Monitoring methodology: surface area of mussel and Pacific oyster beds

Annual mapping of mussel and Pacific oyster bed contours in the Wadden Sea was performed on the basis of a common definition (de Vlas et al., 2005) as agreed upon by the Trilateral Monitoring and Assessment Group (1997). Contours of mussel beds were determined by a combination of aerial surveys, aerial photographs, and by walking around the bed with a hand-held GPS device following the TMAG protocol (de Vlas et al., 2005). In Schleswig-Holstein, intertidal mussel beds were monitored by BioConsult SH and IFAÖ since 1998; in Lower Saxony by NLPV and NLWKN since 1994 (except for 1995 and 1998); in the Netherlands by IMARES and MarinX since 1995. In all three areas, according to the TMAG protocol, beds qualify as mussel beds if the percentage cover by mussels is 5 % or more. In the period 1999–2013, most beds consisted of blue mussels (Mytilus edulis) and Pacific oysters (Crassostrea gigas). Region specific survey details are described below.

A.1 Schleswig-Holstein

In Schleswig-Holstein, aerial photographs were taken annually (except for the years 2006 and 2009) with scale of 1:15,000 or 1:25,000. The photographs were checked visually and the contours of ascertained mussel and Pacific oyster beds were digitized and included in a GIS. Results were combined with field surveys. Each year about 40 % of the bed locations were visited. For those locations where field surveys were not possible, aerial photographs provided the only source of information. For the beds not visited in 2006 and 2009 it was assumed that the contours were the same as the year before. It is noted, however, that during the field campaigns in 2006 and 2009 new beds and the disappearance of beds were observed.

A.2 Lower Saxony

In Lower Saxony, contours of mussel and Pacific oyster beds were determined on the basis of black and white aerial photographs with a scale of 1:15,000. Since 2006, true color photographs with a scale of 1:20,000 were used. Photographs were analyzed with a stereoscope and contours of mussel beds were digitized and included in a GIS. Since 2010 digital orthophotos were used and digital mapping of the contours was done on screen. Results were validated in the field.

A.3 The Netherlands

In the Netherlands, aerial inspection flights were carried out in spring prior to the field survey to assess whether the beds that were mapped in the previous year were still present, whether there were new seed beds, and whether large portions of beds had disappeared. Locations where changes were observed during flights were prioritized during the field surveys. The available time was usually not sufficient to map all mussel beds by means of GPS. The percentage of bed locations that were surveyed in the field ranged between 40 % and 95 %. For beds not visited it was assumed that the contour is the same as in the preceding year if the aerial survey confirmed the presence. When a bed is visited in a following year, the contour in the missing year is estimated on the basis of the contours before and after the missing year. The polygon vector files describing the contours of the mussel beds were imported into a GIS.