Investigation Of Ecological Environment In Oyster Aquaculture Areas And The Factors Affecting The Growth Of Cultchless Triploid Oysters

In this study,a one-year water quality and ecological monitoring were conducted in the Crassostrea gigas aquaculture areas to determine the key factors affecting the ecological environment of the oyster aquaculture areas.On this basis,we explored the suitable ecological aquaculture mode and aquaculture density of cultchless triploid Crassostrea gigas,compared and analyzed the growth of cultchless triploid Crassostrea gigas,conjoined oyster and diploid oyster.In order to construct the aquaculture technology of cultchless triploid Crassostrea gigas based on ecosystem level.(1)The physical and chemical factor phase,microbial phase and algae phase of Crassostrea gigas aquaculture areas were studied to explore the change rule and relationship of each index,to provide reference for scientific cultivation of Crassostrea gigas.From September 2019 to August 2020,samples were collected from 9 stations in Shandong Yangma Island,Zhifu Bay and Rushan aquaculture areas and we detected the changes of physical and chemical factors,bacteria and phytoplankton.The nutrient limitation of aquaculture areas was judged by N/P,the nutritional status and pollution level of seawater were evaluated by nutritional status index and Nemerow environmental quality comprehensive index.Phytoplankton evaluation index were used to evaluate the phytoplankton community structure,and the correlation analysis was conducted by SPSS24.0.The results showed that:the water temperature,pH,salinity,dissolved oxygen and COD of the three aquaculture areas met the second class standard of mariaquaculture;the inorganic nitrogen of the three aquaculture areas ranged from 0.0079 mg/L to 0.8739 mg/L,with an average of 0.1594 mg/L,and the inorganic nitrogen content of the three aquaculture areas exceeded the second class standard of mariaquaculture in December2019 and January 2020,the total phosphorus content of the three aquaculture areas was on the high side,while from November 2019 to January 2020 in Yangma island and Zhifu Bay aquaculture areas,and in September 2019 and December to January 2020 in Rushan aquaculture areas,the active phosphorus content exceeded the second class standard for mariaquaculture.The average N/P of Yangma Island,Zhifu Bay and Rushan were 53.72,26.96 and 29.40,respectively.Yangma island and Rushan were in a state of phosphorus limitation for a long time.Evaluation of nutritional status index showed that the three aquaculture areas were in the state of oligotrophic from March to August 2020;The evaluation of Nemeiro’s comprehensive environmental quality index showed that the seawater quality of individual stations was general and was slightly polluted,and the seawater quality of other aquaculture areas was better and in a relatively clean level.The bacterial content was low as a whole,the number of Vibrio increased abnormally in Zhifu Bay and Rushan in April and July 2020,and the number of heterotrophic bacteria increased abnormally in the S7 station of Rushan in July 2020.The average values of phytoplankton density in the three aquaculture areas are 16.37×10~4cells/L,11.71×10~4cells/L and 20.06×10~4cells/L,respectively.The results showed that the phytoplankton density was high from March to August in 2020;79 species of 44 genus in 5 phylum were detected in the three aquaculture areas.There are 51 species in 27 genus of Diatom,and 10species in 4 genera of Pyrrophyta among them,accounting for 77.26%of the total species;the main dominant species in the three aquaculture areas are Skeletonema costatum,Chromulina sp.,Isochrysis sp.and Chroomonas sp.;The average of phytoplankton diversity index was 2.097,the average of evenness index is 0.812,and the average of richness index is 1.875.The correlation analysis showed that there were significant differences among water temperature,dissolved oxygen and chemical oxygen demand.There was a significant positive correlation between water temperature and Vibrio and heterotrophic bacteria,between phytoplankton and water temperature,and between phytoplankton and active phosphorus.The above results showed that except for a few nitrogen and phosphorus indicators in winter,the other indicators were in line with the aquaculture standards,the eutrophication degree and pollution degree was low,the bacterial content was low,the phytoplankton was rich and the community structure was stable,which was suitable for oyster aquaculture,but there have been in a state of phosphorus restriction for a long time in Yangma Island and Rushan aquaculture areas.(2)In order to explore a suitable ecological aquaculture mode,the experimental aquaculture areas of shellfish-ginseng-fish interculture(including oyster aquaculture area,sea cucumber aquaculture area and oriental puffer fish aquaculture area)were set up in the sea area of Zhifu Bay,and the control aquaculture areas for single cultured oyster and sea cucumber.From June 2019 to June 2020,various ecological indicators of the aquaculture areas were tested in the middle of each month,the water quality was evaluated by trophic state index,and phytoplankton community was evaluated by dominance index,diversity index,evenness index and richness index.Finally,the growth of aquacultured organisms in each aquaculture area was compared.The results showed that:the chemical oxygen demand,ammonia nitrogen,total phosphorus and active phosphorus contents of the control group were significantly higher than those of the three experimental groups;the ammonia nitrogen content of the control group was significantly higher than that of the experimental group;the eutrophication degree of the three experimental groups was lower than that of the two control groups;The number of Vibrio and heterotrophic bacteria in the sea cucumber aquaculture area of the control group was higher than that of the 3 three experimental group.Compared with the experimental groups,the proportion of diatom dominant species in the control group decreased,while the proportion of cyanobacteria increased;the diversity index,evenness index and richness index of phytoplankton in the three experimental groups were higher than those in the control groups;Compare with the oyster control group,the survival rate and output of the experimental group increased by5.2%and 10.7%respectively;And compare with the sea cucumber control group,the survival rate and output of the experimental group increased by 5.1%and 8.1%.The above results show that the ecological indicators of the aquaculture areas under the interculture mode are significantly better than those of the single aquaculture areas,the survival rates and output are significantly improved.(3)In order to explore the effect of different aquaculture densities on the growth of oysters,we compare the growth of oysters under different stratification densities and cage spacing.In mid-August 2019,select oysters with the shell height of 3 to 4 cm,and set 4groups of different stratification densities:20,40,60,and 80 per layer,with cage spacing of 0.5m.After 2 months of aquaculture comparison,the four growth traits of shell height,shell length,shell width and body weight in the 4 different stratification densities all showed the characteristics of decreasing with the increase of stratification densities,20 per layer group and 40 per layer group are obviously better than 60 per layer group and 80 per layer group,but there was no significant difference between 20 per layer group and 40 per layer group,and between 60 per layer group and 80 per layer group(P<0.05).In mid-October 2019,select oysters with the shell height of 7～8 cm,and set up 4stratification densities groups:10,15,20 and 30 per layer;set 3 cage spacing groups:0.3,0.5 and 0.8 m.After 7 months of aquaculture comparison,under the same cage spacing(0.3 m,0.5 m and 0.8 m,respectively),the four growth traits of shell height,shell length,shell width and body weight decreased with the increase of stratification density;Most of the traits showed 10 per layer group and 15 per layer group>20 and 30 per layer group since November,and some traits were still 20 per layer group>30 per layer group(P<0.05)in May;Although the former was higher than the latter,the difference was not significant between 10 per layer group and 15 per layer group.Under the same stratification density(10,15,20 and 30 per layer),the shell height,shell length,shell width and body weight of three different cage spacing groups increased with the increase of cage spacing;Although the differences of the growth traits in the first few months were significantly different,or even there was no difference,they showed the 0.8 m group and0.5 m group>0.3 m group(P<0.05)from April to May;Although the former is higher than the latter between 0.8 m group and 0.5 m group,the difference is not significant.The above results show when the oyster shell height is 3～4 cm,the stratification density of 40per layer and the cage spacing of 0.5 m is appropriate;When the oyster shell height is 7～8cm,the stratification density of 15 per layer and the cage spacing of 0.5 m can achieve the best aquaculture effect.(4)To compare and analyze the growth of cultchless triploid oyster,conjoined triploid oyster and cultchless diploid oyster,and to explore the aquaculture technology of cultchless triploid oyster adapted to the ecological environment.The results of through 15months cultivation showed that there was almost no difference in shell height,shell length,body weight and soft part weight between cultchless triploid oyster group and conjoined triploid oyster group from July to August in 2019.The former was higher than the latter in September,but the difference was not significant.The cultchless triploid oyster group was significantly higher than the conjoined triploid oyster group from October(shell length in December)to September next year(P<0.05).There was no significant difference in shell height,shell length,body weight and soft part weight between cultchless triploid oyster group and cultchless diploid oyster group from July to August in 2019.From September to next September,the cultchless triploid oyster group was significantly higher than the cultchless diploid oyster group(P<0.05).From July to September in 2020,the soft part weight of conjoined triploid oyster group and cultchless triploid oyster group was significantly higher than that of cultchless diploid oyster group(P<0.05).The meat yield of the three groups was low from September 2019 to March next year,and increased to the peak from April to June.The meat yield of the cultchless diploid oyster group decreased significantly from July to September,which was significantly lower than that of the two groups of triploid oysters(P<0.05).After 15 months of aquaculture,the cumulative survival rates of triploid oysters was significantly higher than that of diploid oysters(P<0.05).From April to June in 2020,the gonads of diploid oysters developed obviously,and the visceral mass decreased rapidly after spawning and ejaculation from July to August.However,there was no change of gonad maturation and spawning and ejaculation in the soft part of triploid oysters.The above results showed that the shell height,shell length,body weight,soft part weight,meat yield and survival rate of cultchless triploid oyster were significantly higher than those of conjoined triploid oyster and cultchless diploid oyster.