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

less than that of bivalves, partly because its fat contenl
is only 1.7% (Napolitano & Ackman 1989) compared
with around IO 1 /} in bivalves Tellina tenuis (Ansell &
Trevallion 1967). Mstilus (Dare & Edwards 1975) and
Macoma (Beukema & de Bruin 1977). Fat content also
varies regionally within a species; for example, the fat
content oi Macoma in the Baltic Sea is iw ice as high as
in ihe Wadden Sea (Pekkarinen 1983). Such a regional
variation in fat content may explain why the energ)
density of marine invertebrates from the northern
region is. on average, higher than those from more
southern shores. Thus Wacasey & Atkinson (19X7)
found a grand mean of 22.7 kJ g ' for many invertebrate
species from the Canadian Arctic. Brev el
ul. (1988) arrived al a mean of 23 k.l g ' lor invertebrates
from the Baltic Sea. ln contrast, this study
found a mean of 21.8 kJ g ' for the Wadden Sea. while
Dauvin & Joncourt (1989) found a value of only 20.5
kJ g ' in the English Channel.
In conclusion, ihe seasonal, regional and species
variation in the energy density of estuarine invertebrates
is not very large but. at 10%, might be
enough lo explain a diet shift of predators which might
otherwise be difficult to understand if the simplification
is accepted that prey weight is assumed to
be equivalent to fcxxl value. But as will be show n in the
next section, the variation in flesh weight of prey of
constant si/e is much larger and thus likely to be
ecologically more important.
Seasonal variation in bod) weight at the same
length
The seasonal and annual variations in (he condition of
FOOD SUPPLY HARVESTABLE BY WADERS
the four most important bivalve species in our stud)
area have already been described (Zwarts 1991):
Mamma, S, mhicitlaria, Mya and Cerastoderma of
similar size coniained. in May and June. 1.7 to 2.1
tunes as much flesh as in February and March. This
section therefore deals only with the seasonal variation
in the llesh weight of other benthic species. All
available weight measurements were combined and
the average weight per cm (in worms) and mm (in
bivalvesi M/C class calculated. The common regression
between weight and body length was calculated
for these average weights (Table 1). The slope of the
weight-size regression differed seasonally in Mytilus
as also found by Bayne & Worrall (1980). Craeymeersch
et al. (1986) and Cayford & Goss-Custard
(1990). The regression equation for Carcinus closel)
resembled that already published by Klein Breteler
(1975). The equation for Corophium was similar to
that given by Birklund (1977). but the predicted
weights were somewhat below those given by Boaies
& Smith (1979) and Miiller & Rosenberg (1982) and
somewhat above those of Goss-Custard (1977a) and
Hawkins (1985). A comparison was not possible in the
worm species in view of the lack of standardisation in
the measurement of body size. Weight measiiteiiienis
were expressed as deviations from the mean for each
size class predicted by the [egressions in Table 1.
The average monthlv dev iaiions from the long-term
mean, set to 100. are shown in Fig. 3. The seasonal
variations in Mytilus were very large compared with
those recorded in the four bivalve species mentioned
above. The change in condition is given separately for
two si/e classes, since larger Mussels reach their peak
TaMc I. Fxpiinenti.il relationship belvv ecu body weight (AFDW ot the llesh i.mil rnulv si/e( shell length or worm length, nul carapace width in
Carcinus maeiurs). a and h are ihe intercept imJ slope, respectively, ot the regression: lining AFDW) .le.imsi Inliiim. bin cm in Ihc worm
species). The regressions wen calculated for the mean weights of k size claw, weighted for sample size mi The data are irom 10-12 yean and
all seasons, hut Corophium volutator were ool) collected in summer.
species b range
•
Mytilus eiluli.s ^J.5% 2.840
Nephtys htmbergii
,!iii-rsnnliir
•0.183 2.017
Arenicola marina +1.198
Can inns rnaenns •1925 1S7I
Corophium volutator -5244 2.800
51
0.995 10756 67 2-75 mm
0.962 263 11 2-12 eo
0.996 3586 22 1-13 cm
0.992 1831 13 l-l 3 cm
0.998 772 49 2-61) mm
0.994 526 9 2-10 mm