BOUDEWIJN TJ (1989) The Poehard Aythyaferina as a seedeater in Lake Grevelingen. LIMOSA 62 (4): 169-176.

Common Pochard Aythya ferina The Haringvliet, a former estuary in the southwestern part of the Netherlands, was closed in 1970. From 1972 Pochards visit this freshwater area in autumn in important numbers. Up to 20 000 Pochards can stay here (fig. 2). In 1984, in the evening large groups were seen flying into the direction of the saline lake Grevelingen. In 1985 it was found that these birds fly to Lake Grevelingen in the evening and return to the Zuiderdiep in the early morning. The oesophagus of five Pochards, shot during the morning flight, mainly contained seeds of eelgrass Zostera marina. The oesophagus of the males also contained some tens of Idotea chelipes. Both males and females had eaten some shells (tab. 1). The size of the shells was comparable with the size of the seeds. Probably the shells were passively collected with the seeds from the bottom of the lake. In autumn the numbers of Pochards, both in the Haringvliet and the total northern Delta area, were highly correlated with the area of eelgrass beds in Lake Grevelingen (fig. 3). Since also the arrival of Pochards in autumn coincides with the release of seeds by eelgrass it is concluded that in autumn all the Pochards in the northern part of the Delta area feed on the seeds of eelgrass in Lake Grevelingen at night. Only in 1975, 1977 and 1978 the bird counts of Lake Grevelingen showed important numbers of Pochards during the day. This coincided with a low salinity of the water of lake Grevelingen in these years (fig. 4). In 1986 the Philipsdam in the Krammer Volkerak was finished and the water in the new Lake Volkerak quickly became fresh. In October 1987 the first Pochards were observed in this lake close to the Grevelingen. In future the Pochards of the Haringvliet will probably prefer to stay in the Volkerak during the day. The flying distance will be shortened by about 10 km then. Until now the comsumption of eelgrass by the Pochard was estimated at maximally 0.04% of the annual total production, but this study showed the consumption may be 5-10% of the annual seed production.

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