Fish Resource In The Early Life Stages And Its Relation To Hydrological Factors In Laibin Section Of Xijiang River

Studying the temporal and spatial distribution of ichthyoplankton in rivers and their relationship with hydrological factors is important for evaluating fish reproduction dynamics and resource replenishment.With the regulation of the Datengxia dam closure and the ecological regulation of fish reproduction in Xijiang river,in order to investigate the response of fish reproduction caused by closure,evaluate the effect of ecological operation and guide the conservation of fish resources,this study took laibin section of Xijiang River（the end of Hongshui River and the end of Liujiang River）upstream of Datengxia Dam as the research area and took ichthyoplankton resources as the research object.Two sampling sections were set up in Hongshui River estuary and Silong of Liujiang River respectively.Daily sampling survey was conducted from April 16 to August 31every year from 2019 to 2021,and hydrology factors such as flow velocity,water temperature,p H,transparency,conductivity and dissolved oxygen were measured on-site,and hydrological data such as daily discharge and water level were obtained from Guangxi Water Conservancy Information Network.We analyzed and discussed the species composition,diversity,and temporal pattern of ichthyoplankton resources before and after dam closure and its relationship with hydrological factors in the two river reaches,and then evaluated the possible spawning location and spawning scale of Hypophthalmichthys molitrix,Ctenopharyngodon idella and dominant fish.This study aims to provide basic data for ecological operation,fish spawning grounds and resource protection in laibin section of Xijiang River.The main results are as follows:（1）Survey of ichthyoplankton over three cycles of summer 2019-2021（2019before closure,2020 and 2021 after closure）.The results showed that the ichthyoplankton resources in the Laibin section of the Xijiang River were still abundant,and at least 45 species of fish from 6 orders,13 families and 45 species could complete reproduction in this section of the river.Among them,the main stream Hongshui River contains 35 species of 5 orders,11 families,and the tributary Liujiang contains 6 orders,12 families and 42 species.The ichthyoplankton resources of the two rivers were dominated by Cyprinidae,accounting for about 60%of the total number of species.The main species of fish eggs are Squalidus argentatus and Sinogastromyzon wui.The main species of larvae are Squaliobarbus curriculus,Pseudohemiculter dispar,Squalidus argentatus,Acrossocheilus hemispinus,Siniperca chuatsi,Oreochromis niloticus and Rhinogobius leavelli.The total ichthyoplankton resource runoff through the Laibin section in three cycles is 59.28×10⁸ ind.,of which 18.65×10⁸ ind.in the Hongshui River section and 40.63×10⁸ ind.in the Liujiang section.The dominant species scales in the two river sections are 14.26×10⁸ ind.and 35.9×10⁸ ind.,respectively,accounting for 76.46%and 88.35%of the total resources in the river section.The resources of Hypophthalmichthys molitrix and Ctenopharyngodon idella in the two river sections only accounted for 1.23%and 1.08%of the total resources.（2）Under near natural culture conditions,Sinogastromyzon wui takes about25 days from hatching and development to the end of juvenile stage,during which total length（TL）increases by 7.05 mm.Its early development stage is divided into four stages:pre-flexion larval（0-3d）,flexion larval（3-7d）,post-flexion larval（7-15d）and juvenile stage（15-25d）.However,it takes a longer time for Ptychidio jordani to develop to juvenile stage,which takes about 30 days.During this period,TL increased by 17.39 mm.The development stages were divided into four stages:pre-flexion larval stage（0-4 DPH）,flexion larval stage（5-12 DPH）,post-flexion larval stage（13-15 DPH）and juvenile stage（16-30 DPH）.In the early life history stage,the organs of sensory,feeding and swimming of the two species were rapidly differentiated,and allometric growth was present in both species.Among them,eye diameter and snout length showed positive allometric growth pattern in larval stage,and each fin ray showed positive allometric growth pattern in juvenile stage.（3）By analyzing the shape and microstructure changes of otoliths at the larval and juvenile stages of S.argentatus and S.wui,it was found that the shapes of three pairs of otoliths of the two species were similar after development to juvenile.Among them,lapilli and asteriscus developed from nearly circular to oval,sagitta developed from nearly circular to sickle,and the rate of shape change of sagitta had a better corresponding relationship with the growth rate of fish.Among the three pairs of otoliths,the complete daily increment of lipilli is relatively easy to obtain,and is most suitable for judging the age of larvae and juveniles.The deposition rates of the lapilli were the same,both 1.However,the formation time of the first increment is different.The first increment of the otoliths of the S.argentatus formed on the 3dph and the first increment of the otoliths of the S.wui were formed on the 2dph.（4）By analyzing the Alpha diversity of the ichthyoplankton in the Hongshui River and Liujiang River,it was found that the Shannon-Wiener index（H’）of the ichthyoplankton in the two rivers was similar,and both were relatively low.From an annual perspective,the Margalef index(D_Ma)and Shannon-Wiener index（H’）of the two rivers after the closure of the Datengxia Dam decreased significantly（P<0.05）,while the evenness index increased significantly（P<0.05）.It shows that the closure of Datengxia has had an impact on fish reproduction in the upper reaches of the river.From the perspective of each stage of the summer flood process,the D_Ma and H’index of the summer flood season were significantly higher than Pre-flood and Post-flood stages（P<0.05）.It shows that the Flood stage in summer is the prosperous period of fish reproduction in the Laibin section of the Xijiang River.（5）By analyzing the temporal pattern of Beta diversity of ichthyoplankton in the Liujiang River,it is found that the early resource samples at each stage before and after the closure can be aggregated into 3-4 groups（UPGMA level and PCo A based on Bray-Curtis distance matrix）among them.In Hongshui River,the ichthyoplankton composition of the stages before the closure and the Pre-flood stage in the first year after the closure is significantly different from that of other stages（P<0.05）.In Hongshui River,the ichthyoplankton composition in the pre-flood stage was significantly different from that of other stages（P<0.05）.Spatially,changes in Beta diversity of ichthyoplankton from mainstream to tributary are mainly driven by species turnover.On an annual basis,changes in early resource Beta diversity in the Liujiang section from before the closure to after the closure were also mainly driven by species turnover.（6）The analysis of the relationship between the temporal pattern of ichthyoplankton and hydrological factors showed that the dominant hydrological factors driving fish reproduction were different at different stages in summer,including transparency and dissolved oxygen in Pre-flood stage,water level in Flood stage and water temperature in Post-flood stage.The differences in the ichthyoplankton composition at different stages indicate that fish have different reproductive strategies.The dominant species S.argentatus and S.wui had the longest spawning time,and their spawning dynamics were significantly correlated with transparency,water temperature and water level.The common species Sinibotia robusta and Sinibotia pulchra mainly spawn in the Pre-flood stage,and their spawning dynamics are significantly related to transparency and dissolved oxygen.Common species of Squaliobarbus curriculus mainly spawn in flood,and their spawning dynamics are significantly related to water temperature and water level.（7）Using the RTPT module in Flu Egg,a model was established to predict the possible spawning locations of H.molitrix and C.idella and three dominant species in the Laibin section of the Xijiang River.The results suggest that there are 6 possible spawning locations for C.idella and H.molitrix in the Hongshui River,and 7 possible spawning locations for dominant species,5 of them overlapped.It is speculated that there are 4 spawning positions of C.idella,H.molitrix and dominant species in the Liujiang section.The interannual variation of the possible spawning locations of C.idella,H.molitrix and dominant species indicates that the possible spawning locations of fishes in the two rivers migrated upstream as the dam intercepted.In addition,the spawning scales of all spawning grounds in the two river sections decreased as a whole after the closure,and the spawning scales of C.idella and H.molitrix decreased significantly.The above research results show that there are still many fish spawning grounds with a certain scale in the Laibin section of the Xijiang River at this stage,and the Liujiang section of the tributary has a larger breeding scale than the main section of the Hongshui River.The spawning strategies of most fishes in the two rivers have obvious interspecies specificity and are obviously driven by different hydrological factors.With the closure of the Datengxia Dam,the species number and abundance of ichthyoplankton in the two rivers have declined to a certain extent.However,the spawning phenomenon of economic species such as S.curriculus,Siniperca chuatsi,C.idella,H.molitrix and H.nobilis can still be monitored at this stage.These results are of great significance for ecological operation and conservation of fish resources in the Laibin section of the Xijiang River.