Assessment of the Growth of Juvenile Oysters Crassostrea tulipa (Lamarck, 1819) in the Coastal Waters of the Gulf of Guinea: Case of the Coastal Lagoon in Southern Benin

The coastal zone of the Gulf of Guinea is characterised by the intensive harvesting of Crassostrea tulipa oysters, locally known as ‘Adakpin’ or ‘Atcha’, which serve as a vital source of income for local communities. This study aims to identify the optimal areas, depths, and seasonal periods that favour the growth of juvenile C. tulipa oysters in the coastal lagoon waters of southern Benin. Relatively uniform juvenile oysters were cultured at three depths (surface, mid-water, and bottom) in three production zones (Ahouandji, Dégouè, and Djondji) over the course of one year, covering the four climatic seasons of southern Benin. Juvenile oyster growth (in length, width, height, and weight) was monitored monthly. Simultaneously, key environmental variables (salinity, temperature, pH, water transparency, and dissolved oxygen) were measured in situ to evaluate their influence. A three-way ANOVA revealed that the month of the year had a significant main effect on oyster growth, defining two main growth periods: from October to December 2022 and from March to May 2023. Growth rates decreased during December 2022 to January 2023 and showed no growth from January to March 2023. Growth stopped again from May to September 2023, after the second growth period. Although the main effects of the farming level and production zone were not individually significant, significant two-way interactions were found for ‘Month × Depth’ and ‘Month × Production Zone’. This indicates that the effect of the month on growth depended on both the depth (farming level) and the location (production zone). Survival was highest at the mid-water column (97%) and at the surface (95%). Throughout the study period, survival rates remained consistently high, with the lowest monthly value at or above 80%.