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

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VARIATION IN FOOD SUPPLY OF KNOT
Macoma balihica
Cerastodeniia edule
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JJMMJSN'JMMjSN'JMMJSN'jMMJSN 1
Fig. 5. Growth of five bivalve species at site SJ after spatfall in sumuicr
1979 (1983 for St nihil ularia plana). The si/e classes vulnerable
lo prcdaiion h\ Knol (See fable 2) are indicaied (horizontal grey
fieldl. as well as ihe period during which Knot were present (vertical
grey bars). Average sizes were calculated from the length lie
queue) distribution* in the monthly samples (see 'Methods' i
ten years. The contribution of the 1979 year class of
Macoma lo the biomass harvestable by Knot is evident:
the harvestable food supply was highest alter
four growing seasons and decreased thereafter because
an increasing proportion had passed the critical size
threshold of 16 mm.
On some occasions. Macoma was the only prey
species harvestable hv Knot for example in late summer
of 1981 and 1983. Though Cerastoderma were
available in 1983. Knot ignored them, perhaps due to
the thick shell (Zwarts & Blomen 1992). Macoma
musl therefore have been the staple food for Knot that
passed through ihe study area in late summer, unless
the Mud Snail Peringia ulvae was taken as an alterna-
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tive. This species usually occurred ai densities of
40 000 to 50 000 nv'. though the summer of 1979 was
an exception, when only low densities of 1000 m : occuned
(Zwarts 19881*0 EJowevet direel observations
and faecal analysis showed that Peringia were nol
taken in 1979. 1981. 1983 or 1985. even though the)
were abundant in three of ihe four years. In fact, the
majority of Peringia in the study area were too small
« 2 mm) to be profitable food items for Knot (Zwarts
& Blomert 1992). The distribution paiiern of Knol
over ihe feeding area was another indication lhat Knot
did not feed on Peringia. This prey was superabundant
at the sites situated above mean sea level and in areas
where the clay content of the substrate was above 10%
(Zwarts 1988b). However. Knot ignored these areas
and were usually found on sites below mean sea level
where the clay content of the substrate was less than
This resulted in a negative relationship between
density of Knot and that of Peringia. Considering all
this information together, the conclusion is that Macoma
was the main, and sometimes the only, prey for
Knot in the study area. For thai reason, local and sea
sonal variations only in Macoma are described below.
1977 78 79 80 81 82 83 84 85 86
tig. 6. Biomass (g AFDW m -i ol live bivalve species M sik- \ m
August from 1977 to 19X6. A selection has been made for the range
of si/e classes barvulihlc b) Knol Calidris , annua (see fable 2).
No correction has been made for depth distribution of the prey.