Research On The Relationship Between Key Environmental Factors And Typical Fishery Resources In The Northwestern South China Sea

Marine fishery resources are national strategic resources and an important guarantee for national food security and high-quality animal protein supply.As a big marine country,it is particularly important to build a green and information-based modern marine fishery industry featuring a "blue granary".On the one hand,due to the rapid growth of coastal population in my country in recent decades,the rapid development of industry and economy,the continuous increase of offshore overfishing and environmental pollution,the increasing pressure on fisheries,and the continuous decline of economic fish resources have brought enormous pressure to the development of my country’s fishery industry.On the other hand,the marine environment is the space for marine fish to live and move,and key marine environmental elements include temperature,salinity,turbidity,nutrients,ocean currents,and heavy metal content that affect the spawning and feeding of fish,distribution and agglomeration and other activities also directly affect the economy and quality of my country’s fishery industry.Therefore,the research on the relationship between Marine environmental factors and Marine fishery species,yield and distribution is not only an important scientific issue to understand the relationship between Marine environment and fishery resources,but also of practical significance to strengthen the development of China’s Marine fishery industry and the construction of "blue granary".Based on the Marine environment remote sensing data(sea surface temperature,chlorophyll-A concentration,suspended sediment concentration,wind wave and surge wave,seawater velocity)and fishery resource yield data of the northwestern South China Sea from 2010 to 2019,this study studied the relationship between key Marine environmental factors and typical fishery resources in the northwestern part of the South China Sea.Firstly,based on years of satellite remote sensing data,using the empirical orthogonal function decomposition method,the seasonal temporal and spatial characteristics of key marine environmental elements in the northwestern South China Sea were analyzed.Then,the response of changes in typical fishery resources in the northwestern South China Sea to the marine environment was discussed.Based on the 2012-2018 typical fish resource survey and production data of blue round Decapterus maruadsi in the northwest of the South China Sea,using BP neural network model(Back Propagation Neural Network,BPNN)and generalized additive model(GAM),the blue round Decapterus maruadsi was developed.Prediction and analysis of the resource abundance of Decapterus maruadsi,and further explored the relationship between key marine environmental elements and the abundance of fishery resources in the South China Sea.The main research results of the paper are as follows:(1)The key marine environmental elements in the northwest waters of the South China Sea show significant seasonal changes.The sea surface temperature(SST)in the northwestern part of the South China Sea is distributed in a step belt from north to south,with the highest SST in summer,SST range of 28-30 ℃,and the lowest in winter,SST range of 18-26 ℃;sea surface salinity(SSS)in the north The bay shows a gradual increase from the northwest to the southeast sea area,and the sea area to the east of Hainan Island shows a gradually decreasing trend from the northeast to the southwest.The SSS was higher in winter and spring,ranging from 33-34.1,with obvious step distribution,and lower in summer and autumn,ranging from 32.6-33.6,with weak step distribution;chlorophyll a(CHL-a)concentration and suspended sediment concentration(SSC)were in the northwestern part of the South China Sea has obvious seasonal characteristics,with the highest in winter and the lowest in summer.In the coastal waters,the four-season variation is large,the variation range of CHL-a is greater than 1mg/m3,and the variation range of SSC is 5-50 mg/L.The variation range of SSC is less than 1 mg/L;the wind wave direction(MDWW)and the swell wave direction(MDTS)in the northwest of the South China Sea are the same in the four seasons.Affected by the monsoon,the wave direction is south-north in summer,and northeast-in other seasons.Southwest or East-West orientation.The wave height tends to decrease gradually from northeast to southwest.The wave height is the largest in winter and the smallest in summer,and the swell wave height is generally larger than that of wind waves.The northwestern part of the South China Sea has the highest velocity in winter and the lowest in summer.The four seasons in the Beibu Gulf are relatively small,and the velocity in the southeast of Hainan Island The overall size is relatively large,and a fast-flowing seawater has developed for a long time on the southeast coast of Hainan Island.Whether it is summer or winter,a strong current develops near Vietnam in the central waters of the South China Sea.The EOF modal results in the past 10 years show that the key marine environmental elements in the northwest of the South China Sea have different temporal and spatial characteristics in 2010-2012,2012-2014,2014-2015 and 2018-2019,and the temporal modal coefficients have obvious abnormal fluctuations in 2015-2016 and 2018-2019.(2)The fishery output in the northwest waters of the South China Sea showed an upward trend from 2010 to 2016,and a significant downward trend from 2017 to 2019.Among them,the catches of Trichiurus haumela and Decapterus maruadsi in the three provinces showed a larger downward trend in 2018 than in 2017.The catch of Decapterus maruadsi and Nemipterus vigatus decreased in 2015,and the catch of hairtail and goldfish reached the maximum value in 2016.The analysis shows that in2017,the Ministry of Agriculture researched and issued three reform policies for marine fishery resource management,namely,the “13th Five-Year Plan” period,the “dual control” management of marine fishing vessels,and the management of total marine fishery resources,resulting in a significant reduction in human fishing activities.Probably the main reason for the decline in capture production in 2017-2019.The analysis shows that the temporal and spatial distribution of key marine environmental elements in the northwest of the South China Sea in 2015-2016 and 2018-2019 has different variation characteristics from previous years.When the abnormally high SST,SSS,and CHLa in the northwestern South China Sea,the north-south direction of the MDWW,the decline of the SSC in the coastal waters,and the weakening of the vortex in the far sea in the southeast of Hainan Island and the central part of the South China Sea,it may be unfavorable to the growth of the Trichiurus haumela,Decapterus maruadsi and Nemipterus vigatus in the northwestern South China Sea.(3)The GAM model can achieve a high-precision prediction of the resource abundance of blue round Decapterus maruadsi in the northwest of the South China Sea.Based on the 2012-2018 survey and production data of the Decapterus maruadsi resources in the northwestern South China Sea,the BPNN and GAM methods were used to establish the spring,summer,autumn and winter Decapterus maruadsi fishery resource abundance prediction models.The results show that the GAM model has the highest prediction accuracy and involves with fewer factors,it is the best model.BPNN’s forecasting ability is slightly worse,but in trend,its forecasting results are more consistent in summer and winter.(4)The relationship between resource abundance and key marine environmental elements in the northwestern South China Sea was different in different seasons.In spring,the Decapterus maruadsi CPUE in the northwestern South China Sea was affected by SST,MDTS,MDWW,SSS and Eastern Sea Water Velocity(ESWV)to varying degrees.Among them,MDWW contributed the most,with a contribution rate of 26.4%,followed by SSS,then ESWV;in summer,CPUE is affected by CHL-a,MDTS,SSS,SSC and ESWV to varying degrees,among which,SSS contributes the most,with a contribution rate of 28.00%,followed by ESWV,then MDTS;in autumn,CPUE is affected by SST,MDTS and SSS to varying degrees.Among them,SST has the largest contribution rate,with a contribution rate of 24.00%,followed by SSS,and then MDTS;in winter,CPUE is affected by SST,CHL-a,MDWW,SSC and northward seawater velocity Different degrees of influence,among which,MDWW has the largest contribution rate,with a contribution rate of 15.00%,followed by CHL-a,and then SSC.