From the air to the ocean: revealing cetacean assemblages in the Southeastern Indian Ocean

Cetaceans are highly mobile marine species that pose substantial research challenges due to their wide home ranges, migratory behavior, and low detectability across vast and remote oceanic regions. These realities, combined with the high costs of systematic surveys, have created significant gaps in understanding their spatial distribution, habitat use, and vulnerability to human-induced threats, which are critical for effective conservation management. This study presents the first dedicated aerial transect survey of cetacean assemblages off the western coast of Sumatra in the southeastern tropical Indian Ocean, a region previously understudied despite its recognized importance for marine biodiversity. Conducted between May and July 2024, the survey covered 15,043 km of aerial transects and recorded 77 sightings of 10 species, including the first aerial confirmations of killer whales and pygmy killer whales. With these additions, a total of 23 species has now been documented in the region, representing 68% of Indonesia’s known cetaceans. Our non-metric multidimensional scaling revealed seven distinct cetacean assemblage clusters strongly influenced by seafloor geomorphology and productivity gradients. High-density hotspots, dominated by spinner and striped dolphins, were found predominantly in non-protected areas, with 93% located outside existing or proposed MPAs. The overlap of cetacean habitats with intensive fishing activities and maritime traffic highlights potential risks for species such as killer whales, Omura’s whales, and sperm whales. These findings emphasize the need for targeted spatial protection, adaptive marine spatial planning, and species-specific mitigation measures to complement Indonesia’s 30×45 MPA expansion efforts. Given its exceptional biodiversity and the presence of threatened species such as sperm whales, western Sumatra is a strong candidate for designation as an important marine mammal area, consistent with its status as an ecologically or biologically significant marine area. This study establishes a critical ecological baseline, demonstrating the value of large-scale aerial surveys for informing evidence-based cetacean conservation in Indonesia’s offshore habitats.