A Preliminary Study On The Spatial And Temporal Dynamics Of Early Fish Resources And The Spawning Grounds Of Coilia Nasus In Sutong Section Of The Changjiang River

The Sutong section of the lower reaches of the Changjiang River is located in the tide-sensitive area,subject to strong tidal action and special hydrological environment,and is an essential migratory channel for the Changjiang River migratory C.nasus,while this section possesses many sandbars and shallow distribution,and has a unique main stream-sandbar habitat.In order to understand the spatial and temporal distribution profile of fish resources of early life history stages in the Sutong section and larval and juvenile fish population characteristics of the two major sandbars(Changqingsha and Minzhusha)in this special habitat and the factors influencing them.In this study,a total of 84 days of early resource surveys were conducted in Sutong section from May to August each year from 2018 to 2020,and the main findings were as follows:1.The analysis of the composition of fish resources in Sutong section of Changjiang river showed that:during the survey,a total of 58,718 larval and juvenile fish were collected,belonging to 6 orders and 8 families of 51 species,the most carp family(37 species),accounting for 91.97%of the total number of larval and juvenile fish;Engraulidae ranks second in number,accounting for 4.80%of the total number of samples collected,the remaining 6 families accounted for 3.23%of the total.Relative importance index shows that Sutong section dominant species composition were Hemiculter leucisculus,Hemiculter bleekeri,Coilia nasus,Rhinogobius giurinus and Pseudolaubuca engraulis,of which Hemiculter leucisculus shows absolute dominance(IRI:4821.00),and the number of Coilia nasus larval and juvenile fish is high,with a relative importance index of 182.73 as the dominant species in this section;common species include Xenocypris argentea,Culter alburnus and Siniperca chuatsi.The analysis of the frequency of occurrence of dominant and common species showed that Hemiculter leucisculus which was in the first place,occurred77.35%during the survey,while Rhinogobius giurinus(48.29%),Pseudolaubuca engraulis(41.67%)and Coilia nasus(38.03%).The results of the temporal distribution of the abundance of larval and juvenile fish showed that the high abundance occurred in July,with the highest average abundance of 275.81 ind./100m³in July 2019;the average abundance of larval and juvenile fish in May 2018-2019 was relatively low,with the lowest abundance of 29.45 ind./100m³in May 2018.The results of a study on the aggregation characteristics of Changqingsha and Minzhusha larval and juvenile fish in the key waters of the Sutong River found that the mean abundance of larval and juvenile fish in June and July at each section was higher than in other collection months,with the greatest abundance occurring in July2019 and Hemiculter leucisculus as the first dominant species(S_11#,204.75ind./100m³),and the overall abundance of larval and juvenile fish in the north branch was higher than in the south branch in Rugao section.The clustering analysis based on the abundance of dominant and common species at 14 sites revealed that they were divided into three groups at a similarity level of 84.60,showing significant differences in assemblage structure of larval and juvenile fish in different groups(R=0.76,P<0.05).The values of community diversity characteristics during the survey period showed that Margalef(R)variation ranged from 2.08 to 2.74,Shannon(H)ranged from0.70 to 1.48,and Pielou's(J)ranged from 0.24 to 0.46,all showing an increasing trend,and the community diversity and stability were good.The similarity coefficient(Jaccard's,I)was 0.68,which was between 0.5 and 0.75,indicating that the species were evenly distributed and not significantly different between the south and north chaebols of Changqingsha and Minzhusha.2.The three environments with significant effects(P<0.05)on the abundance of larval and juvenile fish in Sutong section were found to have a temperature range of20.1-32.4?,with an average of 26.69?;a turbidity range of 5.20-124.00,with an average of 48.03;and a transparency range of 0.10-0.60 m,with an average of 0.38 m.The high abundance of larval and juvenile fish was mostly concentrated in June and July,when the lower reaches of the Changjiang River are in their breeding season and also the period of high water flow.The RDA ranking plots showed that Hemiculter bleekeri(SP1),Hemiculter leucisculus(SP2),Pseudolaubuca engraulis(SP4)and Rhinogobius giurinus(SP8)showed positive correlation with temperature;Coilia nasus(SP3),Siniperca chuatsi(SP5)and Culter alburnus(SP6)showed negative correlation with temperature;Hemiculter leucisculus,Xenocypris argentea(SP7),and Rhinogobius giurinus showed positive correlation with turbidity and negative correlation with transparency.The distribution characteristics of species and environment showed disorder due to the special hydrological influence.Analysis of the correlation between the dominant and common species of larval and juvenile fish and environmental factors and habitats in Changqingsha and Minzhusha showed that water temperature(T)and turbidity(TUR)were positively correlated and negatively correlated with transparency(SD)over the range of survey data collected.The flow velocity of the south branch is significantly higher than that of the north branch due to the diversion,and the average temperature of the north branch is 0.19?higher than that of the south branch.At the same time,the shoreline integrity of the north branch was higher,and the abundance of larval and juvenile fish showed that Changqingsha north branch river>Minzhusha north branch river>Changqingsha south branch>Minzhusha south branch river.The results of the above study indicate that the stream-sandbank habitat has some influence on the assemblage structure of larval and juvenile fish,and the north branch,which has a weaker diversion effect in the surveyed section is more suitable for reproduction,fish survival and development of larval and juvenile fish.In summary,it's recommended to strengthen protection and monitoring in the Changjiang River in the presence of the branch waters,fully exploit its role in fish stock replenishment.3.A study on the potential spawning grounds of C.nasus in the Sutong section of the Changjiang River found that a total of 646 fertilized eggs of C.nasus were collected during the survey period,including 328 eggs collected in 2019 accounting for 50.77%of all.The mean abundance of eggs collected in May and June was significantly higher than in the other months of collection(P<0.05),where no fertilized eggs were collected in April.The peak of fertilized eggs collected in each year was on 15 May(4.96 ind./100m³),16 June(26.48 ind./100m³)and 24 May(26.11 ind./100m³),respectively.Pearson correlation analysis showed significant differences in the spatial distribution of the sampled sections(P<0.05),with mean abundance distribution on the right bank(1.42 ind./100m³)>left bank(0.87ind./100m³)>centre of the river(0.69 ind./100m³).The mean abundance and frequency of fertilized eggs of C.nasus collected in the downstream S1(waters of Lvhua Town in Chongming Island)and S2(waters of Xintonghaisha,Haimen City),midstream S5(waters of Gangzha District in Nantong)and upstream S8(waters of Changqing Sha Dian Yuan in Rugao)of the Sutong section were higher than those of other collection sections.The analysis of environmental factors showed that the abundance of fertilized eggs of C.nasus was negatively correlated with temperature(P<0.01),and the abundance of fertilized eggs of C.nasus was higher in the range of 21.1-25.3?and 6.80-7.41 mg/L of dissolved oxygen.The results showed that May and June were the peak spawning months for C.nasus in Sutong section,and the temperature range of 21.1-25.3?and dissolved oxygen range of 6.80-7.41mg/L were suitable for C.nasus spawning.It's hypothesized that the waters in Lvhua Town of Chongming Island,Xin Tonghaisha of Haimen,Nantong Gangzha District and Changqingsha Diangyuan of Rugao are potential spawning grounds for C.nasus.In summary,this study analyzes the distribution of early fish resources and the potential spawning waters of C.nasus of Sutong section by considering the spatial and temporal distribution of early fish resources and combining the water habitat(DO,p H,TUR,etc.)indicators to provide a scientific basis for exploring the resource replenishment mechanism,spawning and nursery grounds of fish in the estuary with strong tidal action.