| Skipjack tuna,Katsuwonus pelamis,is widely distributed in the tropical and subtropical water of the world's ocean,it has a high abundance in west-central Pacific Ocean.The annual catch in recent years were about 250×10~4 t in west-central Pacific Ocean,which account for 60%of the global skipjack tuna catch.Fishing methods for skipjack tuna are mainly purse seine and pole-and-line gear,and the purse seine is the most important fishery.The purse seine fishery in central-west Pacific Ocean was started from mid-1960s,the U.S.,South Korea and Taiwan Province of China took part into this fishery in 1976,1980 and 1982,respectively.China mainland started in 2001.With more than 10 years development,there are 20 large tuna purse seine fishing vessels with annual catch of 10×10~4 t,and China also become the main tuna purse seine fishery countryDuring the development process of purse seine fishery,however,we have to face some important issues,and these issues also restrict the sustainable development and competitiveness improvement for China tuna purse seine in central-west Pacific Ocean:(1)Inaccurate master the central fishing ground lead to the low yields of individual fishing vessel,this value is only account for 70-80%of the highest operational fleet(from South Korea and Taiwan Province of China);(2)management regulation of actual fishing time limitation in the South Pacific islands(total fishing time is 28000 days in west-central Pacific Ocean,annual fishing time for individual vessel in our country is about 250 days),the situation leads to the low availability of effective fishing time,and also the decrease of economic benefits;(3)it is impossible to predict the tuna abundance and spatial distribution in mid-long term,this leads to blindly increase the purchase for the entry-fishing countries and quota.Blindly entry-fishing is not only waste the entry-fishing fee(usually 10000 US dollars per day for each fishing vessel),but also serious influence the catch,thus the total benefits decrease occurred in skipjack purse seine fishery.Thus,I will analyze the above three key problems in this dissertation.I aim to find the relationship between abundance,fishing ground of skipjack tuna and its ambient environment,develop the fishery forecasting analysis and entry-fishing systems model,to improve the accuracy of predicting abundance and seeking fishing ground for skipjack tuna,and establish corresponding scientific fishery forecasting system for skipjack tuna in central-west Pacific Ocean.This study will also support the technology for the development and strength the tuna purse seine in our country.The main results are as follows:(1)Spatial and temporal distribution of fishing ground for skipjack tuna.According to the catch data of skipjack tuna in the west-central Pacific during 1995-2014,using statistical analysis based on the above data with the interval of annually and monthly,detecting the fishing ground variation through catch gravity analysis and cluster analysis.And a total of 22 fishing zones(5°latitude and 5°longitude)with the high catch are selected to analyze the spatial and temporal variation of catch and CPUE for skipjack tuna in central-west Pacific Ocean.The results showed that the annual catch of skipjack tuna is gradually increase with high fluctuation of CPUE,and high monthly catch mainly in the first half year;catch gravity was inhomogeneous in longitudinal direction,four groups can be divided with cluster analysis;monthly gravity position was changed with clockwise,from west to east,from south to north,three groups can be divided with cluster analysis;main catch were focused in the area of 5°S-5°N,120°-175°E,CPUE in extreme years had significant difference with other years,and the CPUE of first half year and second half year had significant difference;135°-145°E is the boundary of longitude variation,no significant difference in latitude variation.The above spatial and temporal variation may caused by the change of warm pool in west Pacific Ocean,and also have some relationship with the entry-fishing strategy in the island countries in Pacific Ocean.(2)The relationship between fishing ground and environmental factors for skipjack tuna.According to the catch data of skipjack tuna purse seine in west-central Pacific Ocean,the spatial-temporal factors(year,month,latitude and longitude)and environmental factors(sea surface temperature,SST;sea surface height,SSH;oceanic nino index,ONI;chlorophyll-a,Chl-a)were selected to analyze the correlation between the fishing ground and the above factors,as well as its relative importance through two different types of models(Generalized additive model,GAM and Boosted regression tree,BRT).The results showed that longitude is the most important factor in the result of GAM,which account for more than 50%of the total,and latitude,year and month decrease by importance;SSH is the most important environmental factor in GAM,and then ONI,SST and Chl-a are less important for CPUE.The result of BRT is similar with that of GAM,spatial-temporal factors are the most important that longitude account for60%of the total importance,and with the decrease of year,latitude and month;ONI is the most important environmental factor in BRT,then the SSH,SST and Chl-a are also less important in the model.As conclusion,the two types of models can effectively reflect the influence of fishing ground.BRT can better explain the result with the training of dataset,and this model can present the result intuitively to us.ENSO induced oceanographic variation will change the abundance distribution of skipjack tuna,so the ONI should add in the fishery forecasting model to improve the accuracy in the future.(3)Establishment of fishing ground forecasting model for skipjack tuna.Firstly,BP neural network was used in the analysis.According to the production statistical data of skipjack in the central-western Pacific Ocean and related oceanographic condition,22BP neural networks are constructed to compare the forecasting effect between CPUE and fishing effort as output factor,respectively.The results showed that the value of minimum residuals in the forecasting models based in fishing effort were all lower than those based on CPUE,which represent that the fishing effort is better considered as the indicator of fishing ground.It is also found that the mean value of fitting residuals decreased as the number of input factors increased,which also show that the selected environmental variables in this study(month,sea surface temperature SST,sea surface height SSH,Nino3.4 index,Chl-a)are the important indicators affecting the distribution of skipjack.The BP neural networks with a structure of 7-5-1 was the most suitable for forecasting fishing ground with a highest accuracy and lowest residuals,as the significance for input factors from high to low are longitude,Chl-a,SST,latitude,Nino3.4 index,SSH and month.Next,22 high catch fishing zone ranged 5°×5°were selected,regard catch data as the index and combine different climatic conditions that divided by the Nino index,to establish the forecasting model of Katsuwonus pelamis in west-central Pacific Ocean based on the different climatic conditions.The results showed that the catch of skipjack fluctuated among different climatic conditions,the monthly total catch and monthly mean catch in El Nino event was the highest,which was similar with La-Nina event,and higher than the normal condition;the catch among different spatial and climatic conditions had difference,the catch in 5°N was higher than 5°S,and the normal condition had the highest catch;La Nina event was the highest between the 145-165°E;La Nina event was the lowest eastward in 165°E;establishing the relationship between catch and SSTA of Nino 3.4,and the above two index had strong relationship,fit well with the Normal Model(P<0.01);the model validation result showed that predict results and actual results fitted well and had significant relationship in all the three climatic conditions.(4)Establishment of entry-fishing analysis for skipjack tuna.According to the production data of skipjack tuna in the west-central Pacific,and the total of 22 fishing zones(5°latitude and 5°longitude)with the high catch are selected to analyze the spatial variation of fishing effort for skipjack tuna in central-west Pacific Ocean.The percentage of fishing effort is also regarded as the index of entry-fishing,and the optimal entry-fishing model between the index of entry-fishing and SST or SSTA is established.The results showed that main fishing efforts were focused in the area of 5°S-5°N in the direction of latitude,accounting for 87.4%of the total fishing effort.Of which the area of 130°-140°E was the major fishing zone area in the longitudinal direction,accounting for 45.08%of the total fishing effort in the 22 fishing zones.The relationships between the index of entry-fishing and SSTA or SST all showed the normal distribution(P<0.01)for each fishing zone.The suitable value of SSTA model was on early-0.25?,and the suitable value of SST model was nearly 29.5?.For the top 10 ranking of entry-fishing index,we find that the actual index and predicted value is the same.It is concluded that this entry-fishing forecasting model can effectively predict fishing area distribution,which could give us a suitable advice for fisheries industries in the future.(5)Influence of extreme climate on the abundance of skipjack tuna and its forecasting model.The study combined with the El Nino/La Nina events(expressed by NINO 3.4 index),the influence of El Nino/La Nina on the abundance index of skipjack is studied by using the time series analysis.The results showed that the yearly and seasonally abundance index of skipjack in periods of 16 years varied greatly,which the maximum and minimum monthly CPUE appeared in February 1995 and October 1997,respectively.The effect of Nino3.4 index on monthly CPUE synchronized(lag 0 months)with the highest correlation.The Nino3.4 index and monthly CPUE has significant negative correlation.Four months(August to February of the next year)of CPUE values were chosen as studying object,and divided 16 years into two periods:1)First period was before 2000.When the strong El Nino year occurs(1997),the abundance index of skipjack was in a lower level.When a strong La Nina(1998)occurs,the abundance index was high.2)The second period was after 2000.In the years of weak El Nino,the abundance index of skipjack fluctuated around the overall mean CPUE value,but still lower than that in the La Nina years'.In conclusion,the temporal and spatial abundance index of skipjack varied with both the changes of marine environment and the primary productivity which caused by the El Nino/La Nina phenomenon.Meanwhile,using annual mean CPUE as an indicator of abundance,analyzing the grey correlation between each environmental factor and CPUE,choosing suitable environmental factor and using in the grey abundance forecasting model.The analyses suggested that the catch of skipjack tuna is gradually increase with year,whereas CPUE dramatically fluctuate within years.SST and SSTA had higher correlation than other factors based on the grey correlation analysis.The and also establish the model based on Sea surface temperature,SST,Sea surface height,SSH,Chlorophyll-a,Chl-a and Sea surface temperature anomaly,SSTA in Nino 3.4,the model's CPUE mean relative error of 6.4752 and correlation of 0.6874 between fitting abundance sequence and predict abundance(P<0.01).So this model could define as suitable model for the predicting of skipjack tuna abundance in central-west Pacific Ocean. |
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