Abstract
The distribution and density of pelagic megafauna (marine mammals, seabirds, elasmobranches and sea turtles) are important indicators of marine biodiversity, reflecting the condition of the underlying ecosystems. Taking advantage of their relative accessibility at the sea surface, a dedicated aerial survey was conducted in the tropical Southwest Indian Ocean to map their distribution, the taxonomic diversity and to estimate their densities for a reference standpoint on the area. This large survey across three ecological sub-regions revealed contrasting spatial distributions: maps of taxonomic richness of marine mammals and seabirds revealed different “hotspots” in the area. Densities were estimated for eight cetacean taxa with small and large Delphininae, or small Globicephalinae dominating, and for seven seabird taxa, with terns and noddies dominating. At the community level, the Southwest Indian Ocean megafauna was structured by the marine environment with strong differences between the Mozambique Channel and the Mascarene Islands, or between shelf and slope/oceanic habitats. Our results illustrate how multi-taxa aerial surveys are relevant and cost-effective monitoring tools for marine megafauna, allowing a community-wide approach.
Highlights
Marine megafauna, defined here as seabirds, marine mammals, sea turtles, and large elasmobranchs, share a number of major conservation issues worldwide
The region of interest is located in the SW Indian Ocean, from the Northern Seychelles Islands, to the Mascarene Islands, and the Mozambique Channel extending over 5 million km2 from latitude 2–24◦S and longitude 40–60◦E (Figure 1)
A total of 1,148 sightings of marine mammals, 16,507 sightings of seabirds, 799 of turtles, and 328 sightings of elasmobranchs were recorded along a total transect length of 89,000 km (Supplementary Tables 2–4)
Summary
Marine megafauna, defined here as seabirds, marine mammals, sea turtles, and large elasmobranchs, share a number of major conservation issues worldwide. These species are subjected to pressures from the increasing intensity of human activity in the ocean (Halpern et al, 2008) Because of their general demographic strategies based on high adult survival and low fecundity rates, marine megafauna are generally characterized by a low resilience, i.e., poor capacity to recover from stressors (Lewison et al, 2004). Owing to their comparatively large size and relative availability at the sea surface these animals can be more monitored than the vast majority of species inhabiting marine ecosystems, in particular in offshore waters. In spite of the fact that the animals’ availability for detection at the surface needs to be corrected for in order to generate robust numerical data, aerial surveys represent a good opportunity to work simultaneously on several taxa of marine megafauna
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