Long-term acoustic monitoring of nonstereotyped blue whale calls in the southern Indian Ocean

• Published in: Marine Mammal Science
• Main Authors: Torterotot, Maëlle, Samaran, Flore, Royer, Jean-yves
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley / Blackwell 2023
• Subjects: Antarctic blue whale; Balaenoptera musculus; D-calls; passive acoustic monitoring; pygmy blue whale; southern Indian Ocean; Antarctic; Austral; Indian; Antarc*; Blue whale
• Online Access: https://archimer.ifremer.fr/doc/00819/93080/99971.pdf; https://doi.org/10.1111/mms.12998; https://archimer.ifremer.fr/doc/00819/93080/

Monitoring the presence of blue whale (Balaenoptera musculus ssp.) stereotyped calls has been a widely used method to assess the different populations' distribution worldwide. All blue whale populations also produce nonstereotyped vocalizations, or D-calls. Here, we monitored the presence of D-calls in long-term records from a large hydrophone array located in the open southern Indian Ocean, using an automated detection method and manual validation of the detections. D-calls were detected at all sites of the array, which extends from 24°S to 56°S, but the majority of them were detected at the two southernmost sites. We observed a latitudinal shift in their seasonal occurrence, with more D-calls in the north during austral autumn and winter and more in the south during austral spring. The geographical occurrence of D-calls compared to that of songs indicates that blue whale acoustic behavior switches from a song-intensive and sparse-D-call emission in the north to song-moderate and more intensive D-call emissions in the south. These findings support the hypothesis that both call types are used for different purposes, as D-calls are mainly detected around foraging grounds and songs in wintering grounds. Monitoring both call types might therefore be a relevant acoustic indicator of blue whale behavior.