Acoustic assessment of the seasonal occurrence and behaviour of Antarctic blue whales Balaenoptera musculus intermedia in the southeastern Atlantic and Southern Oceans

Abstract

With catches of over 360,000 individuals, Antarctic blue whales Balaenoptera musculus intermedia were harvested to near extinction by commercial whaling in the past century. Antarctic blue whales are an important ecological component of marine ecosystems as they ensure the circulation of nutrients in the pelagic environment making such nutrients accessible for primary production. However, their recoveries; distributions; migrations; large-scale response to environmental variabilities are poorly known. This thesis explored the distribution, seasonal occurrence, behaviour and response of Antarctic blue whales to environmental conditions in the high and low latitudes. I used Antarctic circumpolar acoustic data collected from sonobuoys deployed in the austral summers of 1997 through 2009 during the International Whaling Commission’s Southern Ocean Whale and Ecosystem Research (IWC SOWER) line-transect surveys. I also used recent acoustic data from three autonomous acoustic recorders (AARs) deployed between 2014 and 2015; two of these AARs were deployed on oceanographic moorings in the low latitudes and one AAR was deployed on a dedicated mooring in the high latitudes.

Characteristic Z-call and feeding associated D-call of Antarctic blue whales; and sometimes low frequency downsweeping ~28-15 Hz eastern Antarctic fin whale B. physalus calls, were detected using an automated detection template and visual verification methods. I used random forest model to determine pattern of environmental preferences, spatial occurrence and behaviour of Antarctic blue whales. Distance to southern boundary of the Antarctic Circumpolar Current, latitude, longitude and distance from the nearest Antarctic shores were the main geographic predictors of blue whale call occurrence and behaviour during IWC SOWER cruises. Satellite derived sea surface height (SSH), wind stress, wind direction, water depth, sea surface temperature (SST), chlorophyll-a and wind speed were important environmental predictors of blue whale occurrence and behaviour during IWC SOWER cruises. Antarctic blue whale call occurrence and call rates varied significantly in response to inter-annual variabilities of those environmental predictors during those cruises.

Migratory Antarctic blue and fin whales were acoustically detected in South African waters between May and August with fin whales present till November. Diel call rate patterns of both whale species varied between seasons. Wind speed, SSH, SST, chlorophyll-a, time of the day and Ekman upwelling index were important predictors of Antarctic blue and fin whale call occurrence and behaviour off the South African west coast. Off the Maud Rise, Antarctica, call occurrences and rates of Antarctic blue whales peaked in March and were detected throughout the whole year suggesting asynchronous migrations to the low latitudes and part of the population remaining in the Maud Rise during winter. Fin whale calls were only detected in January and March. Wind speed, distance to the sea ice extent, sea surface height, sea surface temperature and time of the day were important predictors of Antarctic blue and fin whale call occurrence and behaviour.

Information emerging from this thesis will improve the management and conservation of these highly depleted species. This thesis provides the first acoustic recordings of Antarctic blue and fin whales in the southern Benguela ecosystem; and provides preliminary information on which to concentrate further research effort to investigate abundance, distribution and seasonality of these large baleen whale populations in both high and low latitudes.