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

ACCESSIBLE PREY ARE OFTEN IN POOR CONDITION
THE MACROBENTHOS FRACTION ACCESSIBLE TO
WADERS MAY REPRESENT MARGINAL PREY
The relationship between relative body condition (deviation from expected mean bcxly weight) and burying
depth was investigated in five macro-zoobenthic species living in a marine intertidal habitat. Body
weight increased with depth when animals of the same si/e were compared. The increase amounted to
51 Y i in the clam Scrobicularia plana, c, 40% in the worm Nereis diversicolor. 25% in the clam Macoma
balthica and 20% in the Cockle Cerastoderma edule and the clam Mya arenaria. Only a part of the prey
was within reach of some feeding wader species. Therefore prey value may be overestimated if one does
not take into account the fact that shallow and accessible prey often have a relatively poor body condition.
Introduction
In most studies it is impossible to determine exactly
the biomass of the prey that are actually eaten by a
predator. Rather an estimate of ihe mean weight of the
average prey in the population is commonly used.
1 low ever, the assumption that the biomass of a prev of
a given si/e class selected bv a predator is die same as
that sampled by the investigator may be wrong. Investigators
sample the prey population at random, while
predators exploit that part of the prey population that is
available, i.e. both detectable and accessible. The body
condition of prey that are available may be lower than
that of the whole prey population since prey which expose
themselves to predation might be hungry (Dill &
Fraser 1984, Lima 1988) and/or be diseased, injured or
weakened (Curio 1976. Fit/Gibbon & Fanshawe
1989).
This paper examines whether prev caught by predators
have a relatively poor body condition. The risk of
fish and benthic animals being taken by predators on.
or above, the surface depends on their own depth beneath
the surface (e.g. Kramer et ai. 1983, Zwarts &
Wanink 1989). Hence the average weight of prey actually
taken by these predators can be estimated provided
that the body condition of prey at different
depths is known (Wanink 19921. This possibility can
be Studied easily in some wader species feeding on estuarine
macro-zoobenthos. Waders do not normally
dig for prey so bill lengih limits the fraction that is ac­
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cessible which, though variable, may be very small
(Hulscher 1973. Reading & McGrorty 1978. Zwarts .V
Wanink 1984. Zwarts & Wanink 1989). In this paper.
the food value of animals vulnerable to predation is
compared to the average of the prey population as a
whole.
Methods
All field data were collected on an intertidal Hat along
the mainland coast ol the province of Friesland. Dutch
Wadden Sea. (53°25' N. 6°04' E) over seven years,
1980-1986. The depth of the benthic animals was measured
at low w aier with the aid of a circular corer as described
elsewhere (Zwarts 1986, Esselink & Zwarts
1989. Zwarts & Wanink 1989). The depth of the
Cockle Cerastoderma edule and ol the clams Mya arenaria.
Macoma balihica and Scrobicularia plana was
defined as the distance between ihe surface of the mud
and the upper edge of the shell. The burrow depth of
the Ragworm Nereis diversicolor was measured as the
distance between ihc surface and the deepest pari ol
Ihe burrow. The actual depth of Nereis was not measured
because the worm moved in iis burrow during
sampling (e.g. Vader 1964. Goerke 1971. Esselink &
Zwarts 1990). Instead, the greatest depth of the burrow
was used as a measure of the depth to which it might
retreat when attacked by a predator. Assuming worms
can detect a predator, there is an inverse relation be-