Otolith elemental fingerprints indicate the importance of estuarine nursery areas for European anchovy in the Gulf of Cadiz.

Understanding which habitats function as nurseries is essential for assessing population connectivity and managing exploited marine fishes. Early life stages often concentrate in estuarine and coastal environments, yet high larval abundance does not necessarily translate into recruitment to adult stocks. Here, we compared otolith chemistry and field-based abundance data to evaluate the nursery role of major estuarine systems (Guadiana, Odiel-Tinto, Guadalquivir, Cadiz Bay) and adjacent coastal areas for European anchovy (Engraulis encrasicolus) in the Gulf of Cadiz during 2016-2018. Using LA-ICPMS, we obtained elemental fingerprints from postlarvae as young as ∼20 days old, representing the first application of this technique at such an early developmental stage for this species. Despite some interannual variability, otolith elemental signatures differed consistently among locations, allowing accurate classification (80.1%) using Random Forest models. Zn, Ba, Cu, P and Pb were the most important predictors and showed positive relationships with ambient water concentrations. Adult anchovies sampled offshore in 2018 were assigned to their most probable nursery origins, revealing that ∼20% of the adult stock had used estuarine habitats, mainly the Guadiana estuary and Cadiz Bay. Importantly, the estuarine contribution did not correlate with postlarval density, particularly in the Guadalquivir estuary, highlighting the role of hydrodynamic processes in decoupling abundance from recruitment success. The heavily mining-impacted Odiel-Tinto system showed elevated metal concentrations in water and otoliths, negligible contribution to the adult stock, and evidence that Ba-based salinity marker can be confounded by contamination. Overall, this approach improves understanding of anchovy dynamics in ICES Division 9a-South.