Spatial And Temporal Distribution Of Portunus Trituberculatus In The North-Central Zhejiang Sea Area Based On Two-Stage GAM Model

Portunus trituberculatus is one of the most important economic crabs in the coastal waters of China,which is also an important dominant species in the east China Sea.It has large market demand and high economic value.Zhejiang has a large of P.trituberculatus,with an average annual output of about 150,000 tonnes.In recent years,P.trituberculatus resources have been overexploited due to increased market demand and overfishing.The yield of P.trituberculatus in Zhejiang province also fluctuated during the process of increasing,which may be caused by many factors,including the changes of fishing behavior and marine environmental factors.In order to realize the sustainable use of P.trituberculatus resources,it is necessary to carry out scientific resource assessment,understand the spatial and temporal distribution of resources,and then implement scientific fishing and management strategies.Based on the fishery resources survey data of offshore bottom trawling in the north-central part of Zhejiang province during April and November 2015-2018,a Two-stage generalized additive model(Two-stage GAM)was P.trituberculatus,the relationship between spatial-temporal distribution of resources and environmental factors was analyzed.Meanwhile,the Akaike Information Criterion(AIC)and the interpretation rate of variance(VE)were used to screen the best-fit models,and the model-fit and model-predicted R2 were used to identify the overfit of different models.The predictive performance of the model was assessed using cross-validation,with each cross-validation process simulated 100 times and the decision coefficient R2 used as an indicator to assess the predictive performance of the model.By comparing the fitting effect and forecasting performance of different seasonal models,the optimal forecasting model of each season is obtained.Finally,the distribution of P.trituberculatus is predicted by using the optimal prediction model of each season.The results:The P.trituberculatus are mainly distributed in the shallow water near the shore,and the P.trituberculatus abundance reaches its maximum at the depth of 3-30m.The abundance decreases from shallow to deep water along the coast,and its spatial distribution has obvious inter-annual and seasonal variations.The P.trituberculatus abundance in autumn was significantly higher than that in spring,and the high abundance in spring was only found in shallow water area,while the high abundance in autumn was also found in deep water area.In 2015,the P.trituberculatus was the largest,mainly distributed in the sea area of 122°00′～123°30′E,and in 2016,the P.trituberculatus abundance was the lowest,which was reduced to the shallow water area near the central coast In 2017,it expanded to the outer sea of the Hangzhou Bay,reached its highest level in 2018,with High P.trituberculatus in the sea east and south of the Zhoushan archipelago,and in autumn,it reached its highest level in 2016,in 2017,the lowest abundance was found in the sea area of 29°30′～30°30′N,and the highest abundance occurred in the sea area near Zhoushan archipelago and north of Hangzhou Bay.The highest abundance in 2018 was found in the Dongtou Sea area.The environmental factors affecting the distribution of P.trituberculatus were different in different seasons.The main environmental factors affecting the distribution of P.trituberculatus in spring were water depth,bottom salt and chlorophyll a concentration The main environmental factors affecting the P.trituberculatus distribution in autumn are water depth and bottom temperature,while the main environmental factors affecting the P.trituberculatus distribution in spring and autumn are water depth and bottom temperature.The optimum temperature for P.trituberculatus in spring is 20-30,and the optimum temperature for P.trituberculatus in autumn is 19～22℃,when the temperature was below 22℃,the P.trituberculatus abundance decreased with the increase of the temperature,and the optimum temperature in spring and autumn was 19～22 ℃,and the abundance increased with the increase of the temperature.The prediction performance of the spring-autumn model is better than that of the spring-autumn model.The optimal prediction model was used to predict the distribution of P.trituberculatus resources in the coastal waters of north-central Zhejiang province.The results showed that the distribution of P.trituberculatus resources varied significantly in different seasons,in general,the resource distribution in autumn is more uniform and the average abundance is higher than that in spring.The high abundance area in spring is mainly in the west near-shore shallow water area,and the high abundance area in autumn is mainly in the east deep water area.Based on the Two-stage GAM model,this study analyzed the spatial-temporal distribution of P.trituberculatus and its relationship with environmental factors in the coastal waters of north-central Zhejiang province,and proposed a prediction model suitable for the distribution of P.trituberculatus resources,the purpose of this study is to understand the distribution of P.trituberculatus resources and provide basic information for the sustainable use of P.trituberculatus resources.