Preliminary Study On Otolith Microchemistry Of Coilia Coilia In Oujiang Estuary

Coilia nasus is one of the important economic fish in China.Since the late 1990s,its resources have been declining precipitously due to many factors such as overfishing,small mesh size,habitat fragmentation and serious environmental pollution.In recent years,although the decline of C.nasus resources has been curbed through conservation methods such as stocking and seasonal fishery closures,its resources are still at a historically low level.Thus,it is important to study the ecological habits,life history patterns and markers of C.nasus before breeding and releasing.In this study,the habitat history,otolith isotopes and strontium markers of C.nasus in Oujiang estuary were analyzed and studied based on the samples collected from drift gill nets in 2021-2022.The main findings are as follows:(1)To grasp the habitat history characteristics of C.nasus in the Oujiang estuary,the strontium and calcium elements of C.nasus otoliths were examined by electronic probe technology using drift gillnet catches.The otoliths were scanned in the sagittal plane to draw strontium element analysis profiles.The results showed that the Sr:Ca ratios of all otolith samples were diverse,showing both similarity and specificity among different individuals.The similarity is that the Sr:Ca ratio in the core region of all otoliths is less than 3,and the specificity is that the Sr:Ca ratio outside the core region varies within individuals.It indicates that all the C.nasus in Oujiang estuary were spawned and hatched in freshwater,and the life histories of individuals after hatching were different.Based on the freshwater coefficient(Fc),the dependence of C.nasus on freshwater can be classified into two categories:early-dependent and moderate-dependent.In addition,the freshwater coefficient shows that there are at least two spawning grounds for C.nasus in the Oujiang estuary.The habitat history of C.nasus in Oujiang estuary is divided into two categories:freshwater-semi-saline water type and freshwater-semi-saline water-saltwater type.The life history patterns are divided into three categories:long-term freshwater-estuarine,short-term freshwater-estuarine,and short-term freshwater-saltwater-estuarine.The results of otolith strontium analysis also further verified the above conclusions.(2)The δ13C and δ18O characteristics within the otoliths of C.nasus in the Oujiang estuary were analyzed by using drift gillnet samples.The results show that δ13C ranges from-17.27‰ to-5.64‰,and δ18O ranges from-4.54‰ to-0.19‰.Stable isotopes of carbon fluctuate more than stable isotopes of oxygen.There was no significant correlation between them and the total length(P>0.05).By building a scatter figure of stable isotopes of carbon and oxygen,we can divide the Oujiang estuary into two groups.The mean value of δ13C and δ18O in the first group are-11.86‰ and-3.97‰ respectively,and the mean value of δ13C and δ18O in the second group are-8.97‰ and-0.92‰ respectively.By comparing with previous data,it is speculated that there are different life histories of C.nasus in the Oujiang estuary,and those in the Oujiang estuary may come from the waters north of the Yangtze estuary.(3)Using strontium chloride hexahydrate solution to immerse C.nasus larvae,five concentration gradients(0,12,24,48 and 60 mg/L)were set for 7 days.It was tested whether it could mark successfully in otoliths and analyzed the effect of strontium on the growth and development of C.nasus larvae.The results showed that there was no significant difference(P>0.05)between the mortality rate of the marker group designed in this experiment and the control group.The larvae of C.nasus were less affected by strontium,and there was no significant difference in the behavior of swimming and feeding among the concentration groups during the labeling period.In all concentration gradients,a stable strontium labeling area could be formed in the otolith,and the labeling success rate was 100%.After labeling,the otolith Sr:Ca ratio of juvenile fish could be restored to the pre-labeling level in the water without strontium addition.After 7 days of tagging at 12 mg/L,the strontium residues in the juvenile fish could be restored to the normal level after 24 days of temporary rearing.This experiment provides an important practice for large-scale tagging of C.nasus.In summary,the study on otolith microchemistry and strontium labeling of C.nasus found that the life history of C.nasus in the Oujiang estuary showed diversity among different individuals and complex habitat biographies;carbon stable isotope fluctuated greatly,and some groups of C.nasus in the Oujiang estuary might come from north of the Yangtze River estuary;the comprehensive labeling results concluded that the optimal concentration of strontium labeling for C.nasus juveniles was 12 mg/L and the labeling time was 4 days.This study further confirms that otolith microchemistry has incomparable advantages in life history studies of migratory fishes and can effectively reveal information about changes in aquatic habitats.In addition,the feasibility of otolith strontium tagging of C.nasus was also verified.As an important tagging technique in release tagging,strontium tagging itself has no additional impact on the survival of juvenile fish,and can eliminate strontium residues in the fish after a period of temporary rearing,so it can be used as a large-scale tagging method for juvenile C.nasus.The results of this study can provide certain reference and basis for the management and conservation of C.nasus resources.