Acoustic detections and sightings of blue whales (Balaenoptera musculus) in the Seychelles, western tropical Indian Ocean (2020-2022)

Historically, the Seychelles archipelago was an opportunistic whaling ground for fleets en route to and from the Antarctic. Soviet whalers illegally killed 500 blue whales near the Seychelles in the 1960s. Since then, no dedicated research has occurred to understand the ecological importance of this region for blue whales. Based on opportunistic sightings, we undertook 2 expeditions to assess the occurrence of blue whales. The overall goals were to determine blue whale distribution, obtain photo-identification data and collect the first acoustic data on this species in this region using a hydrophone deployed for a year. The expeditions consisted of vessel-based visual surveys that focused on the slope habitat (500-2000 m) off the northern portion of the Mahé Plateau. Over the 2 expeditions, a total of 5 sightings of up to 10 animals were seen. The results of our acoustic monitoring off Seychelles demonstrate that blue whales occur there regularly, primarily from December to April, and that the acoustic population identity matches that from near Sri Lanka. Published records and the results of our work suggest that blue whales from the northwestern Indian Ocean are seasonally present in the western equatorial Indian Ocean.