Description: The eastern Atlantic Ocean is considered to provide important breeding and wintering habitats for several migratory cetacean species. The spatio-temporal distributions and migratory behaviors of cetaceans off southern Africa are nevertheless still poorly understood. This study investigated the temporal patterns of acoustic occurrence of baleen whales in a presumed baleen whale breeding area off Namibia using passive acoustic recordings collected between November 2011 and May 2013. Our results show seasonal acoustic presence of humpback whales Megaptera novaeangliae, fin whales Balaenoptera physalus and Antarctic minke whales B. bonaerensis from November to January and from June to August. Their acoustic absence from February to May possibly indicates that most animals migrated to other areas (presumably in higher latitudes) in austral summer to feed. By contrast, Antarctic blue whales B. musculus intermedia were acoustically present throughout the recording period, indicating that part of the population remains at lower latitudes year-round. Our findings support the presumed ecological importance of the oceanic area off Namibia, providing (part of) a suitable cetacean wintering and, possibly, breeding range or migratory corridor. Furthermore, the occurrence of Antarctic blue and minke whales off Namibia, concurrent with their reported acoustic presence in high-latitude feeding areas, adds to growing evidence that baleen whale migration is not obligate but much more dynamic than has long been assumed.

Description: Summertime visual observations suggest that the region around Elephant Island may serve as an important feeding area for fin whales. To explore its year-round relevance, passive acoustic recordings collected northwest of Elephant Island (61°0.88’S, 55°58.53’W) from January 2013 to February 2016 were analysed for seasonal and diel patterns of fin whale 20 Hz calls. Calls were detected year-round, although in some years calls were not present during all months. For all years, fin whale calls were consistently present from March to July for more than 90% of days per month. From August to January, percentage of days with calls varied between years, with presence exceeding 75% of days per month throughout 2014, whereas in 2015 calls were absent in October and November. In 2013, fin whale calling dropped in August and increased again towards October and November. Quantitative analyses of power spectral density for the 20-Hz and 89-Hz fin whale bands, showed that fin whale acoustic power in both frequency bands followed a Gaussian-like temporal pattern, increasing in late January, peaking during April-May and decreasing in late August for all years. A second shoulder peak in PSD seemed to occur during the second part of July showing strongest for the upper fin whale band, followed by a rapid decrease, after which SNR for both bands dropped to zero. Diel patterns in call activity were analysed for a 10-month subset of the data from 2013. Fluctuations in call rates did not follow a diel pattern nor correspond to local insolation. The observed peaks in fin whale call activity correspond to the periods during which fin whale super groups have been observed visually in this region. Our year-round acoustic analysis indicates that the Elephant Island region likely carries an important role for fin whales throughout the entire year.

Description: Here we present a comprehensive description of the acoustic environment approximately 31 km west-northwest of Minstrel Point, Elephant Island, Antarctica at 210 m water depth based on three years (Jan 2013 – Feb 2016) of subsampled (5 min per hour) passive acoustic recordings. Long-term spectrograms reveal a notable recurrence of acoustic environments between years. Fin and Antarctic blue whale calls dominate the low (〈 100 Hz) part of the biophonic spectrum energetically from end of January to late July/early August. November through early January are dominated by leopard seal vocalizations at around 300 Hz. Concurrently, the geophonic spectrum exhibits strong fluctuations between days, both due to storm and tidal influences, causing flow and shackle noise from the instrumentation itself. Manual analysis of every second day of the subsampled data by visual and aural screening (employing short term spectrograms) was used to examine the data in greater detail for additional acoustic contributions and to assign the various acoustic signatures to their sources. Six cetacean and two pinniped species were identified based on their acoustic signatures and analysed for seasonal and diel patterns in occurrence. Anthrophonic signatures were attributed to air guns on 3 % of the analysed days. Vessel noise was noted between 10 and 12 % of days on annual averages, occurring mainly in austral summer and fall with sporadic events throughout the remainder of the year. This work illustrates the value of soundscape studies and provides a first step towards understanding the actual contribution of sound sources in their respective acoustic context and overall local noise budget.