Impact Of Catch Uncertainty On The Stock Assessment Of Indian Ocean Bigeye Tuna (Thunnus Obesus)

Bigeye tuna(BET),Thunnus obesus is a large epi-and mesopelagic species distributed in tropical and subtropical waters of Indian Ocean.Its stock status has been the focus of regional tuna fisheries management organizations.According to the related reports and historical fishing data,stock assessments of Indian Ocean BET have been conducted using ASPIC,SS3,ASPM,ASAP.And it was found that there are several common concerns in the stock assessment of Indian Ocean BET: There are some uncertainty in the stock assessment of the Indian Ocean BET.And in the process of stock assessment,the statistical bias of the original fishery data,the CV on the catch,M,and h will also have several impacts on the results of the stock assessment.Because of a variety of fishing gears and fishing fleet structures,there have some statistical biases in the historical nominal catches of the Indian Ocean BET.However,the impact of this bias on stock assessment has been neglected in recent years.Although the results of stock assessments are more optimistic by using the current fishery data,these influencing factors may affect the stock assessment model to evaluate the true status of the current resources.For the Indian Ocean BET fishery,the IOTC secretariat has developed a more systematic framework for fishery data,and mastered the basic biological data of BET.Therefore,this research is focus on the impact of catch uncertainty on the stock assessment of Indian Ocean BET.First,we conducted the impact of observation error and statistical bias of catch on the stock assessment of Indian Ocean BET,using Age Structured Assessment Program(ASAP)based on fishery-specific catch,catch-at-age,and standardized catch-per-unit-effort data.The results showed that the current stock of BET in the Indian Ocean was neither overfished.The results of base model and eight sensitivity analysis models showed that the observation error of catch had influences on the stock status evaluation.When the bias of nominal catch was assumed to be 15%(i.e.the historical catch was underestimated),the assessment result was basically consistent with the base model(i.e.not overfished).When the bias of nominal catch was assumed to be 20%,the stock might be overfished.Therefore,both the observation error and the statistical bias associated with catch data can have significant impacts on the assessment results,with the later having more impacts.This study highlights the importance of considering both the assumption of observation error and statistical bias in catch data in tuna fishery stock assessment,which has been neglected recently.Meawhile,we used a simulation approach to explore the impact of catch uncertainty(the CV on the catch,M,and h)on stock assessment of BET.The operating model was developed using POPSIM-A software and conditioned based on the results of recent stock assessment with ASAP model.We used ASAP model as estimation model to evaluate the reliability of stock estimates when catch uncertainty was mis-specified.The results from 24 scenarios showed that these catch uncertainty had obvious influence on the reliability of stock assessment.First,two levels of natural mortality values were considered for the current assessment,i.e.,the higher level was assumed for the base case model,and the lower level for the sensitivity analysis.The results showed that low natural mortality resulted in unrealistically pessimistic stock staus.,and the likelihood profile for natual mortality supported that the model preferred a higher natural mortality.Therefore,the models with low M assumption are considered to be significantly biased and not used for futher analysis.Secondly,under the assumption that the CV is small or large,the stock status is more pessimistic than the basic model.This is consistent with the assessment result of the current fishery data.However,it is likely to lead to an underestimation of the MSY and inaccurate assessment results.And the current stock stutas can't be fully predicted and deevelop a fishing plan.Then,according to the study of the steepness parameter h,it has a certain influence on the evaluation results.It may directly lead to a correct judgment of the current stock status.Therefore,in the process of stock assessment in the future,the impact of CV and h should be analyzed and error verification.At the same time,we found that the POPSIM can be used to test the estimation model and It is very important to use POPSIM to create Indian Ocean bigeye tuna simulation population.