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

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1961 Q N. diversicolor
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J F M A M J J A S O N D
Fig. 3, Average seasonal variation in the ISHIV condition ol Msiilus
edulis, given separately for small and largei Mussels, and three
worm species: small samples (< 1(1 animalsl in parentheses. The
average AFDW per size class, predicted hv the regressions given in
Tabic I, were set to 100 and all weight measurements were
expressed as percentage deviation and av eraged per month; n is total
number ol individuals weighed. The trends also are shown for two
separate years in \erei.s diversicolor to show the year-to-year
variation in body condition.
FOOD SUPPLY HARVESTABLE BY WADERS
52
condition later in the year (Fig. 3; Dare 1975, Dare &
Edwards 1975). A decrease in (he body weight of
Mussels in the Wadden Sea after late summer has
already been observed by Everards (1973) and Pieters
el al. (1979). In contrast, Craeymeersch et al. (1986)
found the lowest body condition in June and the
highest in winter in Mussels from the Eastern Scheldt
(SW. Netherlands). A great weight loss in May or June
due to spawning, and a weight recovery after that, was
also noted for Mussels on the British Isles (Baird 1966.
Dare 1975. Dare & Edwards 1975. Bayne & Worrell
1980. Cayford & Cioss-Ctisiard 1990). The geographical
variation in winter condition will be discussed
later.
At first sight, there seemed to be little seasonal
variation in the body condition of Nereis (Fig. 3).
However, when the data for different years were
considered separately, (shown for two years in Fig. 31,
large fluctuations emerged. Moreover, the seasonal
timing of the peak and base weights varied between
vcats by one or two months: the body condition was
usually low in March or April, high in June or July and
again low in July to November. Metiam (1979) found
low body weights in winter, and to a lesser extent also
in May. As spawning occurs when the temperature in
spring rises above 6 °C (Bartels-Hartege & Zeeck
1990). the annual variations in the timing of the decrease
in body condition in spring may reflect annual
variations in the timing of gamete release associated
with temperature. This may also explain the geographical
variation in liming in ihe loss of condition
which in "southern England seems to be in March and
April (Dales 1951. Olive & Garwood 1981). compared
With April and May in the Wadden Sea (Essink el al.
1985) and May in southern Sweden (Moller 1985).
In accordance with de Wilde & Berghuis (1979b).
Arenicola reached a low condition in hue winter and
again in late summer, following a peak in condition in
June (Fig. 3). Beukema & de Vlas (1979) showed that
large worms (common in sand but rare in muddy areas
as our study area) reached their peak weight later in ihe
season. Gamete release occurs in August to November
(Faike & Berghuis 1979. de Wilde & Berghuis 1979b).
which probably explains ihe low body weight in
autumn.
As in Nereis and Arenicola. the seasonal variation
in condition was relatively small in Nephtys hombergh