Acoustic monitoring of blue whales (Balaenoptera musculus) and fin whales (Balaenoptera physalus) in the Mozambique Channel off the northwest coast of Madagascar
Salvatore Cerchio, Tahina Rasoloarijao, Bridget Mueller-Brennan, Danielle Cholewiak
International Whaling Commission
Migratory baleen whales of the Southern Ocean, particularly Antarctic blue (Balaenoptera musculus intermedia), pygmy blue (Balaenoptera musculus brevicauda) and fin (Balaenoptera physalus) whales, move each year into subtropical and tropical waters during the Austral winter, presumably for breeding; however, information for each is highly limited regarding migratory timing, population distribution, and potential breeding habitat. Previous evidence indicated the presence of blue whales off northwest Madagascar, ca. latitude 13.3⁰S, including a pair encountered in 1,800m depth water off Nosy Be, and distant recordings of Madagascar pygmy blue whale song-type from shallow water in December 2014 and 2015. These discoveries prompted an acoustic monitoring project in the deep waters off northwest Madagascar during 2017, in part funded by the IWC SC. Passive acoustic recorders were deployed during four 4-month deployments from December 2016 to April 2018, and two 6-month deployments from April 2018 to March 2019, anchored just off the shelf break at depths ranging from 225-275m. Manual review of spectrograms from the 28 month recording period revealed extensive documentation of SWIO (Madagascar) pygmy blue whale and Antarctic blue whale song-types and fin whale song, in addition to humpback whale and Antarctic minke whale song (not reported on here). SWIO pygmy blue whale song was present bi-modally with peaks of singing activity during May-July (Austral late autumn/early winter) and October-January (Austral late spring/early summer). This pattern suggests a migratory corridor between summer feeding and winter breeding habitat south and north of Madagascar, respectively. Antarctic blue whale song was present throughout the Austral late autumn/winter from May to September (overlapping with the first peak of SWIO pygmy blues), suggesting a winter breeding season aggregation. Sri Lanka blue whale song-type, and a blue whale song-type attributed to the Arabian Sea population (Oman song-type) were detected for short periods between January and May. Fin whale song was present during the late Austral winter, from early August to mid-September. The timing of fin whale song suggests a later arrival than Antarctic blue whales and a lower rate of occurrence and occupancy, potentially representing the northern extent of breeding habitat. Geophysical seismic surveys were detected nearly continuously during a seven month period from October 2017 to May 2018, and have the potential to disturb these sensitive populations of baleen whales.