantly higher than the low dissolved oxygen group.It shows that Atlantic salmon have better adaptability to different dissolved oxygen environments.This might be due to the fact that fish increase the metabolic rate of substances in response to the stimulation of this dissolved oxygen environmental factor.The activities of superoxide dismutase（SOD）and catalase（CAT）in the blood of Atlantic salmon increased with the increase of dissolved oxygen levels,but with the passage of time,the differences between the various dissolved oxygen treatment groups became smaller and smaller.Glutathione peroxidase（GSH-Px）did not respond to different dissolved oxygen at the beginning of the experiment,but with the passage of time.The activity of GSH-Px in the high dissolved oxygen group increased,and then gradually decreased after reaching a peak at 15 days.Then it reached a peak at the end of the experiment,which shows that Atlantic salmon is constantly adjusting its own endocrine levels,responding to unfavorable external environments,and adjusting its homeostasis to eliminate the impact of unfavorable external conditions on itself.After dissolved oxygen stress,the GOT and GPT contents of the low dissolved oxygen group increased and lasted until the 7th day,and then returned to normal levels.This might be due to a certain degree of damage to Atlantic salmon’s liver caused by low dissolved oxygen,but the damage was not significant.5.The effects of dissolved oxygen on the intestinal bacterial community and immunity of Atlantic SalmonIn this experiment,we studied the composition and immune characteristics of the intestinal bacterial community of Atlantic salmon in the recirculating aquaculture system.The experiment set 60%saturation group（L）,100%saturation group（control group,CK）,and 200%saturation group（H）.The results showed that the intestinal bacterial communities of all groups consisted of three bacterial phyla,namely Proteobacteria,Firmicutes and Bacteroides.Among them,Acinetobacter baumannii significantly increased in the L group.Under different dissolved oxygen saturation levels,the classification and distribution of the intestinal bacterial community of Atlantic salmon were very different.We further monitored the immune characteristics of the fish under different levels of dissolved oxygen.The results showed that the number of white blood cells,the level of cortisol,the expression of IL-1β,TLR4 and NOD2 in the L group were significantly higher than the other two groups.Since TLR4 and NOD2 are usually associated with bacterial infections,it is reasonable to believe that the stronger immune response observed in the L group may be related to the higher abundance of Acinetobacter baumannii in the intestines of Atlantic salmon. |
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