Sensitivity of the chub mackerel stock assessment in 2024 to observation and model uncertainty in the Northwest Pacific Ocean

In this working paper, we conducted sensitivity analyses to examine the impacts of observation uncertainty and model uncertainty, which are crucial for the stock assessment and management of chub mackerel in the Northwest Pacific. The sensitivity analysis scenarios with the data up to 2022 or model settings without considering additional process errors showed results that were not significantly different from the base case, indicating that estimates by the base case are relatively robust to these uncertainties. However, scenarios using Japan's indices up to the latest available year, 2023, yielded results that differed significantly from the base case, representing the most pessimistic scenario with recent SSB being low and fishing pressure being high. This was primarily due to the low values of Japan's indices in 2023. Comparative analysis of predictive skill between the base case and this sensitivity scenario demonstrated higher accuracy in short-term forecasting for the sensitivity scenario. Therefore, it is crucial to consider the results of the sensitivity scenario using the latest information from the Japan’s index thoroughly when assessing recent stock status, conducting risk assessments from future projections, and making scientific management recommendations.