Extraction Of Mesoscale Features Based On Remote Sensing And Spatial-temporal Distribution Of Lighting Fishing Boat In South China Sea

The South China Sea（SCS）is located on the south side of the Chinese mainland,the western Pacific Ocean and it is the largest and deepest sea in China.SCS is rich in biological and mineral resources.There are thousands of fish species in SCS,including economic species such as pseudosciaena crocea,mackerel,tuna,grouper,black pomfret fish,silver pomfret etc.It is estimated that SCS has a potential yield of about 6×10⁶⁷×10⁶ t.The natural gas reserves in the SCS are about 1×10¹³m³,and the oil reserves are up to about twenty billion tons,which are 12%of the global petroleum reserves,and about 1/3 of the total oil resources of China.Therefore,SCS play a vitally important role in the field of marine economy,marine ecology,resources and energy protection,military,political etc.in China.In this paper,a series of research about the temporal and spatial distribution of mesoscale front,mesoscale eddy,nocturnal fishing boats as well as their correlation in the SCS and its adjacent area（0°N-30°N,105°E–130°E）was carried out,which are aimed at providing effective technical means and theoretical for fishery activities planning,management and monitoring of fishery behavior etc.The main research contents,methods and conclusions of this paper are as followed:（1）Based on the sea surface temperature（SST）remote sensing data,and according to the morphological character of mesoscale front on SST image,a front extraction method based on image segmentation is established,and is used to extract the mesoscale front of SCS which generated in 2014.Then the frontal length,average frontal intensity and total frontal intensity of SCS in every week are censused.The results of statistical analysis show that:the SST front in the SCS is mainly distributed in the north and northeast of the SCS,and it is stronger in winter than in summer.The total frontal length reaches the highest value（4.6440×10³km）in 4th weeks,and the lowest value（1.0710×10³km）is in 25th week.The maximum frontal value of average strength is 0.0668℃/km,in 5th week,while the minimum is 0.0341℃/km,in 33th week.The maximum frontal total strength（26.3158℃/km）and the minimum（1.4311℃/km）are in 5th and 33 th week,respectively.Before summer,the fronts in SCS move to northeast along the coastline of China,and the fronts in the north and northeast of SCS broken gradually or even disappeared.In summer,the fronts in the northern and northeastern SCS is greatly weakened.Meanwhile,there are series of transient break fronts in Midwest of SCS.Subsequently,the fronts in northern SCS reformed after entering the autumn.（2）The hybrid detection method（HD）was improved and was used to extract the mesoscale eddies of SCS which generated in 2014 based on the sea surface level anomaly（SLA）remote sensing data.Afterwards,this paper research the spatial distribution of eddies in SCS and census their number and total area in every week to analyze the variation of eddies in SCS.The results of statistical analysis show that:there are eddies distribute widely in SCS throughout the year and show a northeast-southwest distribution pattern.The mesoscale eddies with large area are mainly distributed in the east of the Luzon Strait,both sides of the island of Luzon and the east side of Indochina Peninsula.There are more eddies in SCS during winter and spring but less in two seasons in summer and autumn.The highest number of eddies in SCS appeared in 9th week（spring）,and it was 111,while the lowest value appeared in 42th week（autumn）,65,and the number of cold and warm eddies is similar.However,it show opposite variation trend in the total area of eddies.The total area of eddies is greater in summer and autumn.The maximum value of the total area appears in 40th week（autumn）,while lowest value appears in the first week（winter）.Furthermore,the total area of the cold eddies is usually greater than warm eddies,and compared with the warm eddies,the seasonal variation of the cold eddies is more obvious in total area.（3）According to the radiation and morphological characteristics of the nocturnal fishing boat on the VIIRS visible and infrared remote sensing images,a method of nocturnal fishing boats detection that based on the VIIRS nighttime remote sensing data is established,and it is used to detect the nocturnal fishing boat of SCS in 2014,then the distribution density of the nocturnal fishing boat is calculated.At the same time,this paper study the spatial distribution of f nocturnal fishing boat density and the temporal variation about the number of nocturnal fishing boats.The results of the study show that:The number of nocturnal fishing boats in SCS showed obvious bimodal distribution,the first peak appeared in March,where 5227 light spots from nocturnal fishing boats were detected,second peak appeared in September,where 6473 light spots from nocturnal fishing boats were detected.The fishery activities in SCS are mainly carried out in spring and autumn,and it is more dynamic in autumn.The nocturnal fishing boats in SCS are usually block or zonal distribution,and show obvious aggregation.The nocturnal fishing boats in SCS are mainly distribute in the Beibu Gulf,southeast of Central South Peninsula and northeast of SCS.In addition,the distribution of nocturnal fishing boats in SCS show obvious seasonal variation.（4）According to the information of mesoscale fronts,mesoscale eddies and nocturnal fishing boats in SCS which were extracted in this research,this research analyze the correlation between the mesoscale front or mesoscale eddies and nocturnal fishing boats.This study find that the nocturnal fishing boats in northeast of SCS,Taiwan Strait and its adjacent areas are affected by fronts and cold eddy simultaneously,the nocturnal fishing boats usually distribute along the front centerline,and there is usually a concentrated regions of nocturnal fishing boats near the cold eddies in these area.Nocturnal fishing boats in the middle and southwest of SCS are mainly distributed near the cold eddies in winter.However,nocturnal fishing boats in these area are closely related to the warm eddies in summer and autumn,and usually distribute in the dense eddies area.The innovative work and achievements of this research are as follow:（1）Aiming at the fact that the image of marine hydrological factor is characterized by weak edge,a front extraction method based on image segmentation is established.Compared with the traditional front extraction method,the front extraction method based on image segmentation have an advantage on accuracy and continuity of the extraction result,and the front lines extracted by this method are more smooth.（2）This research find that the temporal variation about number and total area of eddies in SCS are different,even the opposite.Furthermore,the cold eddies are similar to the warm eddies in quantity and variation tendency of quantity,while the total area of the cold eddies is usually greater than that of warm eddies,and the seasonal variation of cold eddies’total area is more obvious.（3）The detection method of nocturnal fishing boats based on VIIRS nighttime remote sensing data is established in this research.This method takes into account morphological characteristics and radiance characteristics of the nocturnal fishing boats lighting spots in VIIRS nighttime image.It conduct the preliminary screening of candidate pixels according to the morphological characteristics,this process improves the accuracy of extraction result and reduces the influence of high radiance that cause by moon light or cloud.In addition,it is able to reduce the computation cost of threshold selection,thus improving the efficiency of detection of nocturnal fishing boats effectively.（4）This paper analyze the relationship between the spatial distribution of nocturnal fishing boats and mesoscale fronts or mesoscale eddies.And it is found that the nocturnal fishing boats in northeast of SCS,Taiwan Strait and its adjacent areas are influenced by fronts and cold eddy simultaneously.In the middle and southwest of the South China Sea,the distribution of nocturnal fishing boats in these area are more related to the cold eddies in winter,while during summer and autumn,the distribution of nocturnal fishing boats in these area are closely related to the warm eddies.