Seasonal acoustic presence of blue and fin whales in the Mascarene waters (southwestern Indian Ocean)

In the southwest Indian Ocean (SWIO), the Mascarene region has been identified as an Important Marine Mammals Area, yet the seasonal use of these waters by large baleen whales remains poorly resolved. We analysed passive acoustic data from 16 Ocean Bottom Seismometers deployed from October 2012 to November 2013 across latitudinal and longitudinal transects around Réunion Island and off eastern Madagascar. Songs of fin whales (Balaenoptera physalus), Antarctic blue whales (B. m. intermedia), and SWIO/Madagascar pygmy blue whales (B. m. brevicauda) were manually annotated from spectrograms to quantify hourly acoustic presence per day. Fin whales showed the highest acoustic presence among the three studied taxa, with seasonal presence lasting up to 8 months (April–December) and clear north–south patterns in the onset and end of acoustic presence. Antarctic blue whales were detected less frequently, primarily during austral autumn and winter, with records extending north to 16°S and secondary peaks in October–November consistent with southward movement after wintering farther north. Madagascar pygmy blue whales showed a pronounced bimodal acoustic presence (mid-October–mid-January; March–July/August) and a strong longitudinal structuring, with greater acoustic presence in the western part of the study area. The spatial distribution differed markedly between these two periods, revealing distinct migration pathways through the region during northward and southward movements. Taken together, these results identify the Mascarene waters as a shared seasonal habitat for fin whales, Antarctic blue whales, and Madagascar pygmy blue whales, while highlighting distinct seasonal and spatial patterns among taxa. Given the intense shipping activity in the region, continued and expanded acoustic monitoring is needed to assess long-term whale habitat use, characterize underwater noise, and evaluate its potential influence on the use of these waters, thereby informing future conservation measures.