ZWARTS L (1972) The Grey Lag Geese Anser anser of the brackish tidal habitat the Ventjagersplaten,. LIMOSA 45 (3): 119-134.

The wildfowl reserve? the Ventjagersplaten is situated in the former estuary Haringvliet,. 15 miles south of Rotterdam (Fig. 1). During our observation period (1966-1969) the Ventjagersplaten was still a brackish, tidal area with extensive mudflats; the tidal range was 2.3 yards. The geese were free of disturbance. A dike road intersect the Ventjagersplaten. Before the closure of the Haringvliet (1970) maximal 10,000-11,000 Grey Lag Geese were haunting in or close to the brackish and fresh tidal waters Biesbosch, Hollandsch Diep and Haringvliet; in winter 1,000-6,000 Grey Lags were present. The Ventjagersplaten were one of the most important haunts. The Scirpus fields in this tidal habitat harboured maximal 5,000-6,000 Grey Lags, but after 1959 the Scirpus fields as well as the Grey Lags decreased (Lebret 1964). For the period 1966-1969 the average numbers are given (Table 1). In summer 1968 en 1969 (and 1970; Limosa 43:159) 600-1100 Grey Lags went through their wing moult on the Ventjagersplaten (Table 3 and also Ouweneel 1969). The Grey Lags of the Ventjagerplaten also< grazed in its surroundings, on low grasslands along the river and on arable and pasture land lying on the landside of the dikes. The distance between those feeding areas and the resting places (Fig. 1) was mostly about one and maximal two miles. In tidal habitat the Grey Lags were noctural feeders; they rested between 1;6 h. after sunrise and 21/2 h. before sunset. In the day-time (a seperate group of) Grey Lags were foraging merely on the arable and pasture land. In August and September all Grey Lags present foraged on the Ventjagersplaten, but when the numbers increased part of them started feeding in the surroundings (Fig. 3). In winter small numbers were haunting, partly foraging outside the Ventjagersplaten. It is suggested that, other things being equal, the distance between resting place and feeding area is as small as possible. On the Ventjagersplaten Scirpus maritimus was more common than Scirpus lacustris. In 1966-1969 Scirpus was growing on 8 places (Fig. 2). The geese were feeding in regular groups, and as we made detailed counts and estimated the surface of the Scirpus fields, it was possible to estimate the grazing intensity. The results are given in Table 2. Is is concluded that on the Ventjagersplaten a grazing intensity of 1,500-2,500 geese days/ha (4,000-6,000 geese days/acre) in the autumn had no effect on the coming up of Scirpus next spring; the feeding rate in winter and spring didn't amount to much. With the help of the data of Draayer (1967) it was possible to calculate the grazing intensity in three other Scirpus areas in the tidal habitat of Hollandsch Diep and Haringvliet (Table 4). The total grazing intensity in autumn, winter and spring in these areas was nearly just as great; there wasn't any overgrazing. It is ascertained that the maximal intensity of grazing on the Ventjagersplaten amounted to 3.000-4.000 geese daysJha (7.500-10.000 geese days/acre). Scirpus could not recover from such a severe grazing, however. It is not known whether an equivalent grazing of 3.000-4.000 geese daysJha would produce the same effect, if the geese wouldn't have fed only in autumn, but also in winter and spring. Because the Grey Lags fed especially on the borders of the Scirpus fields, the denudation of the vegetation progressed successively (Fig. 4). Figures 5 and 6 give an impression of a Scirpus field after severe grazing. In 1958-1959 a dike was constructed (called Hellegatsdam) across the Ventjagersplaten. As a consequence of this Scirpus declined locally. It is suggested that the numbers of the geese didn't fall in the same degree, and that therefore the grazing intensity increased; because of this the Scirpus fields disappeared by overgrazing in the course of several years. The relationship between feeding intensity and pattern of dispersion is discussed. It is suggested that a certain regulation of the numbers didn't prevent overgrazing. Greylag Goose Anser anser

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