Recent Anomalously Warm Ocean Conditions and Changes in the Migration and Spawning Distribution of Chub Mackerel ( Scomber japonicus ) in the Western North Pacific

Extreme ocean warming has become increasingly pronounced in the western North Pacific in recent years. Coincident with this, both the egg abundance and the catch per unit effort (CPUE) of chub mackerel ( Scomber japonicus ) in fisheries targeting southward‐migrating adults have decreased. In this study, we examined recent variations in major ocean currents such as the Kuroshio, the Kuroshio Extension (KE), and the Oyashio, together with regional warming conditions characterized by marine heatwave metrics derived from satellite observations. We then investigated their impacts on the changes in migration patterns and spawning distribution of chub mackerel using egg and larval/juvenile sampling and logbook. Warmer conditions off eastern Japan (EJ) were associated with lower CPUE in this region and lower egg abundance in the traditional spawning grounds. In addition, particle‐tracking experiments based on juvenile samples suggested the spawning distribution outside the region traditionally recognized as the main spawning grounds during periods of northward meander of the KE off EJ. These candidate spawning grounds were located within favorable temperature ranges along the periphery of the KE and an anticyclonic eddy and were also consistent with the offshore egg occurrences observed by egg surveys in this region. These results suggest that recent anomalously warm ocean conditions off EJ may influence the southward migration and spawning distribution of adult chub mackerel.