Who lives in the open sea? Distribution and densities of surfacing marine megafauna in three subregions of the South Pacific (New Caledonia, Wallis and Futuna, and French Polynesia)

Context Assessing the distribution and abundance of marine fauna and the ecological status of coastal and pelagic ecosystems is key to biodiversity conservation, but the monitoring of mobile marine species raises multiple logistical and financial challenges.Aims The project describes the distribution, abundance and taxonomic assemblage of several marine megafauna taxa in three subregions of the western and central South Pacific Ocean (New Caledonia, Wallis and Futuna, and French Polynesia).Methods Large-scale aerial surveys were conducted using a standardised multi-taxon protocol, to characterise the occurrence and abundance of marine megafauna over 2.5millionkm2. Analysing more than 122 000km of transects, the densities of 22 different taxa were estimated: seven taxonomic groups of marine mammals (Physeteridae, Kogiidae, Ziphiidae, Globicephalinae, Small Delphininae, Large Delphininae, and Dugongidae), a single group for hard-shelled sea turtles, three groups of elasmobranchs (including whale sharks), and 11 groups of seabirds (including Phaethontidae, Hydrobatidae, Fregatidae and Sulidae).Key results Contrasting patterns of species distribution were found. Marine mammal diversity increases north and west, with a distinct species assemblage in New Caledonia, compared to other subregions. A strong latitudinal gradient was observed across French Polynesia, independent of taxa.Conclusions This study provides the first comparison of marine species assemblages across the three oceanic subregions and sets a regional baseline for the biogeography of marine megafauna in the region.Implications The taxonomic and spatial extension of the results opens up new perspectives for the development of local conservation measures, especially for taxa with already documented population declines.