Buried secrets: What otoliths tell us about sandeel (Ammodytes marinus) connectivity and movements

This study examined the natal sources and post-settlement movements of the lesser sandeel (Ammodytes marinus) in the North Sea using otolith chemistry and hydrodynamic drift modelling to resolve geographic and temporal patterns in connectivity. Significant differences in the chemical composition of the otolith core (Ba, Cu, Fe, Li, Mg, Mn, Pb, Sr, and Zn), representative of the time of hatching, were evident in samples caught in early winter in three major fishing grounds on Tail End, Horns Reef, and Fisher-Jutland Banks. Three distinct natal source signals were identified in each of four cohorts (2016, 2017, 2019, 2020). The relative contribution of these sources varied among areas and cohorts without a consistent geographic pattern. Drift simulations indicated that the origin of the three natal source clusters align with known major spawning grounds in the North Sea, suggesting that interannual variation in current-driven larval drift shapes connectivity. Sandeel from different natal sources but settling in the same areas retained distinct chemical profiles until mid-summer, after which profiles converged until the winter survey. This indicates that sandeel following settlement remain site-attached in their chosen areas until at least the end of their second winter. Overall, this study shows that population dynamics of lesser sandeel in the North Sea are shaped by larval drift linking spawning grounds and settlement areas, with little movement between areas after settlement, where individuals originate from different natal sources, and the strength of the connectivity depends on inter-annual variations in current-related drift patterns during the larval and early juvenile stages.