| Mesoscale eddies are marine mesoscale processes characterized by rapid rotation,which are ubiquitous in the global ocean and have a significant impact on marine ecology.As a short-lived pelagic cephalopod,the squid(Ommastrephes bartramii)population is extremely sensitive to changes in the marine environment.The squid fishing ground is located in the Kuroshio-Oyashio transition zone in the Northwest Pacific Ocean,where eddy activities are intense.However,a comprehensive understanding of how mesoscale eddies affect squid populations in the Northwest Pacific is still lacking.In this study,an angular momentum eddy detection and tracking algorithm based on physical parameters and flow field geometric features established a eddy dataset in the Northwest Pacific Ocean(30°–50°N,140°–170°E)from 2009 to 2018.A total of4675 mesoscale eddies with a duration of more than 4 weeks were tracked,including2637 cyclonic eddies(Cyclonic eddy,CE;also known as cold eddies)and 2038 anticyclonic eddies(Anticyclonic eddy,AE;also known as warm eddies).There are certain interannual and seasonal changes in the number of vortices.The total number of vortices increases in March and October–December,and decreases slightly from March to August,but the overall change is not large.The polarity distribution of eddies may be related to ocean currents and water mass systems.It is found that there are more cyclonic vortices on the north side of the Oyashio front,and more anticyclonic eddies on the south side of the Oyashio front.In addition,the statistical characteristics of the important parameters of the cold and warm eddies are analyzed.The average life cycle of cyclonic vortex and anticyclonic vortex is 12.5 weeks and 13.7 weeks respectively;the average radius of cyclonic vortex and anticyclonic vortex is 44.6 km and 51.3 km respectively;the average rotation speed of cyclonic vortex and anticyclonic vortex is0.12 m/ s and 0.16 m/s;the mean amplitudes of the cyclonic vortex and anticyclonic vortex are-2.38 cm and 2.93 cm,respectively.The average life cycle,scale and intensity of the anticyclonic vortex are higher than those of the cyclonic vortex,and the anticyclonic vortex is dominant in the long-period and large-scale vortex.67% of the vortices have the characteristics of westward propagation,with an average westward movement of 0.44°;54% of the vortices propagate southward,with an average southward movement of 0.08°;The vortex travels slightly farther north than the cyclonic vortex.Based on the 2009-2018 daily tracking data set of eddies in the Northwest Pacific Ocean and fishery statistics,statistical methods were used to preliminarily quantify the relationship between squid fishing grounds and cold and warm eddies.Preliminary analysis showed that the total fishing effort,yield and average CPUE of squid in the warm vortex were higher,and the fishing effort and yield per unit area were also higher.It is believed that there is a higher abundance of squid in the warm vortex.GAM analysis shows that: 1)The relationship between CPUE and longitude is different in cold and warm vortex fisheries.The CPUE in cold vortex fisheries is higher in the west of 150°E and near 158°E,while the CPUE around warm eddy fisheries increases with longitude.It may be related to the different formation mechanisms of fisheries in different sea conditions.2)The CPUE of soft fish is positively correlated with the distance from the center of the cold vortex,and negatively correlated with the distance from the center of the warm vortex.The CPUE of the center of the cold vortex is significantly lower,while the CPUE around the warm vortex has a small change;3)The CPUE of the soft fish is negatively correlated with the development stage of the cold vortex.Correlation,positive correlation with the development stage of the warm vortex,and a significant increase in CPUE at the end of the warm vortex development,presumably related to the short-term upwelling or mixing effect during the weakening and dissipating process of the warm vortex.Based on the Northwest Pacific eddy data set from 2009 to 2018,fishery statistics and high-resolution reanalysis marine environmental data,the scientific mechanism of eddies affecting squid fishing grounds was explored.The statistical results show that the abundance of squid in the center of the warm vortex is significantly higher,while the abundance of squid in the center of the cold vortex is lower.Three key environmental factors affecting the distribution of squid were selected: sea surface temperature(Sea surface temperature,SST),50-meter water layer temperature T50 m,and chlorophyll-a concentration(Chlorophyll-a,Chl-a),to establish a suitable weighted habitat suitability index(HSI)model was used to assess the impact of eddy-induced changes in environmental conditions on squid habitat.The results showed that in the optimal weighted HSI model,the weights of SST,T50 m,and Chl-a were 0.25,0.5,and0.25,respectively,and T50 m was considered the most important factor.The distribution patterns of suitable habitats for squids in and around the vortex well explain the differences in the distribution of squids: the suitable SST,T50 m and Chl-a conditions in the warm vortex increase the abundance of squids,while the high-productivity squids Cold eddies are generally accompanied by unfavorable temperature conditions,which tend to form unsuitable habitats.In addition,it was found that there is a potential interannual variation in the response of the squid fishery to the eddy.Under the unstable background of the Kuroshio Extension(KE),the CPUE of the squid in the warm eddy is significantly higher than that in the stable state of KE.It is speculated that the background Chl-a increases in the KE unstable state,and the combination of suitable temperature conditions in the warm vortex and favorable foraging opportunities drives the increase in the abundance of squid in the warm eddies. |
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