Accumulation of flounder larvae ( Platichthys flesus L.) in the Dollard (Ems estuary, Wadden Sea)

In the context of an investigation of the importance of the Dollard as a nursery for flounder ( Platichthys flesus L.), the process of larval accumulation was studied. The hypothesis was addressed that freshwater discharge is a steering factor in larval transport caused by a larval preference for low-salinity water. Flounder larvae were simultaneously sampled at three locations during five tidal cycles in combination with measurements of physical variables. One location was in the main channel, the other two were in more peripheral channels of the Dollard. One of those is directly connected to a sluice by which freshwater is discharged into the Dollard. Concentrations of larvae varied during the tidal cycle at all locations: high initial values dropped after the middle of the flood to lower levels to increase again at the end of the ebb. In the channel connected to the sluice, significantly lower salinities were observed around low water than at the other locations, which coincided with significantly higher larval concentrations at the beginning of flood. Water transport as well as larval transport at this location showed a flood surplus on all sampling dates. Also, relatively more larvae were transported than expected according to the water transport compared to the other stations. Moreover, there was an ebb surplus in larval transport during some sampling dates at the other locations. The results indicated an accumulation of flounder larvae in the inner Dollard, and particularly in the area affected by the discharge of freshwater. Independent observations of settling flounder larvae in traps also indicated a concentration in the vicinity of the sluice. However, the sluice itself may form an effective barrier for flounder larvae that try to migrate to freshwater.