No abstract

Understanding the seasonal movements and distribution patterns of migratory species over ocean basin scales is vital for appropriate conservation and management measures. However, assessing populations over remote regions is challenging, particularly if they are rare. Blue whales (Balaenoptera musculus spp) are an endangered species found in the Southern and Indian Oceans. Here two recognized subspecies of blue whales and, based on passive acoustic monitoring, four “acoustic populations” occur. Three of these are pygmy blue whale (B.m. brevicauda) populations while the fourth is the Antarctic blue whale (B.m. intermedia). Past whaling catches have dramatically reduced their numbers but recent acoustic recordings show that these oceans are still important habitat for blue whales. Presently little is known about the seasonal movements and degree of overlap of these four populations, particularly in the central Indian Ocean. We examined the geographic and seasonal occurrence of different blue whale acoustic populations using one year of passive acoustic recording from three sites located at different latitudes in the Indian Ocean. The vocalizations of the different blue whale subspecies and acoustic populations were recorded seasonally in different regions. For some call types and locations, there was spatial and temporal overlap, particularly between Antarctic and different pygmy blue whale acoustic populations. Except on the southernmost hydrophone, all three pygmy blue whale acoustic populations were found at different sites or during different seasons, which further suggests that these populations are generally geographically distinct. This unusual blue whale diversity in sub-Antarctic and sub-tropical waters indicates the importance of the area for blue whales in these former whaling grounds.

International audienceIn the Southern Indian Ocean, 2 subspecies of blue whales Balaenoptera musculus spp. occur, the Antarctic B. m. intermedia and the pygmy blue whale B. m. brevicauda. Until the present study was conducted it was assumed that the distribution of these 2 subspecies was delimited by the Antarctic Convergence (52° to 56° S) during the austral summer. Here, we report results from the first year-long, continuous acoustic monitoring of blue whales in mid-latitude waters off the Crozet Islands (46° 25' S, 51° 40' E). Using acoustic data from an International Monitoring System hydroacoustic station, we found that Antarctic and pygmy blue whales occur sympatrically in this area based on detection of their stereotyped calls. Antarctic blue whale calls were recorded year-round, indicating continuous presence in the region, which contradicts the migration paradigm attributed to this subspecies. Three geographically distinct types of pygmy blue whale calls were recorded during the summer and autumn only. The Madagascan call type was the most frequently recorded, while Sri Lankan and Australian call types recorded in this area suggest basin-scale longitudinal and latitudinal movements. During spring and summer, blue whale calls were often associated with higher frequency sounds (D calls), which have been attributed to feeding activity. The summer co-occurrence of blue whale subspecies highlights the importance of this productive sub-Antarctic area as a blue whale hotspot and provides new insights into blue whale seasonal distribution and segregation