Fluctuation Characteristics Of Biological Catches Of Two Main Upper Economic Fisheries In The East China Sea Under Environmental Impacts

With the depletion and severe decline of demersal and near-demersal fish resources,the generations of mackerel（Scomber japonicus）and anchovy（Engraulis japonicus）have changed rapidly,with strong resource self-repair capabilities,and typical r-strategy life history types.Upper economic fishes play an important role in the marine ecosystem and make outstanding contributions to our country’s marine fishing and the supply of aquatic products.The East China Sea is a highly productive fishing area in the world’s oceans,which also is the sea area with the highest utilization rate of offshore continental shelf fishery development in our country.In recent decades,the output of mackerel in the East China Sea has taken 2011 as a turning point,showing a trend of volatility first and then a slow decline.While the yield of anchovy has continued to decline,it has shown obvious fluctuation characteristics.In addition to high-intensity fishing operations,environmental pollution in waters,global warming,and deterioration of the living environment of fish have also caused significant fluctuations in the catches of these two economic fishes.Besides the above factors that have obvious effects on the changes in the catch of fishery resources,this study mainly discusses the differences in the degree of influence and response characteristics of fishing effort and marine environmental factors.According to the 35 years of East China Sea mackerel and anchovy marine fishing yield data from 1985 to 2019,combined with the fishing effort and Sea Surface Temperature（SST）,Pacific Decadal Oscillation（PDO）Index,Typhoon Landing Level（TIOT）and El Ni（?）o 3.4 Index level（TIOE）in this sea area,using the generalized additive model（Generalized Additive Model,GAM）to analyze the fluctuations in the yield of mackerel and anchovy for the relationship between environmental factors,the factors that are significantly related to mackerel and anchovy are screened out.Random Forest（RF）was used to compare the relationship between the changes in the catches of two economic fishes and the aforementioned marine environmental factors.The comparison of yield fluctuations of mackerel and anchovy was analyzed by two models,and the results showed that:1.Through the F test,the fishing effort is significantly correlated with the fluctuations in the output of the two economic fishes in the GAM model,and the fishing effort has a very significant impact on the changes in the output of mackerel(P<2×10^-16);the change in anchovy output did not reach a significant level（P<0.1）.2.GAM has screened four environmental factors,and the fitted model has a cumulative deviance explained rate of 67.5%for the fluctuation of mackerel yield.The largest contribution is sea surface temperature,with a deviation interpretation rate of39.5%;followed by El Ni（?）o index,the deviation explanation rate is 17.8%;the contribution rate of the Pacific Interdecadal Oscillation Index is the smallest,10.2%.The above three environmental factors have reached a significant level on the GAM model（P<0.05）,and the optimized GAM model expression is:In（Y）=s（PDO）+s（TIOE）+s（SST）+ε3.Through the screening of four environmental factors,GAM has a cumulative deviance explained rate of 72.2%for the fluctuation of anchovy yield after the fitted model.The largest contribution is the Pacific Interdecadal Oscillation Index,with a deviation explanation rate of 40%;followed by landing level of typhoon,the deviation interpretation rate is 22.2%,and the contribution rate of sea surface temperature is the smallest,which is10.2%.Among them,the Pacific Ocean Interdecadal Oscillation Index and the typhoon landing level did not reach a significant level（P>0.1）,and the sea surface temperature was significantly correlated with anchovy yield（P<0.05）.The optimized GAM model expression is:In（Y）=s（PDO）+s（SST）+s（TIOT）+ε4.Using the RF algorithm to fit the output of mackerel and anchovy with PDO,SST,TIOE,and TIOT respectively,the study found that when PDO<-10,the output of mackerel remained above 21×10⁴t;when 22.0℃<SST<22.6℃,the output of mackerel showed an increasing trend;when the TIOE remained in the range of-5 to 3,the output of mackerel maintained a relatively high level;when TIOT>37 or TIOT<25,the yield of mackerel showed greater fluctuations.The size of the PDO index directly affects the yield of anchovy,with obvious fluctuations;when 22.2℃<SST<23.2℃,the yield of anchovy maintains an increasing trend;when the TIOE index is negative,the yield of anchovy is above the level of 12×10⁴t.When the value is positive,the yield starts to decline;the year when the typhoon is formed will affect the yield of anchovy.When TIOT>0,the yield of anchovy begins to fluctuate to varying degrees.Exploring the differences between the two models in the comparison of environmental factors and yield fluctuations shows that GAM can more accurately screen out the factors that have significant impacts,remove the weakly correlated factors,and present them as a hyperbola in the final fitting graph;However,RF can contain more factors.Compared with the factors eliminated in GAM fitting,RF can fully express any factor,explore the more complicated relationship between environmental factors and yield,and finally fit the graph.In addition to the curve,the nuances also describe the influence of factors on the yield in the form of broken lines.This study explores the response characteristics of Scomber japonicus and Engraulis japonicus yield in the East China Sea under fishing effort and different marine environmental factors impacts,so as to obtain the difference in the degree of impact of different environmental factors on fishery resources,and provide a basis for assessing the impact of marine environmental factors in the East China Sea on main fishery resources.