Drivers of fish diversity and size spectra across lagoonal habitats of the Toliara reef system (SW Madagascar)

English. peer reviewed

English. Size-spectra and taxonomic diversity are important indicators to study the dynamics of communities and to inform conservation in aquatic ecosystems. To date, few studies aimed at modelling the distribution of reef fish diversity at small spatial scales (∼10 km) have been done. Based on a one-year fish sampling using trawl nets, we described and modeled the distribution of fish diversity and their size spectra across various coastal habitats in the Toliara reef system (SW Madagascar). Our data revealed a significant spatial and seasonal change of taxonomic diversity. Beta-diversity was mainly driven by species turnover with up to 75 %, indicating the important conservation value of the sampled sites. Fish size spectra slope emphasized the high dominance of small-sized fish on the sites close to the shoreline indicating their potential nursery zones. By combining environmental and geographic data with the use of the machine learning algorithm Random Forest, we demonstrated the geographic position of sites and associated habitat features were the foremost drivers of alpha-diversity and size spectra patterns (50-64 % of variation). The use of Generalized Dissimilarity Models revealed sediment cover was the main predictor of species turnover patterns (62 % of explained variation), with a rapid increase of species replacement within the first 2 km from the coast. Beyond a confirmation of the role of habitat features on biodiversity, our study emphasized the importance of distance to the shoreline on the distribution of fish diversity across the lagoon. This unexpected driver is discussed in the light of various anthropogenic factors including fisheries and sedimentation.