Invasion of Pacific oysters (Crassostrea gigas) in the Wadden Sea : competitive advantage over native mussels

Pacific oysters have been introduced into the northern Wadden Sea by aquaculture in the 1980s and subsequently established. As oyster larvae need hard substrates for settlement, they are overgrowing resident epibenthic mussel beds (Mytilus edulis). The impact of C. gigas on the native ecosystem was assessed by investigating the population development and the scope for coexistence with M. edulis. The invasion of C. gigas started slowly with abundances remaining low for about 15 years. Only a succession of three summers with anomalous high water temperatures led to a massive population increase. Field experiments revealed differential settlement of both species and faster growth of C. gigas compared to M. edulis. A high survival rate of juvenile oysters was presumably caused by low predation pressure by the main benthic predators, shore crabs and starfish, which both strongly preferred mussels to oysters. As C. gigas is well adapted to the recipient ecosystem and competitive superior to their native congeners, the development of massive oyster reefs on the expense of mussel beds is expected