Mytilid larval appearance and settlement at offshore wind farm sites in the German Bight—a trial to estimate multi-use potentials for bivalve seed collection and grow-out cultivation

To obtain information on the occurrence of mussel ( Mytilus spp.) larvae in the German Bight (North Sea), plankton sampling was carried out at 12 test locations in offshore areas 10–40 nautical miles offshore and along a transect extending from a nearshore area of the Weser estuary near Bremerhaven to an offshore site near Helgoland in the years 2003, 2006, 2008, 2011–2013, and 2014–2015. Furthermore, the settlement of Mytilus spat was estimated on test moorings equipped with suspended artificial collectors. Larvae were distributed heterogeneously in time and space, with the highest numbers recorded in May of each sampling year (80–25,000 larvae m −3 ). Lower larval numbers were observed in the offshore areas (>20 nautical miles off the coast) compared with those in inshore locations (<20 mi). The numbers of settled Mytilus spat (ranging from 0 to 1,100 mussels m −1 ) were low offshore compared with the inshore data of several tens of thousands of settled spat·per meter collector. Water samples were also obtained to assess the nutrient concentrations, chlorophyll contents, and the particulate organic carbon (POC)/total particulate nitrogen (TPN) ratios as potential indicators of the availability and quality of food for blue mussels. The measured environmental parameters indicated favorable food conditions for mussel growth in the offshore areas. However, further detailed studies on larval distribution and settlement dynamics are needed to fully assess the potential of offshore areas for seed mussel resources and grow-out cultivation.