Investigations on the factors affecting broodstock conditioning of the European flat oyster Ostrea edulis Linnaeus, 1758 - Development of effective hatchery techniques to produce native oyster spat-on-shell for restoration

The shortage of seed oyster supply has been recently identified as the major limiting factor for scale up of Ostrea edulis restoration across Europe. Despite the potential and feasibility of hatchery production, the capability of producing large quantities of native oyster seed in hatcheries is still limited and insufficient to meet the growing demand for restoration. The present study aimed to address some of the main limitations to the successful broodstock conditioning of O. edulis in hatchery settings. The novel non-destructive technique developed in this research (shell drilling and biopsy), was found to be suitable for the repeated sampling of oyster gonads, facilitating future monitoring and manipulation of sex ratios in hatcheries, without sacrificing remnant depleted stocks. The use of algal paste as a cost-effective alternative to live microalgal diets was successfully tested on O. edulis, with oysters showing potential adaptation to new diets after only 4 weeks of exposure (absorption efficiency > 50%), and high allocation of lipids (69 ± 0.7%) and fatty acids (70.5 ± 8.1%) in the gonads, except for Thalassiosira pseudonana (60.5% and 26.5% respectively). The stock density commonly used in oyster hatcheries and recommended by current hatchery guidelines was found to be the most suitable for O. edulis broodstock conditioning, promoting low mortality rate (10%) and high fertility (9.6 ± 3.32 x 10 6 larvae), and potentially maximising parental genetic contributions (inferred sex ratios). The accurate filtration rate measurements acquired in this study promote a better understanding of O. edulis circadian feeding behaviour, contributing to the development of more efficient hatchery breeding programmes, as well as predictive models for the estimate of ecosystem services. As part of this research, a small-scale research hatchery was also successfully established to provide a reliable source of locally adapted and potentially disease-resistant native oyster seed for restoration in the Solent.