Eye lens isotope tag reveal migration as a driver of Japanese sardine synchrony

Tatsuya Sakamoto, Motomitsu Takahashi, Kotaro Shirai, Satoshi Kitajima, Toyoho Ishimura

Understanding population fluctuations of broadly distributed marine fishes remains difficult, partly because they often consist of cryptic mixtures of individuals originating from geographically distinct nurseries that experience different environment pressures. However, resolving these spatiotemporally changing mixing processes had been challenging as conventional techniques are highly resource- and time-consuming. Here, we present a high-throughput approach using archival stable isotopes (carbon and nitrogen) in fish eye lens centers as a reliable natural tag of nursery origin. Applied to over 2000 Japanese sardine in the western North Pacific, a species long assumed to comprise two distinct subpopulations, the age-0 isotope baselines showed clear and temporally consistent geographical variation, allowing robust classification of recruitment source. Analysis of adults collected between 2019 and 2023 revealed the recruits from the offshore Pacific were dominated not only the major fishing ground in the Pacific but also the spawning grounds in the Sea of Japan and East China Sea, directly contradicting the long-standing two-stock model. These trans-boundary migration of Pacific recruits explain the century-long synchrony observed between putative subpopulations, and alter the ecosystem structure at the destination. This scalable method is applicable across mobile marine fauna, providing a crucial tool for spatial ecosystem management under rapid climate change.