Population Structure And Management Strategy Evaluation Of Blue Shark (Prionace Glauca) In The Pacific Ocean

Blue shark(Prionace glauca)is an important bycatch species in tuna fisheries in the Pacific Ocean,it is an apex predator which plays an important role in the marine ecosystem.The increase of fishing pressure,and the climate change impacred on the blue shark's growth and distribution,which put forward a challenge for the sustainable development of blue shark in the Pacific Ocean.The population structure,as well as the population dynamics of blue shark are unclear,and relative large of uncertainties produced by the stock assessment,calling for a study which these uncertainties cause the blue shark's biomass change in the furthure in the Pacific Ocean.In this study,we analyzed the population structure of blue shark in the Pacific Ocean based on the samples derived from the Chinese longline tuna fishery vessels that were collected from 3 locations in the Pacific Ocean.Monte Carlo was used to simulate the blue shark fishery in the Pacific Ocean which gave us an insight into this species population dynamic.Surplus production model and age-structure model served as assessment models,and FMSY,40/10,constant catch and constant fishing mortality as harvest control rules in the management strategy evaluation was used to assess the population of blue shark in the Pacific Ocean.Results obtained from this study are as follows:(1)Partial mtDNA Cytb gene and CO?gene were used to analysis blue shark samples from 3 locations derived from the Chinese longline tuna fishery vessels in the Pacific Ocean during 2011 to 2014.3 variable sites and 4 mtDNA haplotypes were detected among study samples for each gene.The haplotypes diversity is 0.693 and nucleotide diversity is 0.00100 for Cytb gene,and the haplotypes diversity and nucleotide diversity for CO?gene is 0.624 and 0.00126 respectively.These results showed a high haplotypes diversity and a low nucleotide diversity.The AMOVA tests among all the samples revealed that the genetic variation mainly occurred within population and a small part of variation(3.94% for Cytb gene,and 2.16% for CO?gene)occurred among population.FST analysis proved to be non-significance differences among every two stocks,which indicated a frequency genetic exchange and a poor genetic structure.Neutrality tests showed non-significant negative values of Tajima's D,significant negative values of FU'S FU and the unimodal nucleotide mismatch distribution curve occurred corroborating to the population's expansion.A further study indicated that this expansion occurred about 0.21 to 0.29 million years ago.(2)We employed generalized additive models(GAMs)to analyze the population structure of blue shark using new biological data(fork length,right clapper length,feeding level,sex and genetic data(study above))and environmental indices(sea surface temperature,month,longitude and latitude)collected from 2011 to 2014 by the Chinese longline tuna fishery observer program.Fork length was significantly related to position(latitude and longitude)and sex,and it was positively related to sea surface temperature.In general,the fork length from eastern Pacific Ocean was larger than from western Pacific Ocean,and the fork length from males' stock was larger than from females' stock.The estimated relationship between sea surface temperature and fork length was roughly linearly increasing until 29?,point at which it plateaued.No relationships were found between fork length and feeding level,month and genetic data.Right clapper length was significantly related to position and sea surface temperature,and it was positively related to fishing months.In general,right clapper length from eastern Pacific Ocean was larger than from western Pacific Ocean,and the right clapper length measured during September was larger than other months included in this study.The estimated relationship between sea surface temperature and right clapper length was roughly linearly increasing until 29.3?.No relationships were found between right clapper length and feeding level and genetic data.Our results demonstrate a morphological difference over the range of the observed data,but the genetic data imply a panmictic population.We explore several hypotheses(one population,two population and meta-population)to explain these observations,two of which are closely related to climate change.Given the risk of overharvesting the species associated with assuming incorrect population dynamics,future management strategies should be designed to be robust to scenarios in which the structure and dynamics of the population are poorly known.(3)Surplus production model served as an assessment model,Monte Carlo method to simulate the blue shark fishery in the Pacific Ocean.According to the population structure studied from the second chapter,biological parameters sites from previous studies,and the management strategy evaluation was presented to evaluate the different harvest control rules which included Fmsy,40/10,constant catch and constant fishing mortality.The results showed that: 1)the relative error of biomass and fishing mortality under meta-population are larger than those under one signal population and two independent populations;2)the relative error of biomass and fishing mortality from high parameter population were larger than those from low parameter populations,which might caused the overestimate of study parameters;3)constant fishing mortality harvest control rules leads the biomass of blue shark in pacific ocean is smaller than the true Bmsy,which indicated it is to be a failure management measure for the blue shark's sustainable development in the Pacific Ocean;Fmsy and 40/10 harvest control rules produce a high total allowable catch,but it leads the biomass of blue shark in pacific ocean under the Bmsy after a period time of high catch.This strategy,it achieves a sustainable exploition of blue shark only within a period of shortterm,but it will failure in a long-term;constant catch harvest control rule obtain a reasonable catch,and the biomass of blue shark in Pacific Ocean keeps in high level and increasing.From the study above,constant catch(constant catch set as 2.3×107 individuals)harvest control rule makes a more reasonable management strategy for blue shark managememt evaluation among 4 harverst control rules applied into this study.(4)Based on the conclusion made by the third chapter,age-structured simulation model is presented for evaluation of management strategies under the control rule of constant catch for different population scenarios.The results showed that: 1)when the true population defined as one population,and the stock assessment based on one population,which will achieve a high recruitment and keep increasing,and it indicated to be benefit for sustainable managing blue shark in the Pacific Ocean.But,the stock assessment based on two independent population leads the recruitment decline.2)When the true population defined as two independent,and the stock assessment based on two independent population,which will achieve a high recruitment and keep increasing,and benefit for a sustainable unitization.However,the stock assessment based on one population leads a high recruitment within a short-term,and decreasing in the long-term.3)When the true population defined as meta-population,and stock assessment based one two independent population,which will achieve a sustainable recruitment near 7.5×106 individuals,small than the initial recruitment.While stock assessment based on one population,which leads to a higher recruitment than initial recruitment within a short-term,and then decreasing obviously in a long-term.More management strategies should take full consideration for sustainable development of blue shark in the Pacific Ocean.