| Summary: | Biological invasions are a leading cause of biodiversity loss and cause extensive economic losses. Since its introduction to Europe, the invasive Pacific oyster (Magallana gigas) has successfully established far-reaching populations in Scandinavia. This achievement can in part be attributed to the species’ high tolerance to a wide range of environmental conditions. However, the temperature range for survival and reproduction of this species has not been fully evaluated and is poorly understood. The relatively cold climate of northern Europe was initially deemed unsuitable for Pacific oyster populations as potential habitat. This would prove to be a vast underestimation of the species’ adaptability, however, since populations are now widely distributed in the region and their range still expanding. Nevertheless, exposure to severe winter conditions has been demonstrated to cause mortality events and change the contemporary distribution of oyster populations. Such conditions are common in Scandinavia and their effects on the oyster population are yet to be assessed. The purpose of this study was to estimate the extent to which mortality occurred among the Swedish Pacific oyster populations as a consequence of the relatively severe conditions during the 2020/2021 winter. Additionally, these surveys sought to investigate the relationship between winter mortality and the oyster settlement depth distribution. Our observations showed that survival probability generally increased with greater settlement depth. This was in accordance with previous findings. Furthermore, we found that there was considerable variation between geographical locations, with survival rates ranging between 63-98%. The average survival rate across all sampling sites was calculated to 87%. This suggested that the mortality of Pacific oysters in the winter 2020/2021 was influenced by other environmental factors in addition to depth. This project primarily aimed to provide an estimate of the relationship between the settlement depth of oysters and ...
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