| The small yellow croaker,Larimichthys polyactis(Perciformes,Sciaenidae,Larimichthys),is a demersal fish,and is extensively distributed in the East China Sea,Yellow Sea,Bohai Sea,and waters west of Korea.It is one of the target fishery species which are commonly caught by bottom-trawling,canvas stow netting,set-netting,and gillnetting fisheries in China,Japan,and Korean Peninsula.This species have recently become the most commonly overexploited fish in China.Study on fish population and migratory is essential to understand fish ecology and resources,and the results will paly important roles in conservation of overexploited fish stocks.So far,the traditional capture,morphometric analysis,and advanced molecular approaches have been used to investigate the population biology and migration of small yellow croaker in the East China Sea,Yellow Sea,and Bohai Sea and many research achievements have been obtained.In contrast,the knowledges of resources dynamics are very limited on the Sciaenidae species in waters of Jiangsu Province.In this doctoral dissertation,the resource variations and utilization of L.polyactis in sea waters of Jiangsu Province were analyzed comprehensively based on large quantities of data from long-term capture surveys and otolith morphometrics,microchemistry studies.It is hoped to integratedly and objectively grasp the resources dynamics and migratory characterastics of this important commercial fish.Monthly capture survey data of L.polyactis from the southern Yellow Sea in 2003 and 2013 suggested that L.polyactis might migrate from Shawai fishing ground to Dasha fishing ground in spring(March-May)and distribute in the central and western Dasha fishing ground in April and May.Moreover,L.polyactis in Dasha fishing ground might migrate from west to east to offshore wintering grounds in autumn(September-November)and reach the central Dasha fishing ground in October and November.In winter(December-February)L.polyactis might enter western waters of the Shawai fishing ground.The distribution of this fish in the southern Yellow Sea seemed consistent with those reported in previous literature,i.e.,the stock migrated between the wintering grounds in the west of Jeju Island and Lvsi spawning grounds.During the decade of 2003-2013,average body length of adult L.polyactis experienced a tendency from decreasing to increasing.The sex ratios(the female to male ratio)were less than 1 in both two years.The structural composition of L.polyactis was investigated in overwintering ground of the southern Yellow Sea,by 10 gillnets with various net heights(6 m and 10 m)and mesh sizes(35 mm,40 mm,45 mm,50 mm and 55 mm).The age 0+ and age 1+ groups dominated the catchs(i.e.,42.63% and 52.80%,respectively).The sex ratio was found to be size and age dependence in the gillnet catches.Number of males decreased with the increase of fish body length and age of the catches.Nets of 10 m height caught selectively more age 0+ and male individuals than those of 6 m height.The present study suggested that the selective fishing of 10 m height gillnet was less effective for preventing the recruitment fishing and sex-selective fishing of the croaker.For further understanding the recruitment process of L.polyactis in coastal waters of Jiangsu Province,this study systematically investigated the spatial and temporal patterns of juvenile recruitments and their dispersal using data from the setting-nets catches.The abundance of juveniles progressively increased in the middle of May,and reached a peak during the June to the middle of July(with the juveniles accounting for the 90% of the catches),and then this ratio decreased less than 20% until the early-middle of September.The start of recruitment events was observed generaly around 9 May,with a peak around 16 May.The end of the events was usually around 12 September.The duration of juvenile recruitment events could last about 4 months.The changes in otolith(sagitta)shape from juvenile to adult L.polyactis showed a lobed process.The cauda of otolithic central sulcus curved toward the ventral margin.An exponential correlation was observed in the otolith morphological parameters(e.g.long,wide,circumference,square)/fish body length relationship.In juveniles,these parameters would increase with the increase of body length.Furthermore,the parameters were significantly different between the different ontogenetic stage groups of the juveniles with different body length,likely associating with the formation of otolith accessory growth centers from larva to juvenile.However,there were show no significant changes of the parameters between adults(body length?130 mm),and the otolith of might acquire its definitive shape in L.polyactis at maturity stages(body length?130 mm).An electron probe microanalyzer(EPMA)was applied to analyze the element Sr and Ca microchemistry patterns in otoliths of L.polyactis from sand ridges of Jiangsu's sea waters.Generally,the highest Sr concentration was found in core areas while a decreased concentration was found in the remaining areas.Based on the above results,the habitat history of L.polyactis could be reconstructed.Namely,high salinity habitat seemed to be necessary for the hatching and post-hatching development,while decreased salinity habitat seemed to be preferable for the growth and development at the remaining stages of life history.Some individuals in this study had even experienced much low salinity habitat in early life history period.We compared the Sr:Ca ratios of L.polyactis with those of Miichthys miiuy(in the same ecological niche with L.polyactis)and Pampus argenteus(in the different ecological niche with L.polyactis)caught in the same water areas.The salinity habitats of M.miiuy and P.argenteus reconstructed by otolith microchemistry coincided with the salinity regimes used in their aquaculture,indicating the Sr/Ca ratios for different salinity regimes could be utilized to reconstructe the salinity habitats of L.polyactis from coastal waters of Jiangsu Province.The otolith microchemistry of L.polyactis from sand ridges of Jiangsu's sea waters at different development stages of juveniles showed that the high Sr/Ca ratio occurred from the core to 15-20 ?m,corresponding to the time from the hatching around 5-day-old.The Sr/Ca ratio regime shifted remarkably at both the 15-20 ?m(5-day-old)and 400 ?m(35-day-old)outside of otolith core,implying the variations of different salinity habitats.Based the results of capture survey and otolith microchemistry,the hypothesized migration pattern of the juveniles would be that the croakers dispersaled from the early of April to the early of May,then inhabited in the radiative sandy ridge to feed,and followed migrating outwards until the late of June.The spatiotemporal distribution of these earliest stages of L.polyactis might depend upon local hydrodynamic processes of the sand ridge areas at the time of spawning and initial development.Inter-annual variation of otolith Sr/Ca ratios of L.polyactis were compared by a decade(2003,2013)and a two year(2012,2013)period's individuals.All otoliths of L.polyactis showed the similarity Sr/Ca ratio patterns with high values around the core area whereas the low values in remaining part.These results implied the elemental geochemical signatures in the otolith were relatively stable across years.In summary,the present study speculated the dispersal patterns and migration characteristics of L.polyactis in sea waters of Jiangsu Province,based on the results of macro capture surveys and micro otolith determination.At the same time,this study further demonstrated the feasibility and potential application of otolith microchemistry in reconstruction of habitat history for marine fish(including L.polyactis).All the achivements will establish a sound foundation for the future corresponding researches. |
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