Norbert Dankers, Andr

1
Norbert Dankers, André Meijboom Elze
Dijkman IMARES - Texel PO Box 167 1790 AD Den
Burg (Texel) The Netherlands norbert.dankers_at_wur.n
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Mussel beds Recognisable landscape in the Wadden
Sea
In the past, intertidal mussel beds were a normal
feature in the Waddensea. In the 1980s they
disappeared almost completely and this has been
attributed to fisheries activity and a period of
limited spatfall. Since 1992 no fishery was
practiced in the intertidal, and beds are slowly
recovering. In order to follow this development,
all beds are mapped on a yearly basis, and some
selected beds are studied and mapped in great
detail.
For the fieldwork a standardized protocol is used

• Walk along the edge of a mussel bed, and register
  (regularly, e.g. every 5 seconds) your position
  with a GPS. The edge of the bed is determined
  according to three guidelines
• Decide whether the structure is a mussel bed
  according to the definition (beds or patches,
  not loose lying individuals or small clumps).
  Patches should be larger than 1 m, or when
  smaller cover more than 5 of the surface in
  order to be considered part of the mussel bed
• 2. The 25 meter rule
• In the case of more or less continuous beds that
  have indentations or open spaces, the bed is
  considered to be continuous if the gap across the
  indentation or space is less than 25 metres. If
  the distance to the nearest large patch or
  combination of smaller patches is larger than 25
  m then it should be considered as a separate bed.
• (see figure as an example)
• 3. The 5 rule
• At least 5 of the substrate should be covered
  by mussel patches. This means that the distance
  between individual patches is, on average, less
  than 3.5 times the diameter of the patch.

Foto Klaas Kruier van m.s. Harder
To help the mapping and monitoring, it was
important to adopt a common definition of a
mussel bed, so that structures and borders
could be identified consistently. The accepted
definition is A mussel bed is a benthic
community structured by blue mussels. It may
consist of a spatially well defined irregular
collection of more or less protruding smaller
beds, which may be called patches, separated by
open spaces. This description includes young beds
with a high abundance of small mussels. The
described structure may not be so distinct in
young or recently settled beds (spat fall).
The perimeter of a mussel bed is mapped by
walking around the edge of the bed using a GPS to
log position every 5 seconds. In the figure
(left) an example is given of a bed near
Schiermonnikoog. The arrow indicates the position
given in the border of the figure
With careful management of fisheries, it is
possible for mussel beds to fully recover. The
distribution of mussel beds in the past (1976) is
shown in the map below.
Overview of the Waddensea south of
Schiermonnikoog with the position of bed 603
The next step is to display the recorded track in
a GIS system (Arc-Info). For the bed in this
example, this has been done over a number of
consecutive years, and it becomes clear whether a
bed is stable and how the area of the bed changes.