Season and seagrass: drivers of fish assemblage structure in the Banc d'Arguin, Mauritania

The Banc d’Arguin is the most ecologically significant coastal wetland of West Africa, a UNESCO Marine World Heritage area with one of the most extensive seagrass areas on Earth, used by many marine species as breeding and/or feeding habitat. However, little is known about the subtidal biodiversity supported by these extensive seagrass meadows. This study aimed to assess the influence of subtidal seagrass vegetation on fish assemblages, using beach seines to compare vegetated and unvegetated habitat. Effects of season and site were also assessed. We sampled fish communities predominantly composed of juveniles (98.7 %). Specifically, we analyzed differences in abundance, species richness, diversity, evenness, and assemblage structure across these factors. Season was the most influential driver of fish assemblage structure, even more than habitat, as expected in a nursery area where fish reproduce seasonally. Notably, four species - Atherina boyeri, Eucinostomus melanopterus, Mugil capurrii, and Chelon dumerili - accounted for 53.7 % and 49.2 % of the variation in assemblages across habitat and season, respectively. Moreover, four times as many species were found exclusively in seagrass habitats compared to unvegetated areas, underscoring the critical role of seagrass meadows in the Banc d’Arguin in supporting species that depend on such shallow sheltered habitats, enhancing regional biodiversity, and contributing to the sustainability of fisheries.