Population Structure and Growth Dynamics of the Mangrove Oyster Crassostrea Tulipa in the Casamance Estuary (Senegal)

Abstract

The mangrove oyster Crassostrea tulipa is an important ecological and fishery resource in West African estuaries, but information on its population dynamics remains limited in inverse estuarine systems. This study examines the population structure, growth, and natural mortality of C. tulipa in the Bliss Kassa Islands of the Casamance Estuary (southern Senegal), an environment characterized by strong spatial and seasonal variability in salinity. Oyster sampling was conducted monthly from December 2021 to November 2023 at three sites (Diogue, Hitou, and Niomoune) distributed along a downstream–upstream gradient. Water temperature and salinity were measured concurrently during each sampling campaign. Size–frequency analysis showed clear spatial differences in population structure. Diogue was dominated by small-sized individuals, indicating high recruitment and strong post-recruitment mortality, whereas Hitou exhibited a wider size range, including large oysters exceeding 70 mm. Niomoune populations were mainly composed of intermediate-sized individuals. Three overlapping cohorts were identified over the study period, suggesting continuous or multi-modal recruitment. Growth parameters were estimated from size–frequency data using the von Bertalanffy growth function. Asymptotic shell length (L∞) ranged from 48.49 to 57.80 mm, and growth coefficients (K) from 0.222 to 0.367 yr⁻¹. These values indicate relatively slow growth compared with other West African populations. Natural mortality (M), estimated using Pauly’s empirical model, varied between 0.565 and 0.746 yr⁻¹. M/K ratios indicated intermediate life-history strategies across sites. However, the truncated size structure observed at Diogue suggests additional mortality sources, likely related to harvesting pressure. The results highlight the combined influence of local hydrological conditions and anthropogenic pressure on the population dynamics of C. tulipa in the Casamance Estuary. Site-specific management measures, including minimum harvest sizes and mangrove habitat protection, are recommended to support sustainable exploitation of this resource.