Robert, Chloé2025-04-242025-04-242025-04-24978-91-8115-224-1 (PRINT)978-91-8115-225-8 (PDF)https://hdl.handle.net/2077/86062Flat oysters, Ostrea edulis, are native to European waters. Wild populations have declined in the past decades, mainly due to overfishing and the introduction of parasites. Oyster farms have been extensively developed, with regular translocations of stocks between countries resulting in the homogenisation of genetic diversity. However, Swedish populations in the Skagerrak and Kattegat are exceptions, as fishing laws have historically protected them from overharvesting, and Swedish waters have been free from the parasites found in other European stocks. However, oysters in this geographic area have not been well-studied and although largely protected from external threats, they will face challenges in the future, including ocean warming and the proliferation of the invasive Pacific oyster, Crassostrea gigas. This thesis aimed to assess the status of the Scandinavian oyster populations with a particular focus on the Swedish populations. In this process, I investigated four distinct but related aspects of oyster biology to evaluate the effects of potential threats to their resilience and persistence. First, I investigated the current population structure of Scandinavian flat oysters to assess genetic diversity in northern Europe (Paper I). Using genomic tools, I found that the populations in the Skagerrak are genetically homogeneous and form a single panmictic population. This indicates that individuals may be translocated among sites within this water body without risk of genetic change. I also assessed how projected environmental change, specifically increased temperature, affect flat oysters from the Skagerrak (Paper II). During experiments, I monitored physiological responses (feeding and respiration rates) and gene expression. Surprisingly, flat oysters survived short-term (3 day) exposure to temperatures up to 32 °C, suggesting that flat oysters in Sweden may tolerate acute exposure to marine heatwaves, though not necessarily long-term warming. Given that flat oysters are widely cultivated in hatcheries, I investigated the genetic impacts of domestication to assess whether this leads to predictable genomic changes (Paper III). The study included both wild and cultivated oysters from Sweden and from Scotland/Ireland. I found significant genomic divergence between cultivated and wild populations after only three generations, stressing the importance of regularly supplementing hatchery stocks with wild individuals. Finally, as the Pacific oyster has rapidly colonised the Skagerrak, I investigated its adaptive potential to new salinity conditions through crossing experiments (Paper IV). Results showed that this invasive species has become capable of reproducing in low-salinity waters, suggesting a potential to colonise the brackish waters of the Baltic Sea in the future. This thesis contributes to a better understanding of population dynamics, genetic variation, and environmental responses of flat and Pacific oysters in Scandinavia, with direct implications for conservation and aquaculture management.engAquacultureconservationCrassostrea gigasgenomicsglobal changehatcheryinvasive speciesmanagementOstrea edulispopulation geneticswarmingwild populationstext