Extrapolating cetacean densities beyond surveyed regions: habitat-based predictions in the circumtropical belt

International audience

anglais. Aim Our knowledge of cetacean distributions is impeded by large data-gapsworldwide, particularly at tropical latitudes. This study aims to (1) find genericrelationships between cetaceans and their habitats in a range of tropical waters,and (2) extrapolate cetacean densities in a circumtropical belt extending farbeyond surveyed regions.Location Pelagic, circumtropical.Methods Aerial surveys were conducted over three regions in the tropicalAtlantic (132,000 km2), Indian (1.4 million km2) and Pacific (1.4 million km2)oceans. Three cetacean guilds were studied (Delphininae, Globicephalinae andsperm and beaked whales). For each guild, a generalized additive model wasfitted using sightings recorded in all three regions and 14 candidate environmentalpredictors. Cetacean densities were tentatively extrapolated over a circumtropicalbelt, excluding waters where environmental characteristicsdeparted from those encountered in the surveyed regions.Results Each cetacean guild exhibited a relationship with the primary productionand depth of the minimum dissolved oxygen concentration. Delphininaealso showed a relationship with the dominant phytoplankton group. The predictionenvelopes were primarily constrained by water temperature. Circumtropicalextrapolations of Delphininae and Globicephalinae were contrastedbetween ocean basins, with high densities predicted in the equatorial waters ofthe three ocean basins. The predicted densities of sperm and beaked whaleswere lower and more uniform across the circumtropical belt than for the othertwo guilds.Main conclusions Our modelling approach represents a good analytical solutionto predicting cetacean population densities in poorly documented tropicalwaters. Future data collection should concentrate on areas where environmentalcharacteristics were not encountered in our survey regions and where thepredicted densities were the most uncertain. By highlighting cetacean hotspotsfar beyond waters under national jurisdiction, this study can provide guidancefor the delimitation of Ecologically and Biologically Significant Marine Areas.