The Casamance estuary, located in West Africa, has a unique hydrological system, with a partially inverted regime during the dry season characterised by increasing salinity from upstream to downstream. This system is subject to high seasonal variability in precipitation and evaporation, influencing the environmental conditions that govern the biology of benthic organisms such as textit{Crassostrea tulipa}, an oyster of local socio-economic interest. However, quantitative data linking environmental variability to biometric responses of this species remain scarce in the Casamance estuary. It is in this context that this study was initiated. It was carried out at four sites in the Bliss Kassa MPA, namely Diogué, Hitou, Haère and Niomoune. The aim was to monitor seasonal variations in salinity and temperature at the sites, as well as the size distribution, size-weight relationship and condition index of the textit{Crassostrea tulipa} oyster between November 2021 and November 2023. The results highlight the decisive influence of salinity and, to a lesser extent, temperature on the index and populations structure of textit{C. tulipa}. Salinity ranged from 15 to 38.6textperthousand and temperature from 22 to 34$^circ$C, with significant seasonal differences (Kruskal–Wallis, $p < 0.001$). The most stable sites (Niomoune) offer better growth conditions and active recruitment, unlike more inland areas subject to extreme variations. Growth is minimal for this species. This study highlights the adaptation of textit{C. tulipa} to an unstable estuarine environment and the importance of taking these dynamics into account in the sustainable management of oyster resources in Casamance. These findings provide baseline information for defining adaptive management strategies and seasonal harvesting windows within the Kalone--Bliss--Kassa MPA.