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 freshwate r discharge is a steering factor in larval transport caused by a larva l preference for low-salinity water. Flounder larvae were simultaneous ly 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 i s discharged into the Dollard. Concentrations of larvae varied during the tidal cycle at all locations: high initial values dropped after th e 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 s alinities 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 th is location showed a flood surplus on all sampling dates. Also, relati vely more larvae were transported than expected according to the water transport compared tl, the other stations. Moreover, there was an ebb surplus in larval transport during some sampling dates at the other l ocations. The results indicated an accumulation of flounder larvae in the inner Dollard, and particularly in the area affected by the discha rge of freshwater. Independent observations of settling flounder larva e in traps also indicated a concentration in the vicinity of the sluic e. However, the sluice itself may form an effective barrier for flound er larvae that try to migrate to freshwater. (C) 1998 Elsevier Science B.V. All rights reserved.