Résumé
Productivity was shown to structure megavertebrate communities, particularly in tropical oceans where they may be under selective pressure to maintain their foraging success in these low productivity environments. However, overall oligotrophic tropical oceans may encompass contrasting ecosystems. In the western tropical Atlantic, the Antilles and Guiana belong to two different biogeographic provinces and surface chlorophyll concentration is on average 10 times higher in Guiana. The aim of this study was to document megavertebrate communities in terms of encounter rates, species composition, abundance and spatial distribution and to compare them between these two contrasting regions. As part of an aerial census of cetaceans and other megavertebrates, line transect surveys were conducted across the French Antilles and Guiana. Both distance sampling and geostatistics were used to estimate abundance and geostatistics also provided maps of local density. Cetacean encounter rate was 3 times higher in Guiana (1.8sightings/100km) compared to the Antilles (0.6sightings/100km). Moreover, small delphinids strongly dominated in Guiana (representing 83% of the odontocete sightings) with a high abundance of bottlenose dolphins (Tursiops truncatus) (34,965 individuals, CV=0.28). Seabird encounter rate was similar in both regions (4.7sightings/100km in the Antilles and 3.6sightings/100km in Guiana). Charadriiformes, mainly represented by terns, were abundant in both regions whereas Pelecaniformes were only abundant in the Antilles and represented by frigatebirds, tropicbirds and boobies. Additionally, cetaceans and seabirds were mainly distributed close to the islands in the Antilles and were more widely distributed in Guiana. This study provided new information about megavertebrates which are poorly documented in these waters and highlighted differences in communities between these contrasting ecosystems. In particular, odontocete communities appeared to respond to productivity and may be structured by a differential distribution of resources through the water column.
► We conducted aerial surveys to census megavertebrates in the French Antilles and Guiana. ► These two regions are characterized by contrasting oceanography and productivity. ► Cetacean encounter rate was 3 times higher in Guiana and the bottlenose dolphin was particularly abundant. ► Seabird encounter rate was similar in both regions but Pelecaniformes were only abundant in the Antilles. ► Megavertebrates were mainly distributed close to the islands in the Antilles and more widely distributed in Guiana.