waders and their estuarine food supplies - Vlaams Instituut voor de ...

PREDICTING SEASONAL AND ANNUAL FLUCTUATIONS IN THE LOCAL EXPLOITION OF DIFFERENT PREY
Fig. I. Map .'I the Study area along ihe Frisian coast. The food supply was measured in the Nes area (7.3 ha: site N I. The birds were counted
in the Nes area and in twenty counting areas around the Nes area, in total MX) ha ol tidal Hals. See als,. ihe map ol Fig, I"
Methods
Study area
The study was performed between 1977 and 1986 on
intertidal Hats along die Frisian coast in ihe Dutch
Wadden Sea (Fig. I). The study area was intersected
by long rows of poles, these being ihe remnants of
brushwood groynes made in the sixties to enhance sedimentation.
The former groynes delimited the study
area, in total 396 ha. and divided it into 53 subareas.
The eastern subareas were situated around mean sea
level and the clay fraction (< 2 pm) in the substrate was
2-51. The more western subareas were al about 25 cm
above mean sea level and the substrate was soft with a
clay content of 10-20%. For a more extensive description
of the study area, see Zwarts et al. (1992).
The Oystercatchers feeding in this area used several
roosts in the immediate surroundings, but at high
tide the majority were found in the Paesenserpolder or
on the island Engelsmanplaat. In total, about 20 000
Oystercalchers foraged on the tidal flats between the
Frisian coast and the island Engelsmanplaat. We individually
marked more than 3000 birds wilh the aid of
3 colour rings per bird, of which one with bar-codes.
Most were captured with mist and cannon nets on the
tidal roost in the Paesenserpolder.
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Measuring the food supply
Sampling Samples of the macrozoobenthos inhabiting
the mudflats were taken along 27 transects in August
during four years (1977-1980). More detailed data
were collected in the eastern part of the study area,
where 146 plots of 0.1 ha were pegged out around two
observation towers. One to four samples of the benthic
fauna (179 cm-) were taken nearly every month during
seven years (1980-1986. and less frequently in 1978
and 1979) in the 73 sites around one of these towers,
the Nes area, in earlier papers also indicated as site N.
This series of measurements is used here. A comparison
between the Nes data and the samples taken along
the transects covering the entire area showed thai ihe
Nes samples were representative for the area as a
whole, especially for the most eastern part of ihe area
(Zwarts 1988). The measurements in the Nes area
started in December 1977 but. to extend the series of
measurements, we will use for August 1977 the samples
of two of the 27 transects that crossed the Nes
area.
Condition, growth and mortality The laboraiotv
procedures used to determine the biomass of the benthic
animals have been described in Zwarts (1991).
The growth of the bivalves could sometimes be ealeu-