320
J. van der Meer et a1.
the cockle Cerastoderma edule (1.) and the Baltic tellin Macoma balthica (1.),
which are among the most important food sources for two shellfish-eating
shorebird species occurring in the Wadden Sea, the knot Calidris canutus 1.
and the oystercatcher Haematopus ostralegus 1. (Hulscher 1982, 1983; Piersma
et al. 1993).
Between 1970 and 1998, 12 randomly selected transects of l-km length
were sampled each year in late winter (February-March) and late summer
(August). One transect near low-tide level was not taken into account in the
present review, because this plot rarely emerges and is hardly available to
foraging shorebirds. Along each transect 50 cores were taken, 0.019 m 2 each in
winter and 0.009 m 2 in summer. Numerical and ash-free dry biomass density
values were determined for each age-class separately. More details of the
sampling procedure are given in Beukema (1974, 1988, 1993). In the remaining part of this chapter, the term 'recruit' refers to juveniles (i.e. zero-year
class) at the first summer sampling in August. We assumed that the cockle is
sedentary (and hence there is no net migration in or out of the area) after
recruitment. The Baltic tellin is sedentary after the first winter. These assumptions, which enable the estimation of adult mortality rates and production,
were confirmed by surveys with plankton nets suspended in tidal streams in
the Wadden Sea (Beukema 1989) and by studies on the recolonization oflarge
defaunated areas (Beukema et al. 1999). In both cases no adults, and only
post-larvae and juvenile Baltic tellins were observed.
The number of birds that feed on the tidal flats can be estimated from
monthly counts at the high-water roosts, available since the mid-1970s (M.
Otter, pers. comm.). We assume that birds counted on their daytime roosts
represent the population making a living on the Balgzand intertidal. For
oystercatcher, in particular, this is probably a robust assumption. Only few
(unpublished) data on feeding behaviour and diet at Balgzand are available,
but detailed studies have been performed elsewhere in the Wadden Sea
(Zwarts and Drent 1981; Hulscher 1982; Zwarts and Blomert 1992; Zwarts et
al. 1992; Piersma et al. 1993) and in the laboratory (Hulscher 1976; Piersma et
al. 1995).
15.4 Long-Term Variability in Production and Consumption
at the Balgzand
For all cohorts (animals "born" in the same year) secondary production
(elimination) was estimated for each half-year period by the mortalitysummation method (Crisp 1984), i.e. by multiplying the observed decrease in
density (averaged over sites) by the average of the individual mass at the start
J. van der Meer et a1.
the cockle Cerastoderma edule (1.) and the Baltic tellin Macoma balthica (1.),
which are among the most important food sources for two shellfish-eating
shorebird species occurring in the Wadden Sea, the knot Calidris canutus 1.
and the oystercatcher Haematopus ostralegus 1. (Hulscher 1982, 1983; Piersma
et al. 1993).
Between 1970 and 1998, 12 randomly selected transects of l-km length
were sampled each year in late winter (February-March) and late summer
(August). One transect near low-tide level was not taken into account in the
present review, because this plot rarely emerges and is hardly available to
foraging shorebirds. Along each transect 50 cores were taken, 0.019 m 2 each in
winter and 0.009 m 2 in summer. Numerical and ash-free dry biomass density
values were determined for each age-class separately. More details of the
sampling procedure are given in Beukema (1974, 1988, 1993). In the remaining part of this chapter, the term 'recruit' refers to juveniles (i.e. zero-year
class) at the first summer sampling in August. We assumed that the cockle is
sedentary (and hence there is no net migration in or out of the area) after
recruitment. The Baltic tellin is sedentary after the first winter. These assumptions, which enable the estimation of adult mortality rates and production,
were confirmed by surveys with plankton nets suspended in tidal streams in
the Wadden Sea (Beukema 1989) and by studies on the recolonization oflarge
defaunated areas (Beukema et al. 1999). In both cases no adults, and only
post-larvae and juvenile Baltic tellins were observed.
The number of birds that feed on the tidal flats can be estimated from
monthly counts at the high-water roosts, available since the mid-1970s (M.
Otter, pers. comm.). We assume that birds counted on their daytime roosts
represent the population making a living on the Balgzand intertidal. For
oystercatcher, in particular, this is probably a robust assumption. Only few
(unpublished) data on feeding behaviour and diet at Balgzand are available,
but detailed studies have been performed elsewhere in the Wadden Sea
(Zwarts and Drent 1981; Hulscher 1982; Zwarts and Blomert 1992; Zwarts et
al. 1992; Piersma et al. 1993) and in the laboratory (Hulscher 1976; Piersma et
al. 1995).
15.4 Long-Term Variability in Production and Consumption
at the Balgzand
For all cohorts (animals "born" in the same year) secondary production
(elimination) was estimated for each half-year period by the mortalitysummation method (Crisp 1984), i.e. by multiplying the observed decrease in
density (averaged over sites) by the average of the individual mass at the start