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

Table I. Annual variation in the biomass of prev species of Knol
Calidris canutus ig AFDW m -'i.n sue N in August of 1977 to 1986.
Average, lowest and highesi hiomass are given. RSD: relative standard
deviation (SD as percentage of the mean i.
Spei ies Average, g Range, g RSD.%
Macoma 17.22 7.4- 34.5 4(.(.
Cera.sioilenna 20.66 0- 62.3 107.2
Mya 13.20 7ii 23.7 38.6
Si nihil ularia 20.2.1 8.1- 46.3 86.S
Msiilus 1.45 0- 7.9 186.6
Total 73.26 52.X-111.0 26.6
Table 2. Annual variation in the biomass within the range of size
classes harvestable bj Knot Calidris canutus ig AFDW m -: without
depth restriction) al she N in August of 1977 to 1986, Range ol hat
vestable si/e classes are laken from Zwarts & Blomerl (1992). Ai
erage. lowest and highest biomass harvestable by Knot are given.
RSD: relative standard deviation (SD as percentage of the mean).
Species nge, mm Average, ? Range, % RSD, '.
Macoma 6-16 7.81 3.9-14.9 44.0
Cerastoderma 5-12 1.25 (i 6.6 155.7
Mya 7-17 0.33 0- 1.3 153.4
Si nihil ularia 7-14 0.14 0- 1.4 .'00.0
Mytilus 5-20 0.33 0- 2.6 2377)
Total 9.86 3.9-17.7 45.9
biomass. The biomass of these species did not deviate
much, however, from the values found in other, similarly
sheltered areas wilh mixed substrate, situated at
or just above mean sea level (Beukema 1976. Zwarts
1988b).
Knot do not swallow large prey and they ignore the
small si/e classes (Zwarts & Blomert 1992). Table 2
gives the range of size classes harvestable bv Knol and
the corresponding biomass for these size classes at site
N. A comparison of Tables I & 2 reveals that onlv a
small fraction of the total biomass fell within die range
of size classes harvestable by Knot. The exception vv as
Macoma. for which, on average. 449f of ihe total
biomass may be considered as harvestable by Knot.
VARIATION IN FOOD SUPPLY OF KNOT
292
Moreover, of all the potential prey species, the annual
variation in harvestable biomass is least in Macoma.
The large annual variation in total food supply was
caused by the erratic occurrence of summer recruitment
and winter mortality (Beukema et al. 1978,
Beukema 1979. 1982. Beukema & Essink 1986, Desprez
«?/ al. 1991, Ducrotoy el al. 1991, Essink et al.
1991. Jones & Park 1991. Obert & Michaelis 1991).
There were also large differences among species in annual
growth rale and mortality (Fig. 4). Cerastoderma
grew rapidly but the majority died within two or three
years. There was recruitment in this species during
seven of the ten years, but the only substantial cohort
originated in 1984. No spatfall of any significance occurred
in Scrobicularia during ihe ten years after 1976.
when a strong year class was recruited (see also Essink
et al. 1991). Many of these animals were still alive in
1983 and 1984. In Mya as well, there was heavy spatfall
during only one of the ten years, viz. 1979. Recruitment
was variable in Macoma. but the variation
was not as large as in the other species: in six of the ten
years there was recruitment, with a considerable spatfall
occurring in three of them.
Food of Knot
After the first growing season. Mya reached A si/e of 5
lo 10 mm and grew io about 25 mm by the end of their
-econd summer (Figs. 4 & 5). This means that the majority
of the first-year animals were ignored by Knot
since they were too small and therefore unprofitable,
while by the end of the next growing season they had
become too large to be sw allowed. For ihe same reason,
onlv die largest Cerastoderma and Msiilus in their
first year and the most slowly growing individuals in
their second year were potential prey for Knot. However.
Knot were able to feed on Macoma over a much
longer period, as it took a cohort of ibis species four
years to pass through the 'window of predation" digs.
4&5).
The potential importance of Macoma as prey is also
clear from Fig. 6. which shows the annual variation in
biomass of the si/e classes harvestable by Knot. The
other prey contributed, on average, only 2\ c