Spatial-Temporal Distribution Of Abundance And Stock Assessment Of Skipjack Tuna(Katsuwonus Pelamis)in The The Western And Central Pacific Ocean

Skipjack tuna（Katsuwonus pelamis）is an oceanic pelagic fish widely distributed in the oceans of the world’s temperate tropics,which is mainly targeted by purse seine fishery and pole-fishing fishery.Skipjack tuna fishery occupies an extremely important position in the world’s tuna fishery.It is a very important economic fish.The Western and Central Pacific Ocean（WCPO）is one of the main fishing grounds for skipjack tuna.There are many domestic and foreign researches about the stock of skipjack tuna by purse seine,and they involves many biological and ecological aspects,which includes the age determination and growth measurements,living habits,habitat and the impact of environmental factors on the skipjack tuna,etc.It is noted that the FADs set by purse seine for skipjack tuna caused widespread concern in the Regional Fisheries Management Organization（RFMO）in recent years.The closed fishing period on the use of FADs in the WCPO has been gradually extended.To a extent,this affects the fishing operation of Chinese purse seine fleet.In order to promote the sustainable development of skipjack tuna resources while improving fishing efficiency,this paper apply the fishery data of the skipjack tuna in the WCPO from 2014 to 2019,and analysize the spatial and temporal distribution of skipjack tuna and the changes in the center of gravity according to the fishing set types of purse seine.The stock status was also evaluated by the Grey Relational Analysis method for skipjack tuna.Using the fishery catch data and environmental data such as sea surface temperature and salinity based on GAM in the WCPO from 2010 to 2019,the effect of environmental factors on the fish distribution was analysized.In addition,GAM models with standardized CPUE（catch per unit effort）for different fishing set types were also applied.Stock assessment for skipjack tuna in the WCPO based on Bayesian state-space production model was conducted.The results are presented as followings:（1）The fishing grounds for the fish in the WCPO mainly distributed in the coastal waters at 0°N-10°N,5°S-15°S.Significant differences of catch,effort,CPUE and fishing ground were identified among different fishing set types.The fishing ground of free schools set type distributed in coastal waters of Papua New Guinea,Solomon Islands and Nauru.However,the fishing grounds of natural-log type and FADs schools type were wider,with additional distributions in the coastal waters of Micronesia and Kiribati.Significant differences of fishing ground and catch were found among different fishing set types.The high CPUE（higher than 20 tons/net）of free schools set type mainly located in the area of 135°E and 160°W-180°W,and CPUE of natural-log schools set type（over 30 tons/net）focused in 170°E-175°E and 180°W,and CPUE of the FADs schools set type（40 tons/net）concentrated in150°W-180°W.The results from free schools set type data analysis revealed more high for CPUE in 2015 and 2016,but low CPUE values in 2018 and 2019 respectively,and the FADs schools set type fishery indicated that there were some high CPUE area in the southeast waters in 2015 and 2019 respectively.Grey Relational Analysis revealed that the status of skipjack tuna free schools set type resources in 2019 was the best with the comprehensive association degree higher than 0.8,followed by 2016,the worst in 2017 with the association degree of comprehensive lower than 0.4,while the results of natural-log set type and FADs schools set type fisheries indicted that the resources were relatively stable among years from 2014 to 2019.The results of the study analyzed the fishing efficiency of different fishing set types and the distribution of skipjack tuna stocks,and provided a reference for improving China’s skipjack tuna fishing efficiency in the WCPO.（2）The rate of deviance about CPUE is 31.9%.The importance of influencing factors of fishery distribution in descending order is listed as following:longitude,latitude,salinity,temperature,month,year.The factor that provides the largest contribution is longitude and the contribution rate amounted to 16.3%.It is noted that CPUE increases with the longitude in the range of 135°E-175°E,150°W-180°W.Followed by latitude,CPUE was high on 0°-5°S and the salinity among environmental factors also has a great influence on the distribution of CPUE.The optimum salinity ranged about 34.5-35.0,and the optimum temperature is in the range of 29.5°C-31°C.It is noted that CPUE trend increased at first several year and then dropped during 2010-2019.The highest CPUE appeared in 2016.By monthly statistics,CPUE dropped rapidly from January to September,increase from September to December.Standardized CPUE from GAM model provided important parameters for the fish stock assement.（3）The results of the assessment of the skipjack tuna stock showed,the median of carrying capacity amounted to 7.54×10⁶t and 95%confidence intervals of carrying capacity was 4.78×10⁶t.The median of intrinsic growth rate arrived at 0.626 and 95%confidence intervals of intrinsic growth rate was 0.421.The B_MSY was 3.02×10⁶ t and MSY was1.56×10⁶ t.The current stock status was neither overfishedand nor subject to overfishing.As the prediction results indicated,if the total allowable catch（TAC）was set in 2019（1.46×10⁶ t）,the stock status would remain healthy until 2030.The skipjack tuna stock assessent results of this paper are basically consistent with the result by the Western Central Pacific Fisheries Commission.According to the results of this study,in order to maintain the sustainable development of skipjack tuna resources,it is recommended that the allowable catch is set to 1.46×10⁶ t or lower.