Using acoustic telemetry to locate flatfish spawning areas: Estuarine migrations of turbot Scophthalmus maximus and European flounder Platichthys flesus

Estuaries are complex environments, which 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 Roskilde Fjord using acoustic telemetry. For turbot, migratory behaviour was coupled with salinity measurements to estimate likelihood of successful spawning. Turbot in the Roskilde Fjord are stocked fish, whereas the European flounder represents a naturally occurring population. Data suggested that the two species exhibited different migration behaviours towards the spawning season. The migratory behaviour of turbot indicate that turbot remained in the southern parts of the Roskilde Fjord where successful development of turbot 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 the 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 in the Roskilde 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.