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Estuaries are complex environments that provide important nursery areas for several fish species, but anthropogenic activities as well as low salinities may affect fish reproductive potential. This study investigated spawning migrations of turbot (Scophthalmus maximus) and European flounder (Platichthys flesus) in the estuary of Roskilde Fjord using acoustic telemetry. For turbot, migratory behaviour was coupled with salinity measurements to estimate likelihood of successful spawning. Turbot inRoskilde Fjord are stocked fish, whereas the European flounder represents a naturally occurring population. Telemetry data suggested that the two species exhibited different migration behaviours towards the spawning season. The migratory behaviour of turbot indicates that they remain in the southern parts of Roskilde Fjord where successful development of eggs and larvae may be limited by low salinity. In contrast, the majority of European flounder migrated towards more marine waters prior to the spawning season, and only a minority of the fish remained inside Roskilde Fjord during the spawning season. Consistent with previous studies, the present results indicate that European flounder perform partial spawning migration. Thus, European flounder may utilize a diversity of spawning areas, including the brackish waters inRoskilde Fjord estuary as well as more marine waters with elevated salinities. Our results are important for future management of spawning areas, recruitment dynamics and selection of suitable turbot populations for stocking. • Monitoring spawning migrations with acoustic telemetry. • Estuarine-Marine connectivity facilitated by spawning migrations. • Contrasting migration behaviour in two estuarine flatfish; European flounder and turbot. • Maladaptation limits success of marine fish stocking programmes Highlights. [ABSTRACT FROM AUTHOR]