A baseline for biodiversity: spatio-temporal variability of fish assemblages in the pristine and little-known Iguela Lagoon, Gabon

Coastal lagoons in tropical regions are recognized for their high ecological value, functioning as critical nursery, feeding, and transitional habitats for a wide range of fish species. Despite their importance, data on fish assemblage structure and dynamics remain scarce for many systems in Central Africa. This study investigates the spatial and temporal variability of ichthyofaunal assemblages in the Iguela Lagoon, a relatively pristine estuarine ecosystem located in southwestern Gabon, protected under the Ramsar Convention and flanked by Loango National Park and the Iguela Mouth Marine Park. Seasonal fish sampling was conducted across nine stations using trammel nets over four hydrological seasons. A total of 59 species distributed among 30 families were identified during the present study. However, a comprehensive bibliographic synthesis, based on the available gray literature, made it possible to identify 118 species belonging to 46 families within the Iguéla lagoon. Multivariate analyses revealed four distinct assemblage groups shaped by seasonal and salinity gradients: (i) a rainy season-dominated freshwater migrant group, (ii) a dry season transitional estuarine group, (iii) a generalist assemblage occurring year-round, and (iv) a marine-influenced downstream group. Ecological guild analysis confirmed the strong influence of freshwater inputs and salinity fluctuations on species distribution and habitat use. Commercially important species, including Parachelon grandisquamis, Chrysichthys nigrodigitatus, Ethmalosa fimbriata , as well as Lutjanidae and Haemulidae species were abundant, although most individuals were juveniles, underscoring the lagoon’s essential nursery function. The structural complexity of the lagoon, including mangroves and seagrass beds, provides key habitats for juvenile development and contributes to the high biodiversity observed. This study fills a major gap in ichthyological knowledge for Gabonese coastal systems and provides a scientific baseline for conservation planning. The clear spatio-temporal differentiation in fish assemblages highlights the need for adaptive, spatially explicit management strategies to preserve the ecological integrity and fisheries productivity of the Iguela Lagoon in the face of ongoing environmental change. • The Iguela Lagoon in Gabon hosts more than 110 fish species, including more than 20 of high commercial value. • Four distinct assemblage types were identified, structured along clear seasonal and salinity gradients. • Freshwater migrants dominate during rainy seasons while marine species increase during dry periods, reflecting hydrological regime influence. • The high prevalence of juveniles emphasizes the lagoon’s crucial role as a nursery habitat for both estuarine and coastal marine fishes. • Located within a dual-layered protected area and designated as a RAMSAR site, the Iguela Lagoon provides a reference ecosystem for biodiversity conservation and adaptive coastal fisheries management in Central Africa.