Candidate Base Case Scenarios for the 2025 Chub Mackerel Stock Assessment in the Northwest Pacific Ocean

A state-space age-structured (assessment) model (SAM) was used to conduct a stock assessment of the chub mackerel stock in the Northwestern Pacific. This working paper proposes two candidate base case scenarios for the stock assessment this year. The difference between the two base case scenarios is exclusion or inclusion of the latest (2024) abundance indices. The two scenarios obtained almost identical population dynamics. Stock levels were historically high in the 1970s, but declined in the 1980s, were maintained at fairly low levels from the 1990s to the early 2000s; stock levels gradually recovered in the late 2000s and increased rapidly after the occurrence of the strong year class in 2013. However, after peaking in 2017 and 2018 in the scenarios without and with the latest abundance indices, respectively, the stock levels rapidly dropped again. In 2023, the spawning stock biomass was only 16% of the respective peak levels. Neither of the peaks in 2017 (without the latest indices) nor in 2018 (with the latest indices) reached the stock levels observed in the 1970s. We found no serious problems in the model diagnostics, except for the positive retrospective patterns in both scenarios. These patterns were smaller when the latest indices were included in the model. While the estimated population dynamics were generally consistent with the base case in the previous stock assessment, the total biomass, spawning stock biomass, and recruitment in the most recent years were revised downward considerably by the inclusion of 2023 indices, which were not included in the previous base case.