Standardized CPUE of chub mackerel (Scomber japonicus) from the RV Songhang trawl survey in the Northwest Pacific Ocean, 2021– 2026

The nominal CPUE of Chub mackerel (Scomber japonicus) was standardized using fishery-independent trawl survey data collected by the RV Songhang in the Northwest Pacific Ocean between 2021 and 2026. Fifty-four candidate generalized linear mixed models were fitted with the sdmTMB package, comprising all combinations of three response distributions (Tweedie, delta-gamma and delta-lognormal), two spatial random-effect settings and nine nested covariate sets, and were compared using the Akaike information criterion (AIC), convergence and Hessian diagnostics, and five-fold spatial block cross-validation. Among the 20 models that passed all diagnostics, a delta-lognormal model with no spatial random field and with sea surface temperature and chlorophyll-a concentration as covariates was selected. The standardized abundance index was highest in 2021 and 2023, declined in 2024, recovered slightly in 2025 and reached its six-year minimum in 2026. Coefficients of variation ranged from 0.466 to 1.320, peaking in 2026. We recommend that this standardized index could be used as one of abundance indices for the Chub mackerel stock assessment.