Regional migration patterns of young-of-the-year Japanese jack mackerel (Trachurus japonicus) revealed by tissue stable isotope analysis in the Uwa Sea, Japan

Understanding fish migration patterns is crucial for the effective management of stocks. While distinct patterns, such as resident and migratory types, have been suggested for the Japanese jack mackerel (Trachurus japonicus), their specific patterns along the Pacific coast remain unclear. In the Uwa Sea, a key region of the Pacific coast of Japan, potential differences in the migration of young-of-the-year (YOY) fish between the north and south have been empirically recognized. In this study, the migration patterns of YOY Japanese jack mackerel in the Uwa Sea were elucidated. A linear mixed model (LMM) for tissue stable isotope ratios (δ¹³C and δ¹⁵N) was constructed to explore the spatial variation. Seasonal variation in tissue stable isotope ratios, abundance, and fork length (FL) across the Uwa Sea was also compared to examine migration patterns. Our analysis revealed that latitude significantly affected the variation in δ¹⁵N, suggesting that δ¹⁵N variation was influenced by spatial variation in the δ¹⁵N baseline. Significant differences in the abundance and mean FL between the northern and southern areas, as well as latitudinal differences in δ¹³C and δ¹⁵N responses to FL, suggest limited movement of YOY Japanese jack mackerel from the southern to the northern area. While δ¹³C values increased with growth in both areas, a significant increase in δ¹⁵N values was observed with growth exclusively in the northern area. Furthermore, in the northern area, there was a significant correlation between the recruitment (May to June) and post-recruitment (July to April) abundance indices, which were not observed in the southern area. These findings suggest that migration patterns vary regionally, with populations in some areas remaining localized to support subsequent stocks, whereas populations disperse and reaggregate over a wider region in other areas. This underscores the importance of considering regional differences in migration patterns when developing stock management strategies to ensure the sustainability of this species.