This research aims to clarify the spatial and functional role of estuaries for juvenile marine fish. The question is raised which proportion of estuarine migrants contributes to the next cohort, compaired with their conspecific marine residents. Hereto fish have to be assigned to their respective habitats and exchange rates have to be determined. The Scheldt estuary-Southern Bight of the North Sea gradient is used as a model system and the targe fish species is the marine demersal sand goby (Pomatoschistus minutus; Gobiidae). Geochemical trace elements in otoliths (Sr/Ca will be used for the identification and the reconstruction of the individual life history (coupled with aging) of estuarine and marine residents in a spawning population of sand gobied. Quantification of the migrations dynamics between the marine spawning and the estuarine nursey grounds will be assessed using population dynamics and stable isotopes of fish tissues.