| Coilia nasus,belonging to Coilia of engrulidae in clupeiformes,is a rare high-value commercial fish in China.It is considered to be the first of the "three delicacies" of the Yangtze River and one of the "flag species" of the Yangtze River ecological conservation.Poyang Lake is a lake in the middle and lower reaches of the Yangtze River and the largest freshwater lake in China,locating in northern part of Jiangxi Province.Ganjiang River is the largest river in the Poyang Lake Basin.Poyang Lake and Ganjiang River historically play a very important role of distribution area of C.nasus.Due to overfishing,water pollution,water conservancy projects and other human activities,C.nasus in Poyang Lake is facing a serious threat,and its resources are severely declining.No diadromous C.nasus have been reported for quite a long time.Recently,a series of national policies,such as "ecological protection of the Yangtze River" and "ten-year fishing ban on the Yangtze River" have been vigorously implemented,which will play important roles in the restoration and protection of fish resources(including C.nasus)and their habitats in the Yangtze River.Consequently,strengthening the research on dynamics of ecomorphotype(e.g.,anadromous/ freshwater resident long supermaxilla C.nasus;anadromous/ freshwater resident short supermaxilla C.nasus and landlocked long supermaxilla C.nasus)composition and distribution of C.nasus under the new situation will undoubtedly make a unique contribution to the evaluation of the actual effect of the above-mentioned national policies.In this study,otolith microchemical analysis was used to determine the element "fingerprint" of habitat in otolith of C.nasus collected from the Hukou,Lushan(former Xingzi),Duchang water areas of the Poyang Lake,and the Ganjiang River.It is hoped to understand the migratory ecology,habitat history,resource composition and life history characteristics,so as to provide important theoretical basis and technical support for accurately grasping the recovery of C.nasus resource and its dynamics in Poyang Lake and its connected water system under the national policies.The main results are as follows:1.In 2019,10 long supermaxilla C.nasus and 7 short supermaxilla C.nasus were collected from the Hukou water area of the Poyang Lake.After the line transect analysis of otolith Sr and Ca microchemistry from the core to the edge,Sr/Ca ratios of the otoliths of 7short supermaxilla C.nasus were found being all at a low level of <3.Correspondingly,the Sr content level of entire otolithic cross section from the core to the edge in mapping analysis showed a blue color pattern of low Sr level.In addition,the freshwater coefficients of all the otoliths were 1.All these evidences reflected that all the individuals were typical freshwater resident short supermaxilla ecomorphotype C.nasus.In contrast,the Sr/Ca ratios in otoliths of ten long supermaxilla C.nasus could be divided into three phases by the line analysis of otolith Sr and Ca microchemistry.The ratios in the first phase in the core and nearby were always <3.Those in the second phase were usually >3,or even >7 in some individuals.The ratios could be found less than 3 again in the third phase of the otolith edge areas.The corresponding Sr mapping showed the blue pattern with low Sr in the core and nearby,and then the pattern changed obviously to higher Sr content patterns of green,yellow,or even orange and red color.The edge area of the otoliths shows the blue pattern with low Sr again.The otolithic freshwater coefficients of the C.nasus were only 0.48±0.09,which proves that the fish were typical anadromous long supermaxilla ecomorphotype C.nasus.These results reflected that the important ecological function of migratory route between the Yangtze River and the Poyang Lake of the Hukou water area for anadromous C.nasus.2.A total of 19 long supermaxilla C.nasus and 9 short supermaxilla C.nasus were collected in the Lushan waters of the Poyang Lake in 2019.The otolith microchemical analysis of short supermaxilla C.nasus otoliths showed that the Sr/Ca ratios were <3,and the Sr content mapping analysis showed that the entire otoliths presented a blue pattern with low Sr level,coinciding with the freshwater coefficients of 1.00.It is proved that the 9 short supermaxilla C.nasus were typical freshwater resident short supermaxilla ecomorphotype C.nasus.The Sr/Ca ratios of individual otolith by line transect analysis of the long supermaxilla C.nasus suggested that the ratios of each fish could be divided into 3 phases,Sr/Ca ratios in the first phase in the core and surrounded area were <3,those in the second phase were usually >3,and some individuals could be as high as >7.In the third phase of otolith edge,the Sr/Ca ratios were back to less than 3.The corresponding Sr content mapping analysis showed the blue pattern with low Sr in the core and surrounded area,and then the color pattern changed obviously to those of higher Sr green,yellow,or even orange and red colors in the remaining area except that of otolith edge.The pattern of the edge showed low Sr blue color again.The freshwater coefficients for this group of long supermaxilla C.nasus were0.42±0.14.These evidences demonstrated that the 19 long supermaxilla C.nasus were all typical anadromous long supermaxilla ecomorphotype C.nasus.3.The otolith microchemical analysis of 17 long supermaxilla C.nasus collected in2019 in the Duchang water area of the Poyang Lake by line transect analysis showed that the Sr/Ca ratios of each fish could be berifly divided into three phases: the first phase in the core and surrounded area showed Sr/Ca ratio < 3.The second phase in middle area of otoliths presented Sr/Ca ratio> 3 or even > 7.The third phase around otolith edge areas showed Sr/Ca ratio < 3 again.The color map pattern of corresponding Sr content mapping analysis showed the clor patterns for the three phases were low Sr blue pattren,high Sr green,yellow,or even orange,red patterns,and low Sr blue pattren,respectively.The freshwater coefficients of the group C.nasus was 0.38 ± 0.08.These evidences demonstrated that all the 17 fish belonged to typical anadromous long supermaxilla ecomorphotype C.nasus.It is noteworthy that oue results reveal clearly that the Duchang and aforementioned Lushan waters are two of habitats for anadromous C.nasus.However,both waters areas locate at upper reaches of the site of the proposed large Poyang Lake Hydraulic Project.Therefore,it will be critical to keep the connectivity characteristics of the Poyang Lake to the Yangtze River if there are anthropogenic developmental activities in the lake,especially the construction of hydraulic Projects.4.A confirmatory study on distribution of anadromous C.nasus was conducted using otolith microchemistry,based on 21 fishes sampled at the Luziyao section of the Ganjiang River in Jiangxi Province on May 27,2020.The results indicated that 16 individuals were typical anadromous long supermaxilla ecomorphotype C.nasus and constituted as high as76% of the total sampled fish.The proportion is far higher than those reported in literatures till now for C.nasus around the Poyang Lake and its tributaries.The present study firstly confirmed the existence of anadromous C.nasus in the Ganjiang River,and provided objective evidence on the recovery tendency of resource and habitat for this fish under the national policies of ecological protection and corresponding ban on fishing(started on February 1st,2019)in the Yangtze River.In order to rescue protection of resources and key habitats for C.nasus in the Gangjiang River,further studies are urgently needed to more accurately locate the distributing areas and spawning sites,and assess the potential impacts from human activities(e.g.,hydraulic engineering,sand mining of river bed)to C.nasus. |
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