Blue mussel and oyster monitoring

Mussel beds are among the most diverse communities in the Wadden Sea, serving as valuable food source and important habitat for birds, fish and invertebrates.

Mussel monitoring serves to gain an annual overview of the composition, structure and dynamics of the mussel beds and to document the long-term development of the mussel populations.

For this purpose, the eulittoral populations of blue mussels (Mytilus edulis) are recorded annually. In addition, the sublittoral occurrences are investigated selectively.

The Pacific oyster (Magallana gigas) has been spreading increasingly on the eulittoral mussel beds since the 1990s. Since 2006, the species has also been recorded within the framework of this monitoring.

Surveys of eulittoral mussel beds are mainly carried out between May and September. Around low tide, the mussel beds can be reached from the shore or by boat to collect different scientific data. In on-site inspections, the contours of the beds are determined by GPS and the coverage is estimated. Samples are taken and later evaluated in the laboratory to determine abundance, length frequency distribution, biomass and condition index of the mussels.

The species community of four typical mussel beds is documented additionally. In September, the accompanying fauna is sampled to collect data on the structure and dynamics of the endo- and epibenthos.

Twice every five years, complementary digital aerial surveys are carried out on behalf of the Nationalparkverwaltung (National Park Administration) to ensure comprehensive monitoring. These data are used in mussel monitoring for further evaluations. Digital photographs of the entire North Frisian and partly Dithmarschen Wadden Sea are used to verify known beds and to identify freshly colonised eulittoral areas.

Sublittoral blue mussel and oyster populations are surveyed from the ship using a small mussel dredge, checking known sites as well as further areas. This method delivers valuable information about sublittoral sites that could partly be confirmed over several years.

The strategy to monitor mussels has been agreed between Denmark, Germany and The Netherlands in the Trilateral Monitoring and Assessment Program (TMAG 1991) (see Nehls & Büttger 2006).