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					 Bird predation on littoral mussel beds
Mussel beds (Mytulis edulis) are central biogenic structures in the ecosystem of the Wadden Sea. They stabilize
the substrate, improve the water quality and they provide important habitats for many (benthic) species.
Furthermore, they are an essential food source for aquatic organisms like crustaceans and fishes and for many
avian species. In the past, intertidal mussel beds formed a prominent part of the European Wadden Sea.
In the Dutch Wadden Sea though, mussel beds almost disappeared
completely on the intertidal mud flats in the early 1990’s mainly due
to intensive mussel fishery. Since closure of intertidal beds for
fishery in 1993, and after strong mussel recruitment in several
subsequent years, the area covered by intertidal mussel beds has
increased again. The most recent estimates arrive at 1400 hectares
of intertidal mussel beds. However the recovery has been slower
than hoped and mainly occurred in the eastern part of the Dutch Fig.: development of mussel beds in the Dutch Wadden
Wadden Sea. In the western part only a few hectares of mussel beds Sea expressed in fresh weight [mio kg] from 1990 onwards
have reestablished. It is still not well understood why there is such a big difference in the mussel bed recovery and
why the mussel beds have not come back to the western part of the Dutch Wadden Sea. In the West newly
developed mussel beds often disappear after one or two years, when the mussels are hardly larger than 30 mm.
Only a few make it to a stable mussel bed, where regularly new spat settles next to older cohorts, refreshing the
mussel bed.
Main bird predators:
A possible explanation for the discrepancy between east and west could be based in the difference of predator
composition and density and therefore a difference in predation pressure. The main predators for littoral mussels
besides Shore Crabs Carcinus maenas are Common Eiders Somateria mollissima (a), Oystercatchers
Haematopus ostralegus (b) and Herring Gulls Larus argentatus (c). The fourth molluscivorous bird potentially
feeding on mussels is the Red Knot Calidris canutus (d). While Oystercatchers and Eiders primarily prey on larger
mussels, Herring Gulls and Red Knots preferably feed on smaller mussels up to around 20 mm. Thus, the latter
might be responsible for the failure of the mussel bed recovery in the western part.

                           a                              b                             c                              d

The aim of this project is to analyse the predation pressure on a variety of mussel beds both in the east and west
Dutch Wadden Sea by using modelling, experimental and field observation approaches. In particular the
approaches consist of:
1) determining the state of the mussel bed (size, coverage, size composition of mussels and biomass)
2) investigating the composition and density of birds
3) estimating intake rates of the main bird predators (Common Eiders, Oystercatchers, Herring Gulls and Red
Knots throughout the year).
In this project there is a close cooperation with other projects within the MOSSELWAD program, which focus on
mussel attachment and abiotic factors influencing the development and stability of mussel beds. For more
information on the MOSSELWAD program see:


Interested in an BSc / MSc internship about bird predation on littoral mussel beds? Please contact:

        Andreas Waser          mail: or Bruno Ens               mail:

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