Sensitivity analyses of the 2025 chub mackerel stock assessment in the Northwest Pacific Ocean

We conducted sensitivity analyses to examine the impacts of observation uncertainty and model uncertainty in the 2025 stock assessment of chub mackerel in the Northwest Pacific. The analysis showed that the necessary assumptions of biological parameters   to use 2024 fishing year abundance indices do not greatly affect stock abundance estimates. The analyses also show that models with the 2024 indices had higher prediction skill than models without the 2024 indices. We suggest using the most recent abundance indices in the stock assessment considering the robustness and predictability. Our results also suggest that process errors for age-1 and older fish and nonlinearity for age-0 and age-1 indices substantially change stock dynamics such as the strength of the 2013-year class, but these models exhibited bad model performance with respect to fit, prediction skill, and robustness. MSY-reference points were highly sensitive to the choices of data, biological parameters, and stock-recruitment relationship. This highlights the difficulty of using the MSY-reference points, and it may be appropriate to use more robust quantities based on historical SSB estimates as interim and empirical reference points, such as median or quartiles.