Restocking the European flat oyster Ostrea edulis in the Solent – a model for ecosystem service restoration across Europe

European flat oyster Ostrea edulis beds have been reduced by over 90% globally over the last 100 years, through a combination of over-fishing, competition with invasive species, adverse winters and the spread of a lethal invasive parasite, Bonamia ostreae, which is now present in most coastal regions of Europe. Popular interest in Ecological Restoration has meant that O. edulis broodstock availability has replaced monetary investment as the main limiting factor for oyster restoration in the UK. Concerns associated with limited broodstock include the development of a population with reduced genetic diversity, the survival of Bonamia-naive or Bonamia-exposed oysters in an environment dissimilar to their own, and further spread of the parasite and consequential disease, bonamiosis. There are many unknowns surrounding effective restoration. This thesis explored the importance of the hydrodynamic environment, broodstock origin and restoration design (re-laying density and elevation from the seabed) on the physiological performance of O. edulis to further our understanding of how to restore O. edulis to a self-sustaining population. Adult oyster valve movement was influenced by water velocities from 0.19 m s-1, and the threshold for physical displacement of all oyster sizes and weights (from 0.22 m s-1) was found to be lower than many recorded velocities in the Solent. Modelling the trajectory of larvae from given broodstock sites is highly recommended to indicate larval dispersal patterns and identify potential settlement areas where restoration efforts can be focussed. Appreciation of physiological performance of O. edulis was achieved by monitoring the metabolic rate, inferred from respirometry, and clearance rate of the oysters in both their local conditions and when acclimated to common aquarium conditions. Growth, body condition, and B. ostreae prevalence was thereafter recorded. The Solent is not recommended as a location for restoration evidenced by the poor condition of Solent oysters in comparison to those from Loch Ryan, Scotland, and Galway, Ireland, as well as the decrease in body condition observed in Loch Ryan oysters after 2 years of being re-laid in the Solent. However, the rapid development of a rich and diverse community of organisms observed in association with the oyster gabions deployed in the Beaulieu River should encourage restoration of O. edulis to prioritise ecosystem regeneration over commercial oyster production in this area. Undisturbed oyster beds support a rich ecosystem that will directly impact the future biodiversity of coastal areas of UK and Europe. This project identified a potential use for metabolic markers in association with Bonamia-infection, and urges further research into combined cellular and molecular approaches to better our understanding of this parasite-host interaction