Native oysters (Ostrea edulis) are locally and functionally extinct as a habitat throughout much of their historical range. As a result, restoration initiatives are increasing. Our study aimed to collect baseline information for restoration projects in extant oyster beds off western Ireland. Oysters were collected from eight discrete beds in Kilkieran Bay and one bed in Bertraghboy Bay in winter 2023-2024. We examined size frequencies, spat abundance, condition index, and haplosporidian parasite Bonamia spp. prevalence (using standard and real-time PCR techniques) among oyster beds and bays. Oysters in Bertraghboy were generally larger, Bonamia-free, had significantly higher average physiological condition index, but limited spat recruitment. Five oyster beds in Kilkieran Bay had Bonamia ostreae, with three beds in the west having >20% prevalence and two of these beds also having the highest relative frequencies of spat. Oyster beds in eastern Kilkieran Bay had low or no presence of B. ostreae and spat. Our study was the first to assess Bonamia spp. presence in Bertraghboy and a longitudinal gradient of infection in Kilkieran Bay. This study helps identify areas for cultch addition and offers insight into potential connectivity and transmission pathways of B. ostreae, providing important information for restoration interventions and biosecurity.
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