Tatsuya Sakamoto, Motomitsu Takahashi, Kotaro Shirai, Tomoya Aono, Toyoho Ishimura

1. Understanding the sources of recruits is essential for stock assessments of marine fish populations. In 2014 and 2019, schools of Japanese sardine in the Sea of Japan and the East China Sea (SJ-ECS), which arrive in Japanese coastal areas for spawning each spring were shockingly sparse. Abundances of eggs and juveniles also showed abrupt declines, suggesting that sardine reproduction in the SJ-ECS was severely limited during these years. However, in spring of 2015 and 2020 age-1 fish appeared as usual in the coastal areas, along with fish of other ages, challenging the current assumption that sardine in the system is a self-recruiting subpopulation. 2. To test the self-recruiting hypothesis, we analysed the stable oxygen and carbon isotopes (δ18O, δ13C) for otolith areas formed during the first spring and summer in otoliths of age-0 and age-1 sardines in 2010 and 2013–2015 year-classes captured in the SJ-ECS, as indices of temperature and metabolic trajectories. 3. Age-0 sardines generally showed a significant decrease in otolith δ18O from spring to summer, reasonably reflecting seasonal warming in the SJ-ECS. However, the majority of age-1 captured in spring 2011, 2015 and 2016 showed non-decreasing profiles of otolith δ18O, suggesting that the age-0 off the Japanese coast were not the main source of recruitment. The δ18O for summer thus indicates different migration groups: the “locals" growing up off the Japanese coast and the migrating “nonlocals". 4. The isotope ratios of the “nonlocals” overlapped with those of age-0 captured in the subarctic western North Pacific, suggesting that the “nonlocals” may be migrants from the Pacific, or perhaps an unsampled potential northward migration group in the SJ-ECS. Only in 2014 did the majority of age-1 consist of the “locals”, suggesting that the abrupt decline in catches was caused by the absence of the “nonlocals” and accompanying adults. 5. Synthesis and applications Our results highlight the significant uncertainty in the population structure assumed for the current stock assessment models for Japanese sardine. Concentrated investigations on recruitment processes to test and quantify the potential migration groups are recommended to improve the assessment model.