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

prey risk for an animal al a depth of 4 to 6 cm. though
within the reach of the bird's bill, is much less than for
an individual living near the surface (Fig. 1 IB). Optimal
foraging theory (Krebs et al. 1983) offers the explanation:
deep-living prey are ignored because handling
time increases with depth, which makes them
less profitable (Wanink & Zwarts 1985). Scrobicularia
> 30 mm live the greater part of ihe year below a danger
line of 4 to 6 cm (Fig. 5) and in fact individuals
must have a poor condition as well as a low siphon
weight before they expose themselves to a higher predation
risk (Zwarts 19861. The conclusion is that in
summer ihe 'average' Scrobicularia remain just below
the danger line, but that from a point of view of minimization
of predation risk there is no need lo nearly
double the burying depth in winter. This suggests that
other factors may be involved in determining burying
depth.
The Oystercatcher is also an important predator for
large Mya, but it is the Curlew Numenius arquata
which causes a high mortality among individuals living
in the upper 14 to 15 cm (Fig. IIC). Oystercatchers
select Mya > c. 17 mm from the upper 5 to 6 cm,
while Curlews lake Mya > 30 mm present in the upper
14-15 cm (Zwarts & Wanink. 1984). There is thus a
small overlap in size and depth classes preyed upon by
both bird species. When the pari of Mya which is vulnerable
to predation by Oystercatchers or Curlews is
compared to the depth distribution (Fig. 2), it is clear
that risk to be taken by an Oystercatcher or a Curlew is
maximal for Mya of c. 25 mm and 35 to 45 mm, respectively,
which closely resembles the observed size
selection (Zwarts & Wanink 1984). It could also be
shown that Oystercatchers and Curlews exert a heavy
predation pressure on Mya > 2 cm. which makes it reasonable
to assume that the depth/si/e curve is determined
partly by Oystercatchers and Curlews creaming
off the individuals living in the upper 6 and 14 cm respectively.
Most animals > 50 mm live out of reach of
all predators present on the intertidal flats.
Cerastoderma, unlike Scrobicularia and Mya, remain
within reach of most of their predators during
iheu life-time, but by growing, the) gel rid of many of
them. Knot Calidris canutiis prey upon Cerastoderma
< 10 mm (Goss-Custard ei al. 1977b. Ens unpubl.
data), because the profitability of the prey increases
wilh si/e I Sutherland 1982c).
SIPHON SIZE AND DEPTH IN BENTHIC BIVALVES
108
Macoma have, like Cerastoderma and Mya, many
different enemies during the first stage of their life.
Five wader species, which are abundant on ihe intertidal
flats from late summer till spring, are the chief
predators for Macoma which have survived their first
summer (Table 4). In winter most Macoma > 10 mm
live out of reach of Dunlin Calidris alpina, Redshank
Tringa totanus and Knot I Reading & McGrorty 1978).
but a small number of the larger animals remain preseni
in the upper 3 cm (Fig. 4) and hence available for
these short-billed waders. They are. however, ignored
even by the Bar-tailed Godwit Limosa lapponica with
its longer bill. It is very likely thai gape width determines
the upper limit of acceptable prey size.
Contrary to the oilier waders, which swallow Macoma
whole, the Oystercatcher takes the flesh out of
the shell after hammering the shell or prising the bill
between the valves (Hulscher 1982). This species selects
Macoma > 10 mm (Goss-Custard et al. 1977b.
Hulscher 1982). Large Macoma have thus to reckon
with Oystercatcher and Bar-tailed Godw it as ihe only
predators. It is still unknown from which depth layer
the prey are actually taken by these waders, but if the
profitability of the prey decreases with depth as shown
for Oystercatcher feeding on Scrobicularia I Wanink &
Zwarts 1985). il is also likely thai wilh large Macoma
the predation risk is maximal for those animals living
in the upper 3 to 5 cm.
In winter, the depth of Macoma levels off at 4 to 5
cm (Fig. 4) and animals with a heavy siphon enlarge
their depdi till diey reach a depth of c. 4.5 cm (Fig. 8).
This suggests that also in summer the depth refuge is
4-5 cm, but that the reduced siphon size and the need
to feed force most Macoma to move nearer to the surface.
The general conclusion is that benthic bivalves
which attain a safe depth refuge do not increase their
burying depth anymore. The great winter depth of
Scrobicularia deserves another explanation. On European
flats, large Mya have to deal with Curlews as their
only predator and large Macoma with Oystercatchers
and Bar-tailed Godwits only. That is why the upper
limit of ihe depth refuge is set for both species al c. 14
and c. 4 cm. respectively. The Blue Crab Callinecles
sapidus is an important predator for both species in
Chesapeake Bay (e.g. Virnstein 1977. Blundon &
Kennedy 1982b). Large Blue Crabs are able to crack