| The Yichang to Chenglingji section of the middle reaches of the Yangtze River is the main breeding habitat of four major freshwater economic fishes in China.As a large lake in the middle reaches of the Yangtze River,the Dongting Lake plays the role of flood regulation,storage and detention.The compound system of rivers and lakes in the middle reaches of the Yangtze River and its affiliated lakes is a unique freshwater ecosystem in China,which contains abundant aquatic biological resources.In recent decades,the Yangtze River has been affected by damming,overfishing and river barriers,which have led to the degradation of the ecosystem,the increase of endangered species and the accelerated loss of genetic diversity.The fishery resources in the Yangtze River have significantly decreased to the level of"no fish".From 2021,a ten-year ban on fishing will be implemented in key water areas such as the main stream of the Yangtze River,important tributaries,and large river-connecting lakes.How will the total ban affect fish stocks in the middle reaches of the Yangtze River and the east Dongting Lake?Can fish stocks recover?Based on this,hydroacoustic survey was carried out in the middle reaches of the Yangtze River twice(July in wet season and Dry season)and in the East Dongting Lake three times(May in normal season,August in wet season and November in dry season)from 2020 to 2021.In order to understand the spatial and temporal distribution characteristics of fish and the changes of fish resources in the Yichang to Chenglingji section of the middle Reaches of the Yangtze River and the east Dongting Lake in the early stage of the fishing ban,and provide data support for the protection of fishery resources and the evaluation of the fishing ban effect.The main results are as follows:1.The spatial and temporal distribution characteristics of fish from Yichang to Chenglingji in the middle reaches of the Yangtze River:Hydroacoustic survey showed that the fish density of 177.32 ind./1000m~3 in wet season was significantly higher than that of 105.88 ind./1000m~3 in dry season(P<0.05),and the spatial distribution of fish in the two periods was uneven,but fish mainly concentrated in the tail section.(1)In wet season,the fish density in Shishou to Jianli River segment(307.77±84.91ind./1000m~3)and Jianli to Chenglingji(221.53±99.23 ind./1000m~3)was the highest.The second is Yichang to Yidu(210.67±71.88ind./1000m~3)of the upper river.The density of Yidu to Zhijiang section was the lowest(31.49±73.79ind./1000m~3),which was significantly lower than that of Shishou to Jianli River section(P<0.05).(2)During the dry season,the fish density was the highest in Shishou to Jianli(252.27±75.18ind./1000m~3)section,followed by Jianli to Chenglingji(194.28ind./1000m~3)section.In the upper reaches of the Yidu to Zhijiang river,the fish density was the lowest(11.11±73.79ind./1000m~3),which was significantly lower than that in Shishou-Jianli and Jianli to Chenglingji river segments(P<0.05),while there was no significant difference among other regions(P>0.05);In vertical distribution,fish density in two periods showed surface>middle>bottom.(3)The mean values of fish target strength in wet season and dry season were-46.16d B and-48.59d B,respectively,with no significant difference.However,in wet season,the target intensity in Yidu to Zhijiang section was significantly lower than that in Jianli to Chenglingji section.In dry season,the Zhijiang to Jingzhou river section is significantly smaller than the Jianli to Chenglingji river section.(4)A total of 1033 fishes and 27 species were collected in the Jianli River section in wet season,with an average body length of 23.73cm ranging from 7.1cm to 48.5cm,and an average body weight of 466.39g ranging from 11.1g to3820.0g.2.Spatial and temporal distribution characteristics and resource assessment of fish in the East Dongting Lake:(1)The average density of fish in normal season,wet season and dry season were 138.55±34.17 ind./1000m~3,293.80±127.77 ind./1000m~3,150.20±137.89 ind./1000m~3 respectively.The fish density in wet season was significantly higher than that in May and November(P<0.05).The mean values of target intensity were-54.30±2.01 d B,-51.83±3.02 d B and-46.48±6.21 d B,respectively,and there was significant difference in TS value between normal and dry season(P>0.05).(2)At the daily scale,the average densities of day and night were117.53±44.49 ind./1000m~3 and 159.56±66.45 ind./1000m~3,respectively(P>0.05).In wet season,the average densities of day and night fish were 179.03±93.32 ind./1000m~3and 408.56±241.99 ind./1000m~3,respectively,with significant differences(F=4.498,P<0.05).There was no significant difference in target intensity between day and night(P>0.05).(3)In horizontal distribution:fishes were mainly distributed in the northeast lake area and the eastern Lake area in normal season,in the eastern Lake area and Lujiao in wet season,and in the Yanshan to Junshan,Yueyang to Yanshan and the eastern Lake area in dry season;In vertical distribution:fish are mainly distributed in the upper and middle layers of water in normal season,and in the lower and middle layers of water in wet season.(4)A total of 2117 fishes and 26 species were collected in the east Dongting Lake in wet season,with an average body length of 20.28cm,ranging from 3.9cm to48.5cm,an average body weight of 199.33g,ranging from 9.7g to 1873.2g,and a total weight of 172.95kg.3.The resource density volume method was used to estimate the fish resources during the day and night in the East Dongting Lake during the high water season.The calculation results were that the fish resources were about 7.69×10~8 ind.and 1.78×10~9ind.respectively,and the total weight of the fish resources was about 2.424×10~4 t and4.623×10~4 t.Comparing before and after the fishing ban,the average TS of fish in the East Dongting Lake before the fishing ban(-44.66±3.03d B)was greater than the average fish TS after the fishing ban(-46.48 d B),and the average fish density in the East Dongting Lake before the fishing ban(67.90±53.50 ind./1000m~3)is less than the average fish density in 2020 after the fishing ban(150.20 ind./1000m~3).This study shows that although the fishing ban strategy in the Yangtze River Basin lasted only two years,the role of fish population recovery in the middle reaches of the Yangtze River and the East Dongting Lake has been highlighted.The restoration of fishery resources is a slow process.Therefore,while implementing a comprehensive ban on fishing,it is necessary to continuously track the changes in the amount of various economic fish resources and the restoration effect of key habitats under the attention of the fishery management department.This will provide effective help for the rejuvenation of economic fish in the Yangtze River Basin,the protection of endemic and rare species and the construction of ecological civilization in the Yangtze River Basin. |
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