Mapping a native oyster bed in the Dutch Voordelta: a new opportunity.

Supervisor: Pauline Kamermans (Wageningen Marine Research, Netherlands)
Wild oyster beds and reefs have, in many regions of the world, virtually disappeared. This is particularly true for the European flat oyster, Ostreaedulis, which has been declared ecologically extinct in European waters. Oysters beds have come under increasing concern due to the recognition of the multitude of ecosystem services they provide (protection of shorelines, habitat creation, filtering of water, protection of juvenile fish, as nutrient sinks, …). Currently many NGO’s, governments and institutions are initiating restoration programs. During the search for new locations for potential O.edulisrestoration, a large oyster bed was discovered in the Dutch Voordelta. The newly discovered bed offers a unique opportunity to study the species in its natural habitat and will give insight into its functioning. In this study different aspects of the bed are discussed, including: mapping and monitoring techniques, tools to assess O.edulispopulation age, and the effect predators and other reef building organisms have on the bed. Firstly, detailed map had to be made for potential MPA creation and to provide references for future development of the bed. To create a detailed map of the area, different mapping tools are assessed based on literature data using a scoring matrix which was developed specifically for this goal. Video-graphic methods are the most suitable for mapping the bed as, when conditions are right, they offer a good compromise between the size of area mapped, detail of the benthos and costs. Thus, a towed video sledge was used in the Voordelta to map the Northern boundary of the bed. Unfortunately, no results were obtained due to positioning difficulties. To assess ecological interactions, relevant reef-building species and their predators were mapped in the Voordelta over a period of 4 years using existing datasets of Wageningen Marine Research (WMR). The results showed that the common starfish, Asteriasrubens, followed the distribution of blue mussels, Mytilusedulis, in the Voordelta, although no significant correlations could be made. Further, in September and October of 2018 two separate benthic monitoring and explorative studies were performed in the Voordelta; one using dredges as a main sampling tool, the other using drop-cams. The studies were compared, with the conclusion that dredges were able to detect more species, mainly due to the ease of sampling large areas. Secondly, to assess the general health of the bed, an analysis of the age composition of the oysters present on the bed was necessary. However, the only certain method of aging oysters is through sclerochronological analysis, which destroys the individual in the process. To assess the age on a larger scale and in a non-destructive manner, a tool was designed linking age to morphometrics of O.edulis. With more data, the tool could be perfectioned to give accurate age of individuals or populations from e.g. photographs. Thirdly, methods of identifying oyster bed ‘allies’, such as species predating on O.edulis’spredators, and thus helping O.edulissurvival are discussed. Facilitating the presence of these allies may help in the restoration of O.edulisbeds in the North Sea. After identifying O. edulispredators and reviewing related literature, the only predator that lacked predators in the Voordelta was A. rubens. The population of common starfish remained relatively low on the bed, suggesting a possible knowledge gap. The possibility of an -as of yet- unidentified ally to O.eduliscan be an explanation for the low density of A.rubenson the bed. Therefore, the use of baited remote cameras is discussed as a potential method for identifying this(ese) ally(s).