Density Distribution And Habitat Model Construction Of The Jack Mackerel(Trachurus Japonicus) In The Northern South China Sea

Jack mackerel(Trachurus japonicus)is a warm-water pelagic fish mainly caught in China's offshore areas.It is one of the fishing objects of light trapping nets,trawl nets and coastal fixed nets.Jack mackerel was once the target of the fishing nets of both Yellow Sea and the Bohai Sea.Thereafter,the catches were rapidly declined.In 1962,almost no fish were caught.However,the catches in the South China Sea had generally increased from the late 1970 s to the early 1980 s.According to the survey of the margin of the northern part of South China Sea in the late 1970 s,the density of jack mackerel in the depth of 90-200 meters was higher,and the estimated resources were about 27,134 tons.It was reported in 2016 that the output of jack mackerel in the South China Sea was 25,000 tons,indicating that the jack mackerel in the northern South China Sea are relatively stable and have a great potential for further development.Thus,a scientific and rational assessment of the status of jack mackerel and the spatial layout of fisheries is an important basis for the sustainable maintenance and management of this valuable resource.Therefore,the data used for study was from eight cruises seasonal survey data of fishery resources in the northern South China Sea from 2014 to 2017.Based on geo-statistics theory and spatial interpolation method,we analyzed the distribution characteristics and heterogeneous structural mechanism,and examined the spatial and temporal layout of jack mackerel in the northern South China Sea.At the same time,in order to further understand the ecological characteristics of the species and to improve the fishery management,in this study,we explored the characteristics of the probability distribution of the density of the jack mackerel.In addition,according to the surveys data,we explored the relationship between the jack mackerel and the marine environment,established an optimal Habitat Suitability Index(HSI)model,reasonably analyzed and predicted the habitat of the jack mackerel.The main conclusions are drawn as follows:(1)Temporal and spatial density distribution characteristics of jack mackerel.Due to the non-uniform distribution of jack mackerel in the ocean,the heterogeneous structural features were further analyzed by geostatistical theory.It was found that the spatial heterogeneity of the jack mackerel was higher,and the spatial autocorrelation structural ratio was above 75%.Thespatial variability was dominated by Gaussian distribution,and the spatial correlation distance(variation range)was with obvious seasonal characteristics atabout0.52°.On the other hand,the spatial heterogeneity of the northwest-southeast direction was more significant,indicating that the ocean dynamic process in this direction had an important impact on the migration of the jack mackerel.In addition,based on Kriging interpolation analysis of the spatial distribution,we found that it had obvious characteristics of patchy distribution.Also,jack mackerel was susceptible to extreme climate(e.g.ElNino,La Nina,and other extreme events).(2)Probability distribution of the jack mackerel density.In order to further understand the density distribution patterns of jack mackerel resources and to reveal the ecological characteristics of the species,we explored 11 probability distribution features by Kolmogorov-Smirnov Test(K-S test).The results showed that jack mackerel in continental shelf area of the northern South China Sea did not show obvious distribution patterns,whereas the density in the Beibu Gulf followed the lognormal theory(log normal model and Delta normal model),and the Delta normal model was more suitable for actual distribution of jack mackerel,and further optimized the resource assessment work.(3)Construction and verification of the jack mackerel habitat model.The spatial layout of jack mackerel is regulated by the marine ecological environment.The relationships between the jack mackerel and the marine environment(sea surface temperature,salinity and chlorophyll)were analyzed.The four general methods,including the maximum method,the minimum method,the arithmetic mean method and the geometric mean method,were used,respectively.The habitat suitability index(HSI)model was constructed in all the seasons.The results showed that the geometric mean method was better for jack mackerel HSI modeling and the model accuracy was about 92%,where the largest contribution was SST.The HSI model based on the geometric mean method can better analyze and predict the jack mackerel density and fishery.