Spatio-temporal dynamics for dolphinfish (Coryphaena hippurus) in the waters around Taiwan: Implications for stock assessment and fisheries management

Abstract

Understanding the spatio-temporal dynamics of dolphinfish (Coryphaena hippurus) is essential for effective stock assessment and fisheries management. Owing to its migratory nature, rapid growth, and short lifespan, the species requires management approaches that account for spatial and temporal variability. In this study, two spatio-temporal modelling frameworks, VAST and sdmTMB, were applied to longline fishery logbook data collected in the waters around Taiwan from 2011 to 2023. Standardised indices of relative abundance were estimated using an area-weighted approach that accounted for both fished and unfished regions. Cluster analysis was conducted to classify fishing strategies based on species composition, revealing a dominant dolphinfishtargeted cluster concentrated in the eastern offshore region. Both spatio-temporal models detected consistent seasonal patterns, with peak abundance in the second quarter and a decline in the third, aligning with known migratory behaviour and oceanographic influences. Spatial hotspots were identified primarily in the northeastern waters influenced by the Kuroshio Current. While general agreement was observed between the models, sdmTMB produced higher spatial contrast and slightly greater abundance estimates in the early years. Differences in model structure, spatial resolution, and data coverage were found to influence predictive outcomes. Limitations regarding logbook data quality, spatial bias, and the absence of environmental covariates were acknowledged. Nevertheless, the results demonstrate the utility of spatio-temporal modelling in deriving robust abundance indices and identifying biologically and operationally significant fishing grounds that represent key habitats and high-effort zones for dolphinfish fisheries in the northwestern Pacific. These findings provide a strong basis for integrating spatial dynamics into stock assessments and support the development of adaptive, spatially explicit management strategies in the western Pacific.