Study On Spatial-Temporal Pattern And Environmental Stress Effects Of Juvenile Hypophthalmichthys Molitrix In The Upper Reaches Of The Yangtze River

The upper reaches of Yangtze River is one of the river ecosystems with the richest freshwater fish biodiversity in China.With the construction and operation of the Jinsha River tertiary hydropower station,the upper Yangtze River section has been artificially divided into reservoir groups and natural flowing river sections below the dam.In terms of ecosystem,the reservoir groups provide more suitable habitat conditions for fish nursery and growth,and the natural flowing river section below the dam meets the natural reproduction needs of most fish,especially drifting spawning fish.The natural conditions of fish habitats in the upper Yangtze River have changed due to the construction of cascade hydropower station,reduced river connectivity,and continued changes in hydrological and hydrodynamic conditions,which have affected fish reproduction,growth and nursery.It has been shown that as the distance from the dam increases and the inflow of tributaries along the river decreases,the impact of dam operation on the habitat of the river section below the dam is gradually weakened,and the impact on fish reproduction and growth is gradually reduced.Based on the survey data of early fish resources and juvenile fish resources in different reaches of the upper reaches of the Yangtze River,this paper analyzes the spatial and temporal distribution characteristics of the breeding and supplementing populations of Hypophthalmichthys molitrix,and uses the GAM model to determine the key environmental factors affecting the spatial and temporal patterns of fish.Finally,based on the detected concentrations of heavy metals in aquatic ecosystems and the enrichment characteristics of juvenile silver carp in different river sections,we combined indoor experiments to simulate the effects of environmentally relevant mercury exposure on the growth of juvenile silver carp,aiming to provide a scientific basis for the recovery and conservation of silver carp populations in the upper reaches of Yangtze River.The main research results are as follows:1.Spatial-temporal pattern of H.molitrix eggs and larvae.The generalized addition model（GAM）based on Tweedie distribution was used to analyze the main environmental factors affecting the spatial and temporal distribution pattern of the H.molitrix,based on the early resource survey data from the upper reaches of the Yangtze River from May to July of 2017 to 2019 and form May 2019 to January of 2020.The results were as follows:（1）The density of early resource fish eggs showed a consistent trend from 2017 to2019.Spatially,no H.molitrix egg was collected at YB closest to the dam in the three river sections（YB,LZ and JJ）,whereas the density of H.molitrix eggs was highest at JJ,which was the farthest section from the dam with the highest flow and water temperature,and low transparency;temporally,the density of fish eggs in June was significantly higher than that in May and July,suggesting that June was the peak breeding period of H.molitrix in the upper reaches of the Yangtze River.The density of larvae also showed a similar distribution pattern.The density of larvae at FD was the highest（LZ:0.003±0.01 ind./1000 m³;JJ:0.06±0.11 ind./1000 m³;FD:0.69±1.74ind./1000 m³）,and the peak occurred the latest（JJ:June;FD:September）.These results indicate that the low temperature water,low flow and high transparency water released by the dam operation seriously affect the natural reproduction of H.molitrix in the river section near the dam,while the degree of dam impact on fish reproduction is weakened by the increasing distance of the dam and the inflow of tributaries along the river.（2）Generalized additive model analysis showed that the main environmental factors affecting density distribution of H.molitrix eggs and larvae were discharge,distance from the dam,transparency and water temperature（P<0.05）.Both distances from the dams and water temperatures were affect the spatial distribution of H.molitrix eggs and larvae.The distribution of H.molitrix egg density is concentrated when the water temperature is 21-23°C,the flow rate is 6000-14000 m³/s and the transparency is less than 50 cm.The distribution of larvae was concentrated when the water temperature was 18-25°C.（3）The correlation analysis between breeding size and hydrological indicators showed that the duration of flood rise and the rate of flow increase had a significant positive response to the breeding size of H.molitrix（P<0.05）.It is beneficial to the natural reproduction of H.molitrix in the upper reaches of the Yangtze River when the duration of water rise lasts for 4 days or more and the cumulative increase of flow increase was 5030 m³/s and above.（4）The spawning area of H.molitrix showed a tendency to migrate upstream in the post-dam of the Three Gorges Dam,with the upper reaches of the Yangtze River from Baisha Town to Rongshan Town were the main spawning grounds of H.molitrix.As a result,it is suggested that the river section farther away from the dam,especially the Chongqing-Luzhou section above the reservoir tail,be protected as the key breeding area for the H.molitrix to maintain and grow the supplemental population.2.Spatial-temporal pattern of juvenile H.molitrix.Based on the fishery resources survey and hydrological environment data from the upper reaches of the Yangtze River to the Three Gorges Reservoir Area from September 2019 to January2020,we used the generalized additive model（GAM）to screen the main environmental factors affecting their distribution characteristics.The results showed that 73 fish species were collected,belonging to 6 orders,16 families and 54 genera.Among them,10 species are endemic to the upper Yangtze River,accounting for 13.70%of the total number of species.In addition,5 exotic species are included,accounting for 6.85%of the total number of species.According to index of relative importance（IRI）showed that Hypophthalmichthys molitrix and Cyprinus carpio were the dominant species.The density of juvenile H.molitrix resources in the upper reaches of the Yangtze River has significant spatial distribution characteristics,mainly distributed in the Fengdu River section of the Three Gorges Reservoir.The GAM model results showed that the main environmental factors affecting the distribution of juvenile H.molitrix density were velocity,plankton density,distance from the dam and water temperature,among which the maximum velocity deviation rate is 57.7%.With increasing velocity,the resource density of juvenile H.molitrix decreased overall and concentrated in the range of 0.10-0.55 m/s.The plankton density and distance from the dam had a significant linear relationship with the density of H.molitrix resources,that is,with the increase of plankton density and distance from the dam,the resources density of juvenile silver carp showed an upward trend.The resource density of juvenile H.molitrix increased slowly with increasing water temperature,and was concentrated in the range of 18-21.5°C.3.Heavy metal enrichment characteristics and risk evaluation of juvenile H.molitrix.The gill,liver and muscle samples of juvenile silver carp were collected from different river sections in the upper reaches of Yangtze River from June to August 2021,and to assess the heavy metal contamination status and risk level of fish.The results showed that all heavy metals were detected in all samples except for Cd and Hg which were below the detection limit in muscle and gill,respectively.According to the national food safety standard（GB 2762-2017）,there was an exceedance of the standard except for Cr and Hg.The average concentrations and ranges of the seven heavy metals in juveniles silver carp are as follows:42.339 mg/kg（0.085-209 mg/kg）for Cu,14.710mg/kg（1.67-51.5 mg/kg）for Zn,0.504 mg/kg（0.009-3.893 mg/kg）for Pb,0.214mg/kg（0.042-1.730 mg/kg）for Cr,0.078 mg/kg（0.003-0.285 mg/kg）for As,0.054mg/kg（<LOQ-0.295 mg/kg）for Cd and 0.020 mg/kg（<LOQ-0.111 mg/kg）for Hg.The average concentrations of the seven metals follow the trend Cu>Zn>Pb>Cr>As>Cd>Hg.Liver heavy metal levels were higher than those of the gills and muscle in all fish tissues.The single factor pollution index showed that except Cr and Hg,all heavy metals are contaminating for fish,and Pb was the main prevalent contaminant element.The liver and gills of H.molitrix were mildly polluted,and three sections of the Fengdu was mildly polluted according to the integrated pollution index.4.Toxic effects of exposure to environmentally relevant concentrations of mercury on the growth of juvenile H.molitrix.In this study,we systematically evaluated the bioaccumulation,growth parameter,antioxidant status,hormone levels,and GH/IGF axis toxicity of juvenile H.molitrix after exposure to environmental-related concentrations of Hg²⁺（0,0.05,0.5,5,and 50μg/L）for 28 days.Results showed that the mercury bioaccumulation levels in the liver increased in a concentration-time dependent manner.Exposure to mercury resulted in shorter body length,lighter body weight,and significant decreases in weight growth rate（WGR）and specific growth rate（SGR）in juvenile H.molitrix.The serum levels of growth hormones（GH and IGF-1）and thyroid hormones（T3 and T4）were significantly decreased,and the expressions of GH/IGF axis-related genes（gh、ghra、ghrb、igf 1）were significantly downregulated after 7,14,and 28 days of Hg²⁺exposure.Correlations between the growth parameters and growth hormones or expression of genes in GH/IGF axis further suggested that environmentally relevant concentrations of Hg²⁺could have adverse effects on growth.In addition,superoxide dismutase（SOD）activities,catalase（CAT）activities,and glutathione S-transferase（GST）activities,the levels of reduced glutathione（GSH）and malondialdehyde（MDA）were significantly increased,whereas the glutathione peroxidase（GPx）activities decreased significantly when juvenile H.molitrix were exposed to environmentally relevant concentrations of 5 and 50μg/L Hg²⁺.Also,the expression levels of oxidative stress-related genes（sod 1、cat、gstt、hsp70）were significantly altered.Taken together,the results indicate that Hg²⁺exposure leads to growth inhibition and oxidative stress in juvenile H.molitrix.In conclusion,the riverine habitat in the upper reaches of the Yangtze River provides breeding habitat for H.molitrix,and the slow-flow habitat provides nursery and growth habitat for H.molitrix.Discharge,distance from the dam and water temperature are the main environmental factors influencing the spatial and temporal pattern.The growth of juvenile H.molitrix was inhibited by the environment related mercury concentration greater than 5μg/L.