Study On The Spatiotemporal Distribution,Habitat Preference And Environmental Pollution Stress Effects Of Silver Carp Larvae And Juvenile In The Middle Reaches Of The Yangtze River

Early life stages of fishes are a crucial aspect of their life history and directly affect the replenishment of fish resources.The“river-lake”complex habitat in the middle reaches of the Yangtze River is an organic unity composed of physical（water temperature,water depth）,chemical（water quality）,and biological（plankton,aquatic plants）factors,and it is an important place for fish reproduction and habitat.However,since the mid-20th century,a series of hydraulic engineering projects have changed the hydrological conditions and connectivity of the middle reaches of the Yangtze River,resulting in delayed fish breeding time and fragmented habitats for fish reproduction and growth,thus affecting the replenishment of fish resources.Currently,the discharge and residue of new types of pollutants have gradually become new environmental issues in the Yangtze River Basin,However,the mechanisms of their effects on the early growth and development of fish are not yet clear.This study focuses on the silver carp（Hypophthalmichthys molitrix）population,and investigates the early fish resources,fish and habitat factors,and the residue of new pollutants in Dongting Lake and the adjacent main stream of the Yangtze River,in order to study the spatial-temporal distribution characteristics and habitat preferences of silver carp fry,and residue status of new pollutants in the middle reaches of the Yangtze River.The study also identifies the main influencing factors through otolith analysis,GAM model,and pollutant stress experiments,and analyses the impact of key habitat factors such as hydrological conditions,aquatic organisms,and water quality on the distribution and growth of silver carp fry.The study provides theoretical support for the protection of silver carp population resources and the ecological restoration of important habitats in the middle reaches of the Yangtze River.The main results are as follows:1.The temporal and spatial distribution characteristics of silver carp larvae in the middle reaches of the Yangtze River after operation of the Three Gorges Reservoir was analyzed.From May to July 2019,a daily investigation of the fishes of early life history stages was conducted in Jianli（Yangtze River main stream above Dongting Lake）,Yueyang（connecting channel between Dongting Lake and the Yangtze River）,and Honghu（Yangtze River main stream below Dongting Lake）.Based on data on the abundance,age,and hatching date of silver carp fry,the origin of silver carp fry in the Yangtze River main stream below Dongting Lake was analyzed.The results showed that the average density distribution of the four major carp fry in Jianli,Yueyang,and Honghu segments was 0.52±1.23,0.35±0.78,and3.85±5.6 ind./100m³,respectively.The density of the four major carp fry in the Honghu was significantly higher than that in the Jianli,which may be related to the replenishment from the Dongting Lake system.In terms of time,the first appearance of the four major carp fry in the Jianli and Yueyang segment was June 4 and May 16,respectively.The breeding time of the four major carp species in the upper reaches of the Yangtze River,above Jiangli,is delayed by approximately 19 days compared to the Dongting Lake.This delay may be attributed to changes in the hydrological conditions of the Jiangli section caused by the operation of the Three Gorges Dam.The peak period in Jianli segment was concentrated from late June to early July,while in the Yueyang segment,it was concentrated from late May to early June.The peak period in the Honghu segment was scattered from late May to early July.Based on the results,it was hypothesized that the resource replenishment process in the Honghu section,before mid-June,the main source is from the Dongting Lake,while after mid-June,it comes from the upper reaches of the Yangtze River,above Jiangli.Research on the age and hatching dates of silver carp fry indicated that before mid-June,the silver carp larvae in the Honghu section originate from the Dongting Lake water system,while after mid-June,they originate from the upper reaches of the Yangtze River,above Jiangli,thus confirming the hypothesis of the resource replenishment process in the Honghu section.Historically,the reproduction dates of the four major carp species in the Dongting Lake were close to those in the upper reaches of Jianli section.After the impoundment of the Three Gorges Reservoir,due to the effects of cold water discharge and peak flow changes,the reproduction of the four major carp species in the upper reaches of Jianli section was delayed by about 19 days.The silver carp fry in Honghu section mainly came from the Dongting Lake before mid-June and from the upper reaches of Jianli section after mid-June.2.The habitat preferences of juvenile silver carp in the middle reaches of the Yangtze River was analyzed.From July to November 2021,monthly surveys were conducted in the Jiangli section（sandbars,shallow shores,and open water areas）and Dongting Lake（lakeshore zone,connecting channel,and open water areas）to investigate the fish species and habitat factors.The results showed that the length-weight relationships for juvenile silver carp in Jiangli and Dongting Lake were represented by W=0.0225L^2.8882（R²=0.991,n=231）and W=0.0069L^3.2685（R²=0.978,n=359）,respectively.They exhibited negative allometric growth and positive allometric growth.Juvenile silver carp in Dongting Lake showed better growth,possibly due to higher bait abundance.The population densities of juvenile silver carp in the sandbars,shallow shores,and open water areas of Jiangli were（1.09±0.67）,（0.34±0.13）,and（0.01±0.02）g/（h·m²）,respectively,while those in the lakeshore zone,connecting channel,and open water areas of Dongting Lake were（2.10±1.03）,（1.31±0.29）,and（0.38±0.27）g/（h·m²）,respectively,indicating that juvenile silver carp prefer the lakeshore zone and sandbars as suitable habitats in Dongting Lake and Jiangli,respectively.The results of GAM model analysis showed that zooplankton and water depth explained 80.6%of the habitat preference of juvenile silver carp,with zooplankton accounting for 70.1%of the explanation（F=0.021,P<0.05）.The suitable ranges of zooplankton density and water depth in the habitat of juvenile silver carp were 0.1×10⁶-0.5×10⁶ ind./L and 150-300 cm,respectively.Juvenile silver carp tend to choose habitats with abundant bait,shallow water depth,slow flow velocity,and ample aquatic vegetation.Bait and water depth are the main influencing factors,with the preferred ranges being 0.1×10⁶-0.5×10⁶ ind./L for bait and 150-300 cm for water depth.juvenile silver carp in the main stream of the Yangtze River preferentially select sandbars as their growth habitats,while some individuals entering Dongting Lake prioritize the lakeshore zone as their growth habitat.3.This passage analyzes the pollution of OPEs（Organophosphate Esters）in the habitat and their accumulation in silver carp.From October to November 2020,silver carp and water samples were collected in the Jiangli section（main stream of the Yangtze River）and Dongting Lake（connecting lakes）.The concentrations of six OPEs were measured.The results showed that all six OPEs could accumulate in silver carp,with an average total concentration（Σ₆OPEs）of 583.73μg/kg ww.Pearson correlation analysis revealed significant negative correlations（P<0.05）between the concentrations of TDCIPP,TEHP,Σ₆OPEs,and the length,total length,and weight of silver carp.The overall concentration level of the six target OPEs in the water environment of the silver carp habitat was 18.28μg/L on average.Based on the species sensitivity distribution curve derived from the best-fit function,the no-effect concentrations for TCP,TCPP,TDCIPP,TNBP,and TPHP were determined to be 0.06,2.15,0.36,0.76,and 0.07 mg/L,respectively.The results of the environmental risk quotient analysis indicated that TDCIPP had the highest potential impact on the mitten crab population.Six OPEs with high production and usage were detected in both water samples（18.28μg/L）and juvenile silver carp samples（583.73μg/kg ww）.Among them,TDCIPP（RQ=0.0127）was found to have the highest potential impact on the silver carp population.4.A stress experiment was conducted in the field to investigate the effect of TDCIPP on the growth of juvenile silver carp.In this study,the highest risk factor of TDCIPP was selected as the stressor,and concentration gradients were set based on the average concentration of TDCIPP in the field environment（4.63μg/L）.Indoor exposure experiments were conducted from May to July2020,using five environmentally relevant concentrations（0,0.05,0.5,5,and 50μg/L）.The results showed that TDCIPP exposure significantly reduced the body length and weight of juvenile silver carp,indicating a growth inhibition effect.TDCIPP exposure significantly downregulated the relative expression levels of growth-related genes（gh and igf1）,decreased the levels of growth hormones and insulin-like growth factor-1,and inhibited the growth of juvenile silver carp.TDCIPP exposure also induced oxidative stress response in the juvenile silver carp,and the expression of antioxidant genes（cat,sod1,gstt,hsp70）in the juvenile silver carp was significantly upregulated to counteract oxidative damage by altering the activity of antioxidant factors（CAT,SOD,GSH,GPX）.The results of the second-generation integrated biomarker response model showed that the impact of TDCIPP on juvenile silver carp increased with exposure concentration and time,with the concentration effect being greater than the time effect.TDCIPP exhibited a growth inhibition effect on the growth of juvenile carp by downregulating the expression of growth-promoting genes（gh and igf1）,reducing the levels of GH and IGF-1 in the body,and inhibiting the growth of juvenile carp.The concentration effect of TDCIPP was found to be greater than the time effect.In conclusion,the spawning time of the four major carp species in the upper reaches of Jianli section has been delayed to June.The reason for the delayed spawning time is the absence of a significant rise in water level in the upper reaches of Jianli section in May.It is recommended to conduct ecological scheduling of the Three Gorges Reservoir in May,artificially create floods to promote the reproduction of the four major carp species.Furthermore,the delayed breeding time will shorten the growth period for juvenile silver carp before winter.Therefore,it is important to enhance the ecological protection of critical habitats such as lake shorelines and main channel sandbars for juvenile silver carp,providing suitable locations for their growth before winter.Furthermore,as the concentration of organophosphate esters in the water environment of the middle reaches of the Yangtze River has reached the concentration causing effects on fish,it is recommended to include this new type of pollutant in the regular environmental monitoring system as one of the indicators for fishery water quality evaluation.