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  7. 2021 updates on Pacific saury stock assessment in the North Pacific Ocean using Bayesian state-space production models

2021 updates on Pacific saury stock assessment in the North Pacific Ocean using Bayesian state-space production models

    Stock assessment for the North Pacific saury was updated based on the specification (2 base cases and 2 sensitivity cases) agreed in the 7th SSC-PS meeting held in October 2020. The basic model employed in the analysis was the state-space surplus production model as has been used since the SSC-PS01 as an interim stock assessment model. The model can account for process and observation errors in the abundance indices.  Parameters in the models were estimated based on Bayesian framework with a Markov chain Monte Carlo method. The estimation results were diagnosed with respect to shapes of posterior distributions, residual plots, retrospective pattern and predictability of the future population status. The outcomes of stock status and future projection were shown according to the template agreed in the 5th SSC-PS meeting with some modifications to accommodate the data period. 
    As for the combined base case stock assessment result, the 2021 median depletion level was only 19.5% (80%CI=10.8-31.4%) of the carrying capacity. Furthermore, B-ratio (=B/Bmsy) and F-ratio (=F/Fmsy) in 2020 were 0.339 (80%CI=0.212-0.526) and 1.033 (80%CI=0.641-1.625), respectively. For those three years average values, B-ratio over 2019-2021 and F-ratio over 2018-2020 were respectively 0.378 (80%CI=0.238-0.574) and 1.480 (80%CI=0.973-2.187). In addition, the probability of the stock being in the green Kobe quadrant in 2020 was estimated to be nearly 0%, while the probability of being in the red Kobe quadrant was assessed to be greater than 50%. On the weight-of-evidence available now, the current Pacific saury stock is determined to be overfished and subject to overfishing. 
    For population outlook, population dynamics were projected for some scenarios with respect to several levels of reduction/increase of catch as well as status quo. The results showed that increased catch compared to the current level may cause a severe decline in the population size. 
     

    Document Number
    NPFC-2021-SSC PS08-WP03
    Document Version
    1
    Agenda Item
    Review of results and implications to management
    Authors
    Toshihide Kitakado
    JAPAN
    NPFC-2021-SSC PS08-WP03 BSSPM stock assessment report_Japan.pdf