Ecological reef restoration: Consumptive and nonconsumptive interactions among common North Sea predators and European oysters

Julius A. Ellrich¹, Clemens Kozian-Fleck¹, Markus Brand¹, Bérenger Colsoul^1,2, Bernadette Pogoda¹

¹Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Germany - ²Thünen Institute of Fisheries Ecology, Germany

Oyster reefs are marine biodiversity hotspots with several ecosystem functions and services that are declining worldwide. Historic reefs of European oysters (Ostrea edulis) have been decimated by unsustainable fisheries and are nowadays considered functionally extinct. To stop and reverse the associated losses in biodiversity, ecosystem functions and services, several oyster reef restoration projects are conducted in European waters. Within the framework of the RESTORE project on the restoration of European oyster reef habitats in the German Bight (North Sea), this study investigated consumptive and nonconsumptive interactions among common North Sea predators, brown crabs (Cancer pagurus) and European lobsters (Homarus gammarus), and the native oysters for the first time. Field observations found that (i) brown crabs and lobsters co-occur with the oysters in seafloor environments and that (ii) these organisms interact with each other. Manipulative experiments on Helgoland island showed (iii, iv) that both predators consume oysters, (v) that medium-sized to large oysters are safe from brown crabs, and (vi) that large oysters are relatively safe from lobsters. They also showed (vii) that the formation of larger oyster clumps (which are more difficult to handle) and (viii) the presence of common blue mussels (Mytilus spp., which are more profitable) reduce predation on oysters. Furthermore, they showed that the presence of (ix) brown crab conspecifics and (x) lobsters in natural abundances nonconsumptively limit predation on oysters. Finally, they showed that these nonconsumptive predator effects are triggered by waterborne cues released by (xi, xii) both predators. Overall, our study identified several natural mechanisms underlying predator-oyster interactions that constitute fundamental knowledge that is essential to understand predator-prey relationships in oyster reefs and to develop measures to facilitate ecological oyster reef restoration.