Biology And Habitat Characteristics Of The Purpleback Flying Squid Sthenoteuthis Oualaniensis In The South China Sea

The purpleback flying squid(Ommastrephidae:Sthenoteuthis oualaniensis)has high development potential of fishery economy cephalopods resources in the South China Sea,while because of its short life cycle,fast growth,strong fecundity,inhabiting high trophic level ecosystem,complex population structure and environmental sensitivity,it is difficult develop and manage the fishery resources rationally and sustainably.Hence,it is of great scientific and socio-economic interest to understand the biological characteristics and nature habitat of S.oualaniensis in the South China Sea.In this study,the biological samples and fishing yield data of S.oualaniensis was sampled monthly throughout 2018 and February 2019 in the South China Sea.The basic biological characteristics of this squid were reanalyzed,including population structure,reproductive biology,growth,feeding ecology and habitat characteristics.The mantle length frequency distribution was used to analyze the population structure,and morphological characteristics,mitochondrial gene(COI)and ecological features were used to determine the basis of population division of S.oualaniensis.In order to explore the variation of dialy-age growth and the growth rates,growth patterns were established based on the dialy-age estimation of S.oualaniensis otolith after grinding.According to the sex ratio,sexual maturity index and fecundity of S.oualaniensis,the breeding peak and season were investigated.Based on traditional stomach contents analysis and carbon and nitrogen stable isotopes analysis,the feeding ecology and trophic level of S.oualaniensis were analyzed,and the feeding habits at each growth stage were found out while the process of feeding transformation was speculated.Besides,generalized additive model(GAM)methods was used to analyzed the relationship between available environmental factors and catch per unit effort(CPUE)based on the combination of environmental remote sensing data and Catch data.(1)Population structure:The individuals of S.oualaniensis were relatively small,and there were no giant-form individuals in the South China Sea.Corresponding to the appearance of photophore,the purpleback flying squid could be divided into two forms the medium form and the dwarf form,and it was also indicated by significant differences in morphological characteristics.Based on the population genetic structure analysis of mitochondrial gene(COI),it was found that one lineage was totally composed of dwarf individuals without photophores while the other lineage was mainly consisted of individuals with photophores.To be mentioned,in the latter lineage the individuals without photophores were all immature,and it was most likely that photophores had not yet developed.Stable isotope analysis showed that there were no significant differences in?¹³C amoung the four groups,indicating that there were no significant differences in terms of trophic niche.In spite of the morphological differences,two forms of purpleback flying squid mixed together in terms of habitat space,food selectivity and developmental process,with no discriminate in resources management.(2)Growth:The mantle length and body weight power function indicated that S.oualaniensis displayed positive allometric growth in the South China Sea(growth parameter b larger than 3).The daily-age structure of S.oualaniensis(Mantle length range of 41–170 mm)was 45–90 days in the South China Sea,and the lifespan was shorter compared to squid in other seas.According to the mean daily-age of S.oualaniensis,the average daily increment could be calculated as 4 mm/day in the South China Sea.The sample in our study only represented adult S.oualaniensis in the South China Sea,and the logistic growth equation was most suitable to describe its growth pattern.The most suitable growth pattern for female and male purpleback flying squid were von Bertalanffy and power function respectively.The growth rates of mantle length and body weight of S.oualaniensis showed a decreasing trend.(3)Reproductive biology:The first sexual maturity of female S.oualaniensis was in the mantle length range of 90–109 mm and the second sexual maturity was in the mantle length larger than 160 mm,which might be related to the two populations of S.oualaniensis.Sexual maturation of male S.oualaniensis occured throughout growth stage,and no male individuals were found at the mantle length larger than 150 mm,indicating that the males had finished all sexual maturation,and sexual maturation might be an important indicator to separate the two forms.(4)Feeding ecology:S.oualaniensis mainly fed on fishes,followed by cephalopods and crustaceans.Analysis of feeding stages,empty stomach rate,repletion index and food composition of different mantle length S.oualaniensis individuals indicated that the mantle length at 100 mm was a signal of feeding transformation.For instance,the occurrence frequency of fishes and cephalopods increased gradually before this signal,and the empty stomach rate decreased gradually.After this signal,the fishes(Frequency of occurrence is 80.74%)and cephalopods(Frequency of occurrence is 65.68%)were the main food for S.oualaniensis and the empty stomach rate was close to 0.Crustaceans mostly maintained at about 20%in most stage,and the female S.oualaniensis no longer ate crustaceans when the mantle length was large than 160 mm.The variation of ¹³C and ¹⁵N values indicated that the S.oualaniensis had wider food sources and higher trophic level in the second half of the year.Stable isotope analysis of different mantle length groups indicated that feeding transformation may gradually occur in the range of 100–160 mm of female S.oualaniensis.(5)Habitat traits:The sea surface temperature(SST)was the most important factor in the GAM analysis,and the best fitting GAM model explained 62.6%of the variance.In our study,the sea areas with higher CPUE were mainly distributed in 12°–13°N in latitude direction,while the variation of CPUE in longitude direction was not obvious.The results of our study showed that CPUE was higher when SST was in the range of 27–28°C,chlorophyll-a concentration(Chl-a)was>0.18?g/l,and sea surface height anomaly(SSHA)was in the range of-0.05–0.05 m.According to the analysis on suitable habitat conditions of S.oualaniensis,the suitable period for SST,Chl-a and SSHA was in February and March,from July to next March,and from January to March,respectively.Under the influence of high temperature,the study sea area appeared poor nutrition from April to June.