Different migration patterns of European anchovy and sardine around Iberian Peninsula revealed by eye lens isotopes

Small pelagic fish are key components of productive coastal ecosystems, yet their migration ecology remains poorly understood, causing challenges for management. We applied stable carbon and nitrogen isotope (δ¹³C and δ¹⁵N) analyses of eye lenses to investigate movements of European anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) around the Iberian Peninsula. Muscle isotopes showed strong spatial heterogeneity, largely consistent between species and reflecting differences in baseline values. Eye lens centres of small anchovy, and to a lesser extent sardine, also showed clear geographic variation: higher δ¹⁵N off the Atlantic south coast, lowest δ¹⁵N in the Alboran Sea, and lower δ¹³C off the west coast. These patterns persisted across years and fish sizes in anchovy, with only minor outliers, suggesting limited cross regional migration. An exception was the overlap between west coast and Cantabrian Sea values, consistent with connectivity supported by cohort tracking. In contrast, sardine isotopes from the west and south coasts converged into a unimodal distribution with growth, indicating frequent exchange between these regions. These findings support recent revision of stock limits that separate south and west coast anchovy stocks and maintain a single Iberian sardine stock, but they question the current assumption of separation between western and northern anchovy stocks. Eye lens isotopes provide a powerful complementary tool to resolve nursery origin and connectivity, offering new opportunities for fisheries management in shorter time scales than molecular techniques, which is paramount to be able to cope with rapid changes of fish distribution due to climate change, and for spatially explicit management.