| Chub Mackerel(Scomber japonicus) as one of the most important economic fishes in the East China Sea, is the focus of fisheries target of China, Korea and Japan, as well as important species in the marine ecosystems. In recent years, fishing yields of Chub Mackerel in the East China Sea vary widely. And the resources fluctuate, Chub Mackerel may be overfished or in the state of overfishing. In order to make the East China Sea Chub Mackerel resources be at status of sustainable exploitation, scientific and effective management is necessary.Domestic and foreign research papers show that the role of uncertainty in stock assessment is becoming more and more important. In this paper, we will use simulation method to bring the uncertainty into the East China Sea Chub Mackerel stock assessment.Then we will observe how fork data structures which may be lack of large sizes or small sizes impact the results of Chub Mackerel stock assessment. Based on the collected 5311 fork length samples of Chub Mackerel, we will generate data by using Monte Carlo simulation,then to research the impacts of data structure, sample size and other errors on stock assessment by using simulation method. According to the simulated data to calculate its corresponding growth parameters, then to assess the biomass, as well as biological reference points F0.1 and FMAX. Meanwhile to compare the results with results calculated by the original fork length data, at last we will analyze where the errors come from.The simulation results show that the lack of large samples of individuals will have a greater impact on growth parameters estimation. By using the ELEFAN to calculate L? and K, when the maximum fork length group is missing even if with large amount of data, there are still big discrepancies between the results with the real value. While the small size data is missing, if we could ensure the samples contain the largest size fork length data, it is still possible to obtain satisfactory results but with a greater error between the actual results and the maximum value of the fitting index Score.To do stock assessment by using the original sample data and the fork length data with less large individuals of Chub Mackerel. The two sets of data share the similar surplus yield curve, and their maximum sustainable yield(MSY) and fishing effort(EMSY) wereapproximately consistent with each other, about 1.88×105and 72. However, the stock-recruitment curves are very different from each other. The inclusions of stock derived from age groups with errors was between 3,000*106 and 5,000*106, which was too high and will facilitate overfishing in an attempt to improve the state of the stock by increasing fishing efforts. Including groups containing errors suggests that stock and catches in recent decades have shown dramatic cyclical changes. The error-free group, which predicts biomass, stock and catches, will stabilize respectively at 323,000 tons, 158,000 tons and105,000 tons in the last. The results showed that body length data containing errors of assessment will result in a significant impact. Groups with errors showed that the biomass will reach a peak value of about 1.35 million tons in a year, but fall to about 400,000 tons two years later. And it will rise to the peak again in 2 years, as this cycle was significantly different compared to the error-free group.With the simulated data which is lack of large and small individual to find out how they would impact Chub Mackerel biological reference points. By using the fork length data sample with less small individuals, the estimated value of biological reference points F0.1 is significantly larger than the calculated results without any errors. And its value of biomass is far greater than the actual situation of biomass. It can be seen, when the sample with few small individuals will result in overestimation of the stock, and the department of management will increase Chub Mackerel fishing effort leading to status of overfishing.While using the sample with fewer large individuals to estimate the reference point, it turned out the value of F0.1 is only 0.352 and its biomass is 1.2×106 tons. However result of the data without any errors and the data with fewer small individuals are 2.9×106and4.7×106 tons. Thus, to ensure the accuracy of Chub Mackerels fork length is very important to the results of stock assessment when we are estimating the biological reference points with YPR model. In the experimental control group, distribution trends of the biological reference points FMAX and F0.1 are very similar. But under the influence of fork length errors,FMAX changes more dramatically than F0.1 in the same conditions. It also shows that when the environmental changes have a greater impact on sampling fork length data, F0.1could be more stable. |
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