Standardized Abundance Indices for Age-0 and Age-1 Fish of Chub Mackerel from Northwest Pacific Autumn Surveys up to 2026

We standardized CPUE from autumn surface trawl surveys of Pacific chub mackerel using the Vector Autoregressive Spatio-Temporal (VAST) model. Local densities of age-0 and age-1 fish in the Northwest Pacific were estimated for 2005–2026, accounting for spatial autocorrelation and environmental covariates derived from sea surface temperature (SST) and temperature at 30 m depth. High abundance indices were frequently observed after 2013, whereas recent indices were generally low. The age-0 index increased in 2025 before declining in 2026 to a level comparable to that in 2012. The age-1 index declined sharply in 2025 and further in 2026, reaching its lowest level since 2010. No age-1 fish were caught in 2026; a positive standardized index was estimated using random year effects, but this estimate was highly uncertain. Model diagnostics revealed no serious residual patterns. We propose using the standardized indices as age-0 and age-1 abundance indices in the Technical Working Group on Chub Mackerel Stock Assessment (TWG CMSA), with particular attention to uncertainty in the 2026 age-1 estimate.