Study On Fishery Biology And Fishing Ground Changes Of Chub Mackerel (Scomber Japonicus) In The High Seas Of The Northwest Pacific

Chub mackerel（Scomber japonicus）is a warm temperate pelagic migratory fish with clustering habit,phototaxis and vertical movement characteristics.It is mainly distributed in the Western Pacific Ocean.It is rich in resources and has high edible value.The Japanese mackerel fishery has become one of the main fisheries in the high seas of the Northwest Pacific Ocean.At present,the countries and regions that have been fishing for chub mackerel in the area are mainly China,China’s Taiwan,Japan,Russia and South Korea.Japan has the longest fishing history in the region,dating back to the 1950s.The resources and catch of chub mackerel in the sea area have experienced great fluctuations.Since the 1970s,its output has increased first and then decreased,reaching a minimum of2.8×104 t in 1990,but since the year of 2010,the output has increased significantly,reaching a maximum of 53.9×104 t in 2017.On the whole,the age structure of chub mackerel landing in the Northwest Pacific Ocean is relatively reasonable,the yield accounts for a low proportion of the assessed resources,and has great development potential.However,in the statistics of the actual fishery production in the high seas of the Northwest Pacific Ocean of China,the proportion of chub mackerel in the catch has continued to decline in recent years.Since the development of chub mackerel fishery,the changes of its fishing ground have attracted the attention of some fishery scientists,but it has not been deeply analyzed.Based on the understanding of fishery remote sensing,fish ecology and chub mackerel fishery,starting from the fishery biological characteristics and fishery statistical data of chub mackerel,the resource abundance index is standardized,which changes the situation of ignoring the growth state of fish and using empirical parameters to calculate resource abundance in the past.Then,combined with fish growth and accurate resource abundance index,the changes of chub mackerel fishery in the Northwest Pacific are deeply analyzed based on a variety of geospatial models.At the same time,considering the influence of environmental factors,comprehensively evaluate the change of chub mackerel fishing ground.Carrying out the research on the change of chub mackerel fishing ground actively can not only provide data for fishery situation prediction and improve fishery production efficiency,but also provide theoretical support for scientific and effective fishery management.Based on the above research,the main conclusions of this paper are as follows:1)Based on the statistical data from 2014 to 2019,this paper makes a statistical analysis on the catch,the catch per unit effort（CPUE）and the main catch composition.At the same time,the seasonal variation of the distribution of the fishing ground gravity center is discussed through the method of gravity center of catch and cluster analysis.The results showed that:（1）The annual differences of catch yield and CPUE were large,which showed a trend of first increase and then decrease in each year,and the annual CPUE decreased year after year.（2）A total of 8 species of catch were found in the statistical year.Among them,Scomber japonicus was the main catch species,with a frequency of 100%.The Sardinops sagax was the second most frequent occurrence,with a frequency of 87.5%and the rest was small relatively.（3）The center of gravity of the fishing ground gradually moved to the Northeast from April,and turned back to the Southwest after reaching the Northeast end from August to October,showing a certain anticlockwise variation.In terms of years,the center of gravity of the fishing ground gradually moved to the Northeast,the center of gravity moved 0.92°East in longitude and 1.57°North in latitude from 2014 to 2019.（4）The clustering results showed that the monthly yield barycenters could be divided into four categories,one was July,August,September and October,another one was May and November,the others were April and June respectively.At the same time,the annual yield barycenters could also be divided into four categories,the first one was 2016,2017 and 2018,the second one was 2019,the third one was 2014,the final one was 2015,indicating that there were certain inter-annual and seasonal changes in the light purse seine fishing grounds in the Northwest Pacific.2)In order to deepen the understanding of the population structure characteristics of them,based on the samples of chub mackerel collected in the high seas of the Northwest Pacific from 2016 to 2021,the biological characteristics such as fork length,body weight composition,sex ratio,gonad maturity and feeding grade were studied.The results showed that the fork length of chub mackerel ranged from 102 to 400 mm,with bimodal distribution.The dominant group was 190 to 230 mm and 270 to 300 mm.The effect of gender on the fork length was not significant（P>0.05）.In the relationship between fork length and body weight,the growth parameter b was 3.2548,showing allometric growth.Gender had no significant effect on the relationship between fork length and body weight（P>0.05）.The ratio of female to male was 1.74:1,which was significantly different from 1:1（P<0.05）.Sexual maturity was mainly stage II,and fork length and body weight had a significant effect on gonadal maturity（P<0.01）.The curve fitting showed that the fork length of female and male chub mackerel at the first sexual maturity was 298.08±4.32 mm and 287.82±4.26 mm,respectively.The feeding intensity was mainly grade 1 to 3,which was significantly correlated with fork length and body weight（P<0.05）,and gender had no significant effect on it（P>0.05）.The fatness index ranged from 0.64 to 2.45,and increased with the increase of body length and weight.The results showed that chub mackerel grew well in the high seas of the Northwest Pacific Ocean and had good development potential.This study enriches the biological characteristics of chub mackerel in the Northwest Pacific.The results can provide a basis for population identification,life history research,resource development and utilization of chub mackerel in the Northwest Pacific.3)Chub mackerel is the main fishing species in the Northwest Pacific high seas（NWP）fishery and plays an important role in the ecosystem of the sea.In recent years,with the climate warming,the change of fishing power and the implementation of management system,the ashore catch yield of chub mackerel has changed.In this study,in order to understand the resource status of this species accurately,we first estimated the progressive fork length(l_inf)and K value of chub mackerel by using the ELEFAN I method,then obtained the theoretical initial age t₀ and inflection point age t_ip according to the empirical formula.At the same time,the resource status was evaluated by the length based Bayesian biomass estimation method（LBB）.The result showed that the progressive fork length of chub mackerel was 41.53 cm,the progressive weight was 1006 g,t₀=-0.46 a,t_ip=1.73 a,and the growth rate of body weight reached the maximum at this time.LBB analysis showed that L_mean/L_opt=0.99,L_c/L_opt=1.10,M/K=1.77,Z/K=1.83,B/B₀=0.951,B/B_msy=2.8.These parameters show that the total mortality of chub mackerel in the high seas of the Northwest Pacific Ocean is mainly contributed by the natural mortality,and there are a certain number of large individual groups.The proportion of developed biomass in the original biomass is low,and the resource structure is healthy,therefore,it has great development potential.This analysis shows that LBB model is an effective method for fishery resources assessment when there is a lock of fishery data and only fish length and frequency data are available.We believe that the results of this study can provide effective information for the development and management of Japanese mackerel fishery in the high seas of the Northwest Pacific,and at the same time,we suggest that the development of chub mackerel in the high seas of the Northwest Pacific can be appropriately strengthened.4)Based on the fishery production statistics of chub mackerel in the Northwest Pacific of China from 2014 to 2018,combined with the sea surface temperature,sea surface height,chlorophyll concentration,year,month,longitude and latitude,this chapter conducted a standardized study on the CPUE of chub mackerel in the Northwest Pacific based on the generalized linear model（GLM）and the generalized additive model（GAM）.The results showed that the best GLM model based on BIC criteria showed that year,month,longitude,latitude,sea surface temperature,sea surface height,Chlorophyll a concentration and the cross effects of year and month all significantly affected the CPUE of chub mackerel,with a deviation interpretation rate of 59.59%and R2 of 0.3552.The results of the best GAM model showed that all factors also had a significant impact on CPUE,with the variance interpretation rate of 63.71%and R2 of 0.4628.The results of 5-fold cross validation showed that the CPUE standardization of GAM model was better than GLM model,and was more suitable for the CPUE standardization of chub mackerel in the Northwest Pacific.（5）Based on catch data of high sea light purse seine fishery of China from 2014to 2019,this study analyzed the spatial and temporal patterns of chub mackerel fishing grounds in the Northwest Pacific using the global Moran index,Getis-Ord Gi*,center of gravity migration trajectory model,and standard deviation ellipse.The results showed that the catches of chub mackerel mainly concentrated in the area between 39～44°N and 147～155°E,the annual catches showed a trend of first increase and then decrease,the annual CPUE decreased year by year,the monthly catches showed a trend of first increase and then decrease,which maintained at a high level from June to October,and the monthly CPUE increased with month.The spatial autocorrelations of annually and monthly chub mackerel catches were significant,and the monthly autocorrelations were stronger than annually ones,indicated highly aggregated distribution of the fishing grounds.The distributions of chub mackerel hot spots and cold spots showed a certain spatial agglomeration,but their distribution patterns and the areas covered varied apparently with years and months.The gravity center of fishing ground generally showed northwest shift from year to year and,in term of seasonal changes,moved northwest from April to August,and then turned back to the southwest.The annual and monthly shifts in directions of fishing grounds were consistent,showing a southwest-northeast pattern with strong directivity and aggregation.The spatial correlation models used in this analysis presented a new look at spatial and temporal patterns of the chub mackerel fishing grounds,which may provide useful information for the rational development and utilization of the chub mackerel resource.According to the above conclusions,this paper believed that there was still a large space for the development and utilization of chub mackerel resources in the high seas of the Northwest Pacific.Chub mackerel had an absolute advantage in the composition of catches whoese proportion had decreased in recent years.Therefore,the accuracy of fishery prediction should be improved.From the perspective of individual development,the biological characteristics of chub mackerel showed that its population development is healthy,the fork length of the first sexual maturity was large,and there is no overfishing.In terms of resource parameters,the total mortality was mainly contributed by natural mortality,and there were a certain number of large individual groups.The proportion of developed biomass to original biomass was low,and the resource structure was healthy,which had great development potential.The trajectories of the chub mackerel fishery in the Northwest Pacific were similar in each year,but the center of gravity of the fishery tends to moved to the northwest.