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Progress Report: Acoustic Monitoring of Blue Whales (Balaenoptera musculus) and other baleen whales in the Mozambique Channel off the Northwest Coast of Madagascar
Salvatore Cerchio, Tahina Rasoloarijao, Danielle Cholewiak
International Whaling Commission
Migratory baleen whales of the Southern Ocean, particularly Antarctic blue (Balaenoptera musculus intermedia), pygmy blue (Balaenoptera musculus brevicauda), fin (Balaenoptera physalus) and Antarctic minke (Balaenoptera bonaerensis) 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. Recently, SC/66b/SH33 reported evidence for 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 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. Three passive acoustic recorders were deployed during four 4-month deployments starting in December 2016 and ending in April 2018, anchored just off the shelf break at depths ranging from 250-270m. Initial review of data from the first three deployments from December 2016 to November 2017 revealed extensive documentation of both Antarctic and Madagascar pygmy blue whale song-types, fin whales and Antarctic minke whales. SWIO (Madagascar) pygmy blue whale song was present bi-modally with peaks of singing activity during May-July and October-January. This pattern suggests a previously unrecognized migratory corridor between summer feeding and winter breeding grounds south and north of Madagascar, respectively. Antarctic blue whale song was present throughout the Austral winter from June to September (overlapping with the first peak of SWIO pygmy blues), suggesting a previously unrecognized breeding season aggregation. NIO (Sri Lanka) blue whales song, as well as a potentially new, previously undescribed blue whale song was detected for short periods between January and May. Fin whale song was present during the late Austral winter, from early August to mid-September. At times high SNR series of fin whale 20Hz pulses were recorded, which included a single secondary frequency peak at 94-96Hz; this appears to be distinct from the different types previously reported from the Southern Ocean, however further analysis and direct comparisons are necessary. 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. Antarctic minke whale pulse trains representing three distinct song types, were found to be very common in the higher bandwidth. Although a systematic browse above 100Hz has not yet been completed, Antarctic minke whales were present from at least early August to early November, so remaining seasonally later than Antarctic blue or fin whales. In addition, the project has also documented humpback whales (Megaptera novaeangliae), Omura’s whales (Balaenoptera omurai).