Preliminary standardized fishery-independent index of Japanese Sardine (Sardinops melanostictus) based on the Chinese research vessel “Song hang” from 2021 to 2025

This study standardized the nominal index of Japanese sardine (Sardinops melanostictus) using trawl survey data collected by the RV Songhang in the Northwest Pacific from 2021 to 2025. Using the sdmTMB framework, generalized linear mixed models (GLMMs) incorporating different spatial and spatiotemporal random-effect structures were developed to evaluate Tweedie and delta-lognormal distributions for standardization. Model performance was assessed using Akaike’s Information Criterion (AIC), convergence diagnostics, and Hessian matrix evaluation. The Tweedie model with an independent and identically distributed (IID) was selected as the final model for standardization. The standardized abundance index showed a generally consistent temporal pattern with nominal index, with relatively higher values during 2021–2023 followed by a substantial decline in 2024 and reaching the lowest level in 2025. Among the environmental variables examined, chlorophyll-a concentration showed a significant positive relationship, whereas sea surface temperature and sea surface height exhibited negative effects. The results demonstrate that environmental variability and sampling effects should be considered when interpreting survey-based index trends and provide a preliminary standardized abundance index for monitoring Japanese sardine resources in the Northwest Pacific.