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Pacific oysters in Dutch estuaries : causes of success and consequences for native bivalves

(2009) Troost, Karin

Nowadays many non-indigenous species are found in the Netherlands. Among the most successful is the Pacific oyster (Crassostrea gigas). Since its deliberate introduction in 1964 it has been spreading rapidly throughout Dutch estuaries. It became clear that the expansion of extensive oyster beds could have a large impact on Dutch estuaries and indigenous species, especially bivalves such as the cockle (Cerastoderma edule) and mussel (Mytilus edulis). How the oyster managed to establish itself successfully in the Netherlands, and what the consequences are for native bivalves, is investigated in this thesis.
The Pacific oyster possesses all characteristics that are generally attributed to successful invaders, which explains its success. Examples are a fast growth rate and a quick adaptation to different circumstances. Pacific oysters even modify the environment to their own demands by building reef structures. Also, an absence of natural predators and diseases in the Netherlands played an important role.
Consequences for native bivalves largely arise from the filtration of large amounts of water by Pacific oysters. This not only lowers food levels for native bivalves, but also increases mortality rates among their free-swimming larvae. Oyster larvae are filtered as well, but are better able to escape. As yet, negative effects on native bivalves due to lowered food levels only occur in the Oosterschelde estuary. A local disappearance of species due to the Pacific oyster invasion is more likely to occur in the Oosterschelde estuary than in the more food rich waters of the Wadden Sea.

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