The Pacific oyster (Crassostrea gigas) is an invasive species in the Oosterschelde estuary. It was originally brought there to replace the native flat oyster Ostrea edulis which had been greatly reduced in numbers due to diseases and parasites. Because of some hot summers the pacific oyster could spawn, spreading throughout the Oosterschelde estuary. The pacific oyster now spawns in most years and continues to increase in number. This might cause competition with other filterfeeding bivalves that have commercial and ecological importance. That is why it is important to know how the oyster and these bivalves respond to competition for food. The blue mussel Mytilus edulis was chosen for this research because it is easy to use in the set-up. The goal is to find out how the mussels and the oysters respond to low food availability and how they fare in competition. Food availability was influenced by three factors; the biomass of the oysters around the experiment location, the tidal height of the location and the distance from the edge of the oyster bed of the location. For oysters the oyster biomass around the location had a significant negative effect on growth. A high biomass around the location caused lower growth. The distance from the edge of the oyster bed also had a significant negative effect on growth. Locations that were further into the oyster bed had lower growth. For the mussels no significant effects were found. The mussel data was a bit odd, with only condition decreases and very high standard deviations. Due to this no comparison could be made between the two species, thus no conclusions could be drawn about the competition between the two species
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