| Uroteuthis edulis belongs to the Loliginidae family and Uroteuthis,which are widely distributed in the western Pacific Ocean and the coastal waters of eastern Africa.Since the 1990 s,U.edulis has gradually replaced Sepiella inermis and has become one of the dominant economic cephalopod resources in the East China Sea.Swordtip squid grows faster,reproduces throughout the year and lays eggs.It feeds on small crustaceans and scombroid juveniles,and is also preyed by marine mammals.Clarifying the population structure,age,growth,and migration characteristics is the basis of cephalopod fishery biology research and one of the prerequisites for assessing the status of the fishery resources.To this end,this research was sampled in the East China Sea(123 °E～127 °E,28 °N～31.5 °N)from September 2017 to March 2018 and September 2018 to March 2019.The randomly collects swordtip squid samples every month to analyze and compare their individual biological characteristics,Exploring the population structure,daily age composition and mantle length of the swordtip squid through statolith information;and analyze the relationship between the growth of dominant groups and environmental factors;and analyze the composition and differences of statolith trace elements among dominant groups,and combines environmental factors to explore the distribution of potential spawning grounds and feeding grounds of dominant groups in order to better grasp the life of swordtip squid historical characteristics.The main results are as follows:(1)Results have shown that the dominant mantle length and body weight group of male individuals are 100-180 mm and 40-160 g,respectively,females are 80-160 mm and 40-160 g,respectively.There was a significant difference in body weight-mantle length relationship between males during the year(P<0.05).In addition,there was a significant difference in body weight-mantle length relationship between male and female in 2017.Male mature length is mainly concentrated in individuals larger than 120 mm,female is large than 140 mm,and the tendency of 7maturation can be observed under a small mantle length.The mantle length and body weight growth rate of female individuals were faster than males,and the weight growth rate of male and female individuals varied greatly from year to year.According to the comprehensive analysis,the populations of the swordtip squid sampled over the past two years were all spawning populations in spring and summer,and the number of larvae in the spawning population in summer of 2018 is large,and the changes in the habitat environment are the key factors affecting the biological characteristics of the population.(2)Results showed that there were differences in population mantle length and body weight composition between years,but there was no significant difference between genders(P>0.05).It was found by reading the age of otoliths and performing inverse calculation that the two-year dominant day-age group was 190-220 d,and there was significant differences in age composition of male during the year(P<0.05).The inverse calculation results showed that the peak hatching period was from March to May and August.The spring and summer spawning groups are the dominant hatching groups.The mantle length and body weight growth of the spawning group in spring were best fitted with a logistic curve,and the spawning group in summer were best fitted with a Von-Bertanffy and a logistic curve.In 180-270 d,the mantle length and body weight growth rate of spring spawning group was faster than that of summer spawning group.After 240 d,the male mantle growth rate was faster than that of female,and the body weight growth rate was slower than that of female.The age of the 50% maturity of the spawning population in spring is younger than that of the summer spawning group,and male individual is younger than the female.According to comprehensive analysis,the growth differences of different spawning groups may be related to the environmental factors and growth mechanisms of the habitated sea areas.(3)Results show that the statolith growth increments was composed of light and dark rings,the postnuclear zone,dark zone and peripheral zone were divided according to the width and brightness of the growth increments.According to the statolith age,it was found that the spawning population in spring and summer was the dominant cohort,and the morphological parameters of the statolith between the two cohorts were significantly different(P<0.05).Principal component analysis shows that the total length(TSL)and maximum width(MW)of otoliths can be used as indicators to characterize the length and width of statoliths in spring and summer spawning cohorts.The TSL growth function of the spring and summer spawning cohorts was significantly different in two years(P<0.05).The spring spawning cohorts were best fitted with Logistic and logarithmic growth functions,respectively,and the summer spawning cohorts was Logistic and linear functions.The MW growth function of the spawning population in spring and summer showed no significant difference between the two years(P>0.05).The logistic growth function was the best fit and the gender difference was not significant(P>0.05).In age of 180-240 d,the growth rate of spring and summer spawning cohorts both showed a downward trend,and the growth rates of TSL and MW in spring spawning cohorts were greater than those in summer.(4)The relationship between the daily growth of the statolith of the U.edulis and environmental factors.Temperature of 25 m(Temp＿25),sea surface temperature(SST)and zonal velocity(ZV)are the key environmental variables that affect the daily growth of statoliths in the spring spawning cohort(peak in April),the daily increment of statoliths has an effect on SST(20-22 ℃),Temp＿25(24 ℃),SST(25 ℃)and ZV(0.07 m/s)have strong threshold responses.Temp＿25 and SST in the range of24-27 °C and ZV in the range of 0.1 ～ 0.3 m/s were positively correlated with daily otolith growth.SST,Temp＿25 and Mixed Layer Depth(MLD)are the key environmental variables that affect the daily growth of statoliths in the summer spawning cohort(peak in August),and the daily increment of statoliths has an effect on SST(20-22 ℃),Temp＿25(21-23 ℃)and MLD(55-60 m)have strong threshold responses.Temp＿25 and SST in the range of 22-28 ℃ and MLD in the range of 0-50 m were positively correlated with daily otolith growth.Key environmental variables for different spawning cohorts indicated that early growth was influenced by seasonal variations in water temperature,Velocity,and prey abundance.(5)Results showed that calcium(Ca),sodium(Na),strontium(Sr),iron(Fe)and magnesium(Mg)were the top five elements in the spawning cohort in spring and summer,and there were significant differences between the two spawning cohort(P<0.05).Ca,Na,Barium(Ba),Sr,Cobalt(Co),manganese(Mn)and lithium(Li)were selected as the key trace elements in two cohorts.Water temperature,salinity and mixed layer depth(MLD)were the key environmental variables affecting element content changes.In different growth stages of life history,the values of Sr/Ca,Ba/Ca and Li/Ca of the spawning cohort in summer were higher than those in spring,while the values of Na/Ca were lower than those in spring.The values of Sr/Ca and Ba/Ca of the two spawning populations reached the maximum value in the embryonic period,and the differences of Sr/Ca,Ba/Ca and Li/Ca between each growth stage were not significant(P>0.05).The hatching water temperature of spring and summer spawning cohort was 19～24 ℃ and 18～22 ℃,respectively.The spawning ground of summer spawning cohort was more located in the central and northern part of the East China Sea.Therefore,Ca,Na,Sr,Fe and Mg elements are the potential elements of cohort discrimination,and Sr/Ca,Ba/Ca,Na/Ca and Li/Ca can be used as the potential indicators to construct the habitat history of swordtip of spring and summer spawning cohort. |
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