Population Ecology Of Coilia Nasus In The Middle And Lower Reaches Of The Yangtze River Based On Otolith Microchemistry And Population Genetic Studies

Coilia nasus,a small anadromous fish(Order Clupeiformes,Family Engraulidae),mainly distributes in the main stream and connected lakes of the Yangtze River,as well as the sea-connected rivers of the Bohai Sea,the Yellow Sea and the East China Sea in China.This fish also distributes in waters around the Ariake Sea in southwestern Japan.Recently,the resources of Coilia nasus in China have declined severely,it is extremely important to protect its resource populations and habitats.The C.nasus of the Yangtze River Basin includes the types of longjaw tapertail anchovy(C.ectenes),shortjaw tapertail anchovy(C.brachygnathus)and landlocked longjaw tapertail anchovy(Coilia nasus taihuensis).There are subtle differences in morphology among the three fish types.The controversy is still very obvious about the classification status of the three types of C.nasus at levels of different species,subspecies,or ecotypes.The exact population ecology characteristics of C.nasus is waiting to be further explored or confirmed.Those knowledge gaps have caused serious constraints on the resource assessment and habitat protection of this high-value fish and the formulation of corresponding policies.Related problems urgently need to be solved by introducing alternative new approaches.Otolith microchemistry provides a powerful means for analyzing the population structure,life history characteristics and habitat history of migratory fish.This approach can not only invert the environmental conditions experienced by the fish,but also trace the key habitats(such as spawning grounds)where the source populations originated.Likewise,population genetics has significant advantages in assessing the formation mechanism of the spatial population structures and their mechanisms of genetic differentiation.Hence,this thesis combined the approaches of otolith microchemistry and population genetics to systematically explore,for the first time,the life history patterns,migration features,habitat use strategies,population structures,ecological phenotypic compositions and genetic relationships of C.nasus in the middle and lower reaches of the Yangtze River.Its researches could more deepenly grasp the population ecology characteristics of C.nasus in the relevant waters.The main research contents and results of this thesis are as follows:1.Migratory ecology of C.nasus in the Yangtze River EstuaryThe Sr:Ca ratio from the core to the edge of the otoliths combined with the otolith microstructure were used to analyze the life history of adult C.nasus in the Yangtze River estuary in order to reconstruct its migratory pattern.In addition,the freshwater coefficients were used to determine the distance between its original spawning ground and the estuary.Meanwhile,observation of the reproductive maturity degree of the adult C.nasus was taken to determine whether the fish were near or off the breeding area.The results showed that the freshwater coefficients for the type of long freshwater early life history(LF)C.nasus was significantly higher than those for the type of short freshwater early life history(SF),indicating that the former populations originated from a spawning ground far from the Yangtze River estuary.In contrast,the lower freshwater coefficient of SF showed that SF C.nasus were hatched in a spawning ground close to the Yangtze River estuary,and it will also spawn in areas closer to the river estuary.The C.nasus in SF should be from an unknown breeding population of C.nasus in the Yangtze River estuary.2.Migratory ecology of C.nasus from the Zhenjiang section in mainstream of the Yangtze RiverThere are many sandbars and backwater bays along the Zhenjiang section of the Yangtze River.They may be suitable sites for the reproduction of C.nasus.Therefore,the Sr:Ca ratios from the core to the edge were combine-studied with the microstructure of the otoliths to reconstruct the the life history of C.nasus from this section.Most fish had high freshwater coefficients,showing that the C.nasus population might come from the spawning grounds far from the Yangtze River estuary.Their low gonadal maturity stage further indicated that they might continue to upstream moving at the time of sampling.Two individuals had low freshwater coefficients.The otolith core element fingerprints proved that the two individuals might originate from the Poyang Lake.Our results suggested that the Zhenjiang section of the Yangtze River should play important role of migatory route for C.nasus populations from different spawning sites.3.The life history and population structures of C.nasus from the lakes in the middle and lower reaches of the Yangtze RiverThe C.nasus from river-disconnected lakes(e.g.,Gaoyou Lake,Gucheng Lake,and Taihu)and river-connected lakes(e.g.Dongting Lake,Poyang Lake,and Shijiu Lake)were studied for understanding difference on the characteristics of life history and population structures.The results of the microchemical analysis of the otoliths revealed two anadromous C.nasus in Dongting Lake.This was the first time to confirm anadromous C.nasus in Dongting Lake in the past 20 years.The migratory C.nasus in Poyang Lake belonged to anadromous long supermaxilla ecomorphotypes,while the C.brachygnathus in the Dongting Lake,Poyang Lake,and Shijiu Lake belonged to freshwater resident short supermaxilla ecomorphotypes.The C.nasus from river disconnected lakes of the Gucheng Lake,Gaoyou Lake,and Taihu Lake belonged to freshwater resident short supermaxilla or landlocked long supermaxilla ecomorphotypes,respectively.4.The connectivity among C.nasus populations in the middle and lower reaches of the Yangtze River inferred by otolith microchemistryThe core element characteristics of otoliths were focused to study the population connectivity among populations C.nasus in the middle and lower reaches of the Yangtze River.The multi-element fingerprints in the otolith cores generally showed temporal stability for anadromous and freshwater resident C.nasus from the Poyang Lake in 2015,2017,and 2019.The similar core element characteristics of the anadromous C.nasus collected in the spawning site of Lushan water in the Poyang Lake at different years suggested that C.nasus natally return or home to the spawning site every year.The otolith core element fingerprints of the Poyang Lake’s C.nasus in different years could be used as the reference to determine whether C.nasus populations are originated from the Poyang Lake or not.Based on corresponding discriminant analysis,the populations for type of long freshwater early life history C.nasus collected from the the Zhenjiang section and Yangtze River estuary were estamited being originated from the Poyang Lake,C.nasus collected from also originated from Poyang Lake.Those for the type of short freshwater early life history C.nasus from the Yangtze River estuary were believed to be originated from other spawning sites.The origination between population of C.nasus from the Poyang Lake and other lake,e.g.,Gucheng Lake,Shijiu Lake was different.Co-existence of the Poyang Lake originated and non-the Poyang Lake originated populations was found for anadromous C.nasus in the Yangtze River estuary.5.The connectivity and ecomorphotype characteristics among C.nasus populations in the middle and lower reaches of the Yangtze River based on population geneticsMicrosatellites were used to analyze the population genetic structure of among C.nasus populations in the middle and lower reaches of the Yangtze River after confirmed the anadromy of the anchovy fish studied.The results indicated that there was a significant genetic differentiation between the populations from the Yangtze River estuary and the Poyang Lake,being similar to the aforementioned patterns by the otolith microchemical analysis.The tendency suggested that long-generation geographic separation had caused the accumulation of genetic differences between the populations.Furthermore,the mitochondrial gene cytochrome b(Cyt b)and polymorphic microsatellite loci were applied to study the relationship among the five ecomorphotypes of C.nasus of the middle and lower reaches of the Yangtze River.The results demonstrated that the longjaw tapertail anchovy(C.nasus)and the shortj aw tapertail anchovy(C.brachygnathus)should be two independent species in incipient speciation.Both of them have life history diversities of migration and resident.Noteworthily,the reproductive isolation barrier between C.nasus and C.brachygnathus is strong,but it is not totally impermeable.The existence of hybrid individuals was found in the present study between these two closely related sister species.The ecomorphotypes of landlocked and anadromous long supermaxilla C.nasus were believed still to be different ecotypes within the same species of C.nasus,and were not yet at the level of subspecies.In addition,reproductive isolating barriers between C.nasus and C.brachygnathus could be destroyed in the river-disconnected lakes.The corresponding gene flow increased with an asymmetric gene flow from long supermaxilla C.nasus to short supermaxilla C.brachygnathus.6.A wide-area perspective on the characteristics of biogeographical genetic differentiation of C.nasus populations in the middle and lower reaches of the Yangtze RiverThe C.nasus anchovies distribute in coastal areas of the Bohai Sea,the Yellow Sea and the East China Sea in China and the Ariake Sea in Japan.There might be gene exchanges between C.nasus populations in the Yangtze River and in other waters along the coast of China or in Japan.Additionally,the C.nasus populations in Chian and Japan are still believed as different geographic ones of the same species,although there have been few studies on whether there is any recent gene flow and the direction of gene flow about C.nasus in China and Japan.Therefore,the present study used mitochondrial genes and microsatellites to reveal the characteristics of biogeographical genetic differentiation of C.nasus between the middle and lower reaches of the Yangtze River and other waters.The genetic structure of microsatellites and mitochondrial genes reflected that the populations of C.nasus in China and Japan had differentiated with an asymmetric gene flow from Japan along the Korean Peninsula to the coast of China due to historical and geographical events.Gene flows exsited between C.nasus populations between the middle and lower reaches of the Yangtze River and other waters in China.