Responses Of The Distribution Of Skipjack Tuna In The Western And Central Pacific To Environmental Factors In Climate Events

Skipjack tuna,Katsuwonus pelamis,is a highly migratory pelagic fish.It is one of the main fishing targets in the Western and Central Pacific Ocean.Changes in climate and marine environmental factors have a significant impact on its distribution and it presents an emerging challenge to the sustainable management of tuna fisheries.In recent years,abnormal climate changes,El Ni(?)o and La Ni(?)a have profoundly affected the distribution of fishing grounds.At the same time,the performance of the international fish price market is not optimistic,the cost of entering fishing grounds is rising,and the sea area management policy is becoming more and more stringent.By analyzing the impact of changes in the marine environment on the distribution of habitats under different climate events,exploring the most suitable habitat for skipjack tuna can effectively reduce the fishing cost of the fleet and improve the fishing efficiency of the fleet.Understanding changes in the frequency or characteristics of El Ni(?)o events in a changing climate and the relationship between the marine environment and the distribution of fishing grounds is therefore of broad scientific and socioeconomic interest for grasping the distribution characteristics of bonito fishery resources.According to the log data of skipjack tuna seine fishing in the Western and Central Pacific Ocean(140°E～160°W,15°N～15°S)of all fishing vessels in mainland China from 2009 to 2019,combined with sea surface temperature(SST)and temperature of subsurface at 50m depths,sea surface salinity(SSS),net primary productivity(NPP),u-component of current(UCC),and v-component of current(VCC),sea level anomaly(SLA),mixed layer depth(MLD),etc,Maximum Entropy Model(Max Ent)was constructed to model El Ni(?)o,La Ni(?)a and normal years on different time scales.At the same time,according to the established model,the habitat distribution of skipjack tuna fishing grounds were simulated under the La Ni(?)a event at the end of 2017 and the El Ni(?)o event at the end of 2018,and real fishery operation data were superimposed for verification.The results show that:(1)The operating characteristic curve(ROC)is higher than 0.9,so the Max Ent model can be used to predict the distribution of fishing grounds well.Sea surface temperature(SST),sea temperature at depth of 50m(T50)and sea surface salinity(SSS)are key factors affecting the distribution of skipjack tuna.In addition,v-component of current(VCC),mixed layer depth(MLD),net primary productivity(NPP)and u-component of current(UCC)have little effect on the distribution of skipjack tuna.In the moderate Central Pacific El Ni(?)o events,SSS has the highest contribution rate,while in the super Eastern Pacific and weak Central Pacific El Ni(?)o events,T50 has the highest contribution rate.In the moderate Central Pacific(CP)El Ni(?)o events,SSS,T50,sea level anomaly(SLA),and SST are the main environmental factors affecting the distribution of skipjack tuna,and their cumulative contribution rate reaches 63.59%.In the super Eastern Pacific(EP)El Ni(?)o events,T50,SST,SLA,and SSS are the main environmental factors affecting the distribution of skipjack tuna,with a cumulative contribution rate of 83.38%.While in the weak Central Pacific(CP)El Ni(?)o events,T50,SST,and SSS are the main environmental factors affecting the distribution of skipjack tuna,with a cumulative contribution rate of 74.69%.This indicates that SST and T50 of different El Ni(?)o events have different effects on the distribution of skipjack tuna.(2)The response of skipjack tuna fishing grounds to environmental factors is generally similar in different El Nino events,and the optimal range of SST is 29.3～31.2℃,while the optimal range of T50 is 29.3～30.5℃.The optimal range of SSS is33.9～34.5g/m~3for moderate Central Pacific El Ni(?)o,but it is 34.3～35.2 g/m~3for super Eastern Pacific and weak Central Pacific El Ni(?)o events.(3)The moderate Central Pacific El Ni(?)o events are mainly distributed around 160°E in the equatorial Pacific,while the super Eastern Pacific and weak Central Pacific El Ni(?)o events are mainly distributed around 170°E.The center of gravity of fishing ground along the longitude was mainly distributed between 160°E and 175°W,the suitable habitat acreage percentage is different in different El Ni(?)o events.The suitable habitat acreage percentage of the moderate Central Pacific El Ni(?)o events,the super Eastern Pacific El Ni(?)o events and the weak Central Pacific El Ni(?)o events are 24%,28%and 29%.The effects of El Ni(?)o,La Ni(?)a events and normal years of climate change on the distribution and displacement of skipjack tuna fishing grounds are discussed.The results show that:among the three types of events,SST is the main environmental factor affecting the distribution of skipjack tuna,it is contribution rate and replacement importance are about 50%.The contribution rate and replacement importance of SSS,NPP,and MLD are second.In normal years,the fishing grounds are distributed in the 160°E—170°E waters of the equatorial Pacific in general.The fishing grounds moved eastward when the La Ni(?)a event coming,mainly in the waters of 150°E—175°E.During the La Ni(?)a event,the fishing grounds moved westward,mainly located on the sea area of the equatorial Pacific Ocean from 145°E to 160°E.