Extrapolating cetacean densities beyond surveyed regions: habitat-based predictions in the circumtropical belt
Résumé
Aim Our knowledge of cetacean distributions is impeded by large data-gaps
worldwide, particularly at tropical latitudes. This study aims to (1) find generic
relationships between cetaceans and their habitats in a range of tropical waters,
and (2) extrapolate cetacean densities in a circumtropical belt extending far
beyond surveyed regions.
Location Pelagic, circumtropical.
Methods Aerial surveys were conducted over three regions in the tropical
Atlantic (132,000 km2), Indian (1.4 million km2) and Pacific (1.4 million km2)
oceans. Three cetacean guilds were studied (Delphininae, Globicephalinae and
sperm and beaked whales). For each guild, a generalized additive model was
fitted using sightings recorded in all three regions and 14 candidate environmental
predictors. Cetacean densities were tentatively extrapolated over a circumtropical
belt, excluding waters where environmental characteristics
departed from those encountered in the surveyed regions.
Results Each cetacean guild exhibited a relationship with the primary production
and depth of the minimum dissolved oxygen concentration. Delphininae
also showed a relationship with the dominant phytoplankton group. The prediction
envelopes were primarily constrained by water temperature. Circumtropical
extrapolations of Delphininae and Globicephalinae were contrasted
between ocean basins, with high densities predicted in the equatorial waters of
the three ocean basins. The predicted densities of sperm and beaked whales
were lower and more uniform across the circumtropical belt than for the other
two guilds.
Main conclusions Our modelling approach represents a good analytical solution
to predicting cetacean population densities in poorly documented tropical
waters. Future data collection should concentrate on areas where environmental
characteristics were not encountered in our survey regions and where the
predicted densities were the most uncertain. By highlighting cetacean hotspots
far beyond waters under national jurisdiction, this study can provide guidance
for the delimitation of Ecologically and Biologically Significant Marine Areas.