Based On Otolith Microstructure And Otolith Sr/Ca Analysis To Explore The Incubation Period And Early Life History Of Small Yellow Croaker (Larmichthys Polyactis) In Sea Waters Jiangsu Province

Small yellow croaker（Larimichthys polyactis）,one of the most importantly economic and ecological fish species in marine fishery of East Asian countries,has always been attracting the attention of fishermen,fishery scientist and government agency.It is one of the flagship species to explore marine fishery economy and ecology in China.With the increasing development and utilization of L.polyactis over the last decades,it was naturally characterised by miniaturisation of the body,youth in population structure,and expansion of spawning grounds.Therefore,it is urgent to implement scientific and effective management for L.polyactis.Robust information on age and growth at the early life stages is fundamental to exploring the fish population dynamics and is key to sustaining fish stocks by protecting recruitment stock.Therefore,it is essential to explore the incubation period and hatching environment at the early life history of L.polyactis.Accurately and rapidly estimating the age is fundamental to exploring the population structure for L.polyactis.In the present study,45 wild young-of-the-year（YOY）L.polyactis were collected in southern Yellow Sea to compare the three otolith sections,including the sagittal section,frontal section,and transverse section of sagittal otolith.And the von Bertalanffy growth functions（VBGF）of L.polyactis during recent30 years were simultaneously summarized to age backward based on fish body length.Taking the clear annual ring pattern and intuitive ring as the highest standard,the results showed that the annual ring clarity was the lowest in the sagittal section（53.33%）,higher in the frontal section（66.67%）,and the highest in the transverse section（73.33%）.The annual ring of the three sections became more distinct after acid etching.Comparing the age identification results for three otolith sections with the age inversion results from 19 growth equations,the VBGF of L_t=366^{[1-e-0.11777（t+1.76429）]}showed the highest similarity levels with 53.33%for the sagittal section,86.67%for the frontal section,and 86.67%for the transverse section,respectively.The results revealed that the optimal section and VBGF for aging the L.polyactis in southern Yellow Sea is the transverse section and L_t=366^{[1-e-0.11777（t+1.76429）]},respectively.It is suggested that the VBFG should not be arbitrarily used to estimate the fish age based on the fish body length or weight,and we should prefer the VBGF with high similarity verified by age identification.This study provides a scientific reference for fish age identification and age backward inferring.Based on the otolith microstructure of the transverse section,the best section for age identification validated in the above study,262 wild YOY L.polyactis were collected in Haizhou Bay in 2020 and 2021 to explore the incubation period and early life history.The results are as follows:the average diameter of otolith primordia was15.26±2.38μm.The distance between the feeding wheel and the otolith primordium was57.93±5.88μm.The increment was 9.88±1.53μm on the first day after feeding.The width of the daily increment increased significantly during 20～40 days post hatching（dph）,when the L.polyactis in the metamorphosis phase with fastest growing rate.The width of daily increment was relatively stable during 40～65 dph.After the 65 dph,the otolith daily increment width tends to level off.At the 90 dph,the maximum otolith daily increment width was 54.52μm,and the average width was 42.31±5.65μm.The incubation period of L.polyactis was estimated from March to May and the peak incubation was from April 10 to 20 and from March 20 to March 30 for 2020 and 2021,respectively.A comparison of otoliths daily increments of young L.polyactis revealed the presence of a mixed population in Haizhou Bay.After verifying the incubation period of L.polyactis,wild and cultured L.polyactis were collected in Lvsi fishing ground and Rudong aquaculture base,respectively,to further explore its early life history.Based on the relationship between the otolith Sr/Ca and water salinity,the environment they inhabit can be inferred,contributing to explore the early life history.The otolith Sr/Ca in different developmental stages of L.polyactis was determined by electron probe microanalysis（EPMA）.In order to better observe and explore the correlation between the early developmental stages of L.polyactis,STARS（Sequential t-test analysis of regime shift）and nonparametric Mann-Whitney U-test test methods were mainly used.The results were as follows:（1）Otolith Sr/Ca in the core area was higher than other areas in cultured adult,juvenile,larval,and wild young L.polyactis.（2）Significant differences were found among the embryonic,metamorphosis,and juvenile stage（P<0.05）of wild L.polyactis.However,significant differences were found between metamorphosis and larval stage of cultured L.polyactis（P<0.05）.（3）The Sr and Ca elements deposited may be mainly affected by physiological factors for L.polyactis during the early development stage,including the transformation of individual development stage and the synthesis of nutrients.The results of this study are highly important for improving essential basic data on the hatching,growth,and age and further research on the early life history and population dynamics of L.polyactis.