Study On The Spatial-Temporal Patterns Of The Early-Stage Fish Resources In The Jianli To Ezhou Sections In The Middle Reaches Of The Yangtze River

The middle reaches of the Yangtze River refers to the section of the main river from Yichang,Hubei province to the Hukou,Jiangxi province.It is rich in fish diversity,also it's an important habitat and breeding place for a variety economic fish such as Mylopharyngodon piceus,Ctenopharyngodon idellus,Hypophthalmichthys molitrix and Aristichthys nobilis.However,with the operation of the Three Gorges Dam,changes in the hydrological flow pattern of the river section under the dam would definitely have a significant impact on the fish resources under the dam.To grasp the status of the early-stage fish resources under the dam and understand the impact of the Three Gorges Dam operation,In this study,from December 2018 to November 2019,a monthly survey was conducted on the early fish resources in the three river sections of Jianli,Honghu and Ezhou in the middle reaches of the Yangtze River,the content of the investigation is a preliminary study on the species composition,spatial distribution characteristics,seasonal change dynamics and the relationship with environmental factors of the early-stage fish resources(including four major fish)in the middle reaches of the Yangtze River from Jianli to Ezhou.In May and July of 2018 and2019,a survey was conducted on the effects of species composition,resources,spawning grounds and ecological scheduling on fish reproduction in the Jianli River section.The main results are as follows:1.Spatiotemporal pattern of the early-stage fish resources in the middle reaches of the Yangtze River:from December 2018 to November 2019,217 eggs and 251,000 larvae were collected in the middle reaches of the Yangtze River,belonging to 43 species(genera),13 families,6 orders.Larvae peak from late May to middle July.The farther away from the dam,the earlier the larval abundance peaked(Jianli:late June;Honghu:early June;Ezhou:late May).The three dominant species in the three river sections are all three(Jianli:Hemiculter bleekeri,Botia sp.,Pseudolaubuca sinensis;Honghu:Hemiculter bleekeri,Culterinae sp.,Neosalanx taihuensis;Ezhou:Hemiculter bleekeri,Rhinogobius sp.,Pseudolaubuca sinensis).The larval abundance of the Honghu river section(57.18ind./100m~3)was higher than that of Jianli(53.99 ind./100m~3)and Ezhou river section(38.03ind./100m~3).The drifting density of larvae was positively correlated with water temperature,flow rate and water level,and negatively correlated with dissolved oxygen(P<0.01).2.Spatiotemporal pattern of four major Chinese carps eggs and larvae in the middle reaches of the Yangtze River:spatial pattern of the larvae of four Chinese major carps in the middle reaches of the Yangtze river showed that the larvae abundance was the highest in Jianli river section(6.69ind./100m~3),followed by Honghu river section(6.32 ind./100m~3)and Ezhou river section(2.85ind./100m~3).There were obvious differences in the occurrence time of the larvae of four major Chinese carps(Jianli:mid-June;Honghu:late May;Ezhou:mid-May).The peak of drifting density of larvae in Jianli River section occurred in early July,the water temperature was 23.10?,and the runoff of the Yangtze River was 22000m~3/s;Honghu has been relatively high from mid-June to mid-July,during which the average water temperature was 24.88±0.60?and the average flow was 35817±3576 m~3/s;larvae in Ezhou had two peaks from mid-May to late May and mid-June to early July,with water temperatures of 22.48±0.33?,26.47±0.52?,and flow rates of 34100±1900m~3/s and 34267±3278 m~3/s.Density of domestic fish larvae was positively correlated with water temperature(P>0.05),and significantly positively correlated with flow(P<0.05).3.Status of the early-stage fish resources in Jianli River:about 49 species of fish eggs and larvae belonging to 14 families and 7 orders were identified in Jianli river section from May to July in 2018 and 2019,and there were 26 species spawning drifting eggs among these fishes.Hemiculter bleekeri(44.9%of the total numbers)occupies an absolute advantage in the species composition of larvae,followed by Leptobotia taeniops(15.2%)and Botia sp.(12.5%).Cultrinae were competitive during the investigation,which including Hemiculter bleekeri,Parabramis pekinensis,Pseudolaubuca sinensis,and so on.Leptobotiapur purea and Botia sp.appeared mainly in June and July.Squalidus argentatus and Xenocypris sp.were found from mid-May.Rhinogobius sp.appeared in late May.In 2018,it was estimated that the total runoff of fish eggs and larvae at Jianli section in the middle reaches of the Yangtze River were respectively 33.3×10~8 ind.and 324.2×10~8ind.In 2019,there were 35.9×10~8 ind.and 1140.7×10~8ind.respectively.4.Effects of ecological scheduling on fish reproduction:there were two corresponding spawning peaks during the dispatching period in 2018.The spawning scale was 6.2×10~8ind.and4.9×10~8ind.,respectively.The cumulative increase in water temperature and flow during flooding were 20.3-21.5?,6000 m~3/s,and 23.0-24.9?,5900 m~3/s;In 2019,the peak size of spawning in ecological regulation is 2.3×10~8ind.,and the cumulative increase in water temperature and flow is20.3-21.1?and 200 0m~3/s.Jianli section had become the important spawning ground for Parabramis pekinensis and Squalidus argentatus and other fish.Ecological regul ation played an obvious role in promoting the natural reproduction of Hemiculter bleekeri,Parabramis pekinensis and Squalidus argentatus at Jianli section.Spawning quantity was positively correlated with duration of flood and accumulation of flow rate.