Spatio-temporal patterns in Antarctic minke whale (Balaenoptera bonaerensis) vocal behaviour in the Weddell Sea

Abstract

Antarctic minke whales (AMW) have a circum-antarctic distribution and are known to occur in both open and ice-covered waters of the Southern Ocean (SO). They are a pagophilic species and are regularly observed in heavily sea-ice covered areas. Because of this, current knowledge about Antarctic minke whales is scarce and limited to austral summer and ice-free regions where the majority of visual survey data have been collected. Hence, very little is known on their winter distribution in high latitude waters and to what extent the wintering individuals that are occasionally observed in Antarctic waters represent single observations or a structurally present behavioral trait of the species. Likewise, AMW population structure is poorly known. As most baleen whale species, AMWs are known to undertake annual migrations between high latitude feeding areas and low latitude breeding areas. However, available data to date suggest that for AMWs, migrations seem more protracted and complex than in other baleen whale species. The recent identification of vocalizations produced by AMWs (“bio-duck call”) now makes it possible to also use passive acoustic technology to study occurrence patterns and behavior. Passive acoustic monitoring (PAM) offers a versatile technology with which long-term archival data can be collected on sound-producing species using autonomous recording units. Acoustic observation has the additional advantages that it can continue data collection independent of light conditions and under poor weather conditions. Furthermore, it can monitor large areas at the same time, since it collects data omnidirectionally, and it can collect data over extended periods (i.e., multi-year time scales) through the use of autonomous devices. In the scope of this doctoral thesis, I analysed passive acoustic data from high and low latitudes collected in the Atlantic Ocean sector of the Southern Ocean (Weddell Sea) and southern Atlantic Ocean, to investigate spatio-temporal patterns in the acoustic presence and vocal behaviour of AMWs. In Chapter I, I investigate year-round spatial- temporal patterns in AMW occurrence in ice-covered areas. Our analysis was based on a 9-year passive acoustic dataset (2008- 2016) from 21 locations throughout the Weddell Sea (WS). AMW bio-duck calls were detected acoustically at all mooring locations from May to December, with the highest presence between August and November (bio-duck calls present at more than 80% of days). At the southernmost recording locations, bio-duck calls were present up to 10 months of the year. We identified a substantial inter-annual variation in the seasonality of vocal activity correlated to variation in local ice concentration. Our analysis indicates that part of the AMW population stays in the Weddell Sea during austral winter. In Chapter II, I used PAM data from 6 positions located along the WS during 2013 as well as three consecutive years of PAM data (2015-2016-2017) recorded at the stationary coastal acoustic observatory PALAOA to study both spatial and inter-annual variability in the acoustic behavior of AMWs. During 2013, between May and December throughout the WS, we detected 11 different bio-duck call types. The comparative analyses of the AMW bio-duck call repertoire throughout the WS showed that these 11 call types can be attributed to four call type groups. Furthermore, multi-year data from PALAOA on AMW repertoire composition showed an inter-annual difference in bio-duck call type usage. The observed patterns in AMW vocal behavior show that AMW vocal behavior matches the definition of song, exhibiting regional differences and highly dynamic revolutions over time. This study provides new information to potentially explore stock structures and movement patterns using passive acoustic methods. In Chapter III, my work addressed the occurrence of the AMW through bio-duck acoustic detections in the Santos Basin, South-Southeastern Brazil (22º and 28º S / 42º and 48º W), recorded between 12 November and 19 December 19, 2015, utilizing advanced passive acoustic technologies, i.e., an autonomous underwater vehicle (SeaGlider), for sampling. AMW calls were detected during 12 days. We detected and classified 9 different bio-duck calls in Brazilian coastal waters, evidencing a highly diverse acoustic behavior on this potential Antarctic minke whale breeding ground. Finally, in Chapter IV, our study investigated the temporal patterns of acoustic occurrence of baleen whales in a presumed baleen whale breeding area off Namibia. Our results show seasonal acoustic presence of humpback whales, fin whales and Antarctic minke whales 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. 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 previously assumed.

Overall, this PhD thesis highlights the significance of using long-term and large-scale datasets for the investigation of distribution patterns, habitat preferences, behavior and the effects of environmental variation on AMW distribution. The results present new information about fundamental and ecological knowledge of AMWs that can be implemented for the conservation and management of populations and ecosystems.