Base case stock assessment for chub mackerel in Northwest Pacific Ocean in 2024

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 shows the model selection methodology, parameter estimates and model diagnostic results for two base case scenarios (with different settings of M for natural mortality coefficient) in detail. Estimated total biomass and spawning stock biomass (SSB) declined from high levels in the 1970s to low levels in the 1980s, remained at low levels in the 1990s and early 2000s, and increased in the late 2000s; a 2013 strong year-class led to a significant increase in total biomass and SSB, which peaked in 2017 and declined slightly up to the latest year (2022), with recent abundance levels estimated to be lower than those of the 1970s. Model diagnostic showed that a few parameters had strong correlations and large uncertainties with other parameters, and the retrospective analysis showed a moderately large positive bias in total biomass, and there is room for further improvement on these issues.