Trophic Ecology of Rhinoptera marginata in Coastal Waters of Nouadhibou, Mauritania

Résumé fourni par la source

Elasmobranchs play a major role in shaping the structure and functioning of marine ecosystems through their position in food webs. The Lusitanian cownose ray Rhinoptera marginata, a benthopelagic species classified as Critically Endangered on the IUCN Red List, is frequently landed in fisheries off the Mauritanian coast, yet its trophic ecology remains poorly documented. We investigated the diet of R. marginata along the coast of Nouadhibou through stomach content analysis of 308 individuals sampled between March 2022 and August 2023; 135 stomachs (66 females and 69 males) contained identifiable prey and formed the basis of the analyses (disc width range 33–87 cm). Common dietary indices (frequency of occurrence, numerical and gravimetric percentages, Pinkas’ IRI, Hureau’s Q) were combined with multivariate approaches (PERMANOVA on Bray–Curtis dissimilarities, SIMPER, non-metric multidimensional scaling), measures of trophic niche breadth and specialization (Levins’ standardized index Bi, Shannon H′, Pielou’s J′, modified Costello plot), niche overlap (Pianka and Schoener indices), and ontogenetic modelling (generalized additive models). A total of 33 prey taxa were identified, dominated by Bivalvia (86.1% IRI), followed by Gastropoda (6.9%) and Crustacea (6.7%). The species exhibited a strongly specialized feeding strategy (Levins Bi = 0.06; Shannon H′ = 1.42) with high niche overlap between sexes (Pianka = 0.89). PERMANOVA detected significant compositional differences across seasons (pseudo-F = 6.98, p < 0.001) and size classes (pseudo-F = 4.98, p < 0.001) but not between sexes (pseudo-F = 1.87, p = 0.077). The proportion of bivalves significantly decreased with body size (R2 = 0.176, p < 0.001), revealing a clear ontogenetic shift towards more diversified prey in larger individuals. The estimated trophic level (3.13) confirms a mesocarnivore benthic position. These results provide the first comprehensive description of the diet of R. marginata in West Africa and highlight its role as a specialized durophagous predator of bivalve molluscs, omnipresent along the northern Mauritanian coastal ecosystems. By identifying the principal prey, our findings provide a basis for identifying potentially important foraging habitats for this Critically Endangered species and informing future conservation and management strategies.