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

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burying depth (cm)
Kin. I. Cerastoderma edule 20 mm long. Relative body condition
(•. mean ± SE) as a function of burying depth. The histogram
-In ni - ihe depth distribution. Data are pooled for all seasons. The results
of a one-way analysis of variance are given: burying depth explains
a significant part of the variation in body condition.
4 6 8 10 12
burying depth (cm)
Fig. 2. Mya arenaria 40 mm long. Relative body condition (•.
mean ± SE) as a function of burying depth. The histogram shows ihc
depth distribution. Dala are pooled for all seasons. The results of a
ooe-waj analysis of variance are given: burying depth explains a
highly significant pan ol the variation in body condition.
ACCESSIBLE PREY ARE OFTEN IN POOR CONDITION
131
tween burrow depth and predation risk (Esselink &
Zwarts 1989. Zwarts & Esselink 1989).
The animals collected in the field were stored for
up to 24 h in fresh sea water of 4 °C. The length of the
bivalves was measured io the nearest mm along the anleiio
[Hisictiiii avis. The valves were opened by short
immersion in boiling water and the flesh removed. We
noted the infestation of Macoma by the parasitic
nematode Panatrema affinis (Swennen & Ching
1974). Since the length of a Ragworm is variable, we
measured the maximum length of a worm moving in
sea water (Esselink & Zwarts 1989). The llesh was
dried at 70 °C for 2 to 3 days and burned at 550 °C for
2 h to determine ash-free dry weight (AFDW). All animals
were weighed separately. Cracked bivalves and
broken worms were not used. Energy values of prey
were determined by bomb calorimetric analysis of
dried flesh which had heen deep-frozen for a maximum
of 12 months.
Monthly mean body weights were calculated per
cm size class for Ragworms and per mm size class for
bivalves. The body weight of each individual animal
was expressed as a percentage deviation from the
monthly mean AFDW for the corresponding size
class. This deviation from the average is called the relative
body condition of the individual. The relation between
relative body condition and depth was analysed
only for a limited range of si/e classes: 10 to 30 mm in
Cerastoderma, 30 to 50 mm in Mya. 10 to 20 mm in
Macoma. 30 to 40 mm in Scrobicularia and 7 to 14 cm
in V, iris. Nonetheless there was an increase in depth
with body size even within these restricted size classes.
Therefore all depths were adjusted relative to the average
depth of an animal of a size halfway between the
ranges of the size class (see Esselink & Zwarts 1989,
Zwarts & Wanink 1989). After this transformation,
both burying depth and relative body condition were
independent of size, suggesting lhat the conversions
had been made correctly. The data were analysed by
using SPSS (Nie et ul. 1975). Two-way analyses of
variance revealed lhat the relationships between relative
body condition and depth were different between
seasons in Macoma. Scrobicularia and Nereis, but noi
in Mya and Cerastoderma. Accordingly the data for all
seasons were pooled in the case of the latter two
species, but treated separately for each season in the
other three.