What caused the increase in retrospective pattern in the chub mackerel stock assessment in the Northwest Pacific?

In the 2025 provisional assessment of the Northwest Pacific stock of chub mackerel, retrospective patterns quantified by Mohn’s rho for stock biomass and recruitment over the five-year retrospective analysis increased compared to the values in the previous year’s assessment. An investigation into the cause revealed that the primary factor was that all index values for 2023 were lower than the predicted values of the model without 2023 indices, leading to a downward revision of recent stock biomass and recruitment when including the 2023 indices. The one-year shift in the reference period used to calculate Mohn’s rho also contributed to the increase in the values. In contrast, changes in the stock assessment model settings had little effect. It was also found that revisions to catch-at-age data prior to 2022 contributed to a reduction of Mohn’s rho. Retrospective pattens does not necessarily mean estimation bias and rather the update of estimates with new data is a crucial step in stock assessments. For the chub mackerel stock assessment, it is important to include the latest available abundance indices to mitigate future shifts in abundance estimates that would inevitably occur if the latest data were excluded.