Spatiotemporal standardization of Taiwanese longline CPUE reveals cohort-resolved recovery patterns in Pacific bluefin tuna ( Thunnus orientalis )

Abstract Catch-Per-unit-Effort (CPUE) from the Taiwanese longline fishery shows a rapid increase following the introduction of management measures that were designed to rebuild the stock. Age-group spatiotemporal standardization was applied to investigate CPUE patterns and to evaluate whether fishery-dependent evidence was consistent with the success of these management measures. The Taiwanese longline fishery provides the only remaining fishery-dependent indicator of adult Pacific bluefin tuna used in the ISC stock assessment. A reconstructed spatiotemporal dataset for this fishery (2003–2024) was assembled by combining dockside market inspection data, with fork length measured for 95% of landed fish, and independently reconstructed fishing effort from vessel data recorder-derived fishing days. Standardized adult CPUE indices were developed and contrasted using a traditional delta-generalized linear mixed model (delta-GLMM) and a spatiotemporal Vector Autoregressive Spatio-Temporal (VAST) framework. Outputs were generated for approximate age groups using the VAST framework (“VAST-at-age”) to characterize cohort-resolved patterns and associated spatial distribution metrics (effective area occupied and center of gravity) during rebuilding. Age groups were approximated using deterministic length-based assignment. Model evaluation was conducted using descriptive diagnostics (cross-validation and retrospective analyzes) to assess internal consistency. Standardized adult indices showed increasing trends after 2012, with more rapid increases in recent years. Cohort-resolved outputs indicated that recent increases were dominated by younger adult groups (6–8 and 9–11 years), while older groups (15 years) contributed proportionally less over the same period. Together, these results provide a cohort-resolved and spatially explicit description of changes in the adult index and provide fishery-dependent evidence consistent with stock assessment conclusions that reduced juvenile fishing mortality contributed to rebuilding of the stock.