Distribution of blue whale populations in the Southern Indian Ocean based on a decade of acoustic monitoring
Introduction
The Indian Ocean, particularly its southern extent, is one of the oceans with the greatest blue whale acoustic diversity (McDonald et al., 2006). As a response to extensive commercial whaling in both the Southern and Indian Oceans which greatly depleted numbers of blue whales, in 1979 the International Whaling Commission (IWC) created the Indian Ocean Whale Sanctuary (IOWS), the first ever region where commercial whaling was banned for all species (IWC, 1980). Three blue whale subspecies are seasonally present in the Indian Ocean: the Antarctic blue whale (Balaenoptera musculus intermedia) and the pygmy blue whales (B. m. brevicauda and B. m. indica). Antarctic and pygmy blue whales are genetically distinct and differ in body size and acoustic signatures (Ichihara, 1966; LeDuc et al., 2007; Ljungblad et al., 1998). Furthermore, at least three distinct populations of pygmy blue whales have been identified: the Northwestern (NWIO), Southwestern (SWIO) and Southeastern (SEIO) pygmy blue whale populations (Stafford et al., 2011). Whereas the Antarctic blue whale has been declared as an endangered species by the IUCN (International Union for the Conservation of Nature), the status of the pygmy blue whales is unknown and therefore considered ‘data deficient’ by the IUCN (Cooke, 2019). Monitoring blue whales remains a challenge due to the scarcity of individuals and to the extent and location of their habitat, largely encompassing remote and inaccessible regions of the ocean. Moreover, identifying pygmy from Antarctic blue whales by visual observation is very difficult, as they look almost identical, despite the smaller length of pygmy blue whales (Ichihara, 1966). Thus, most of the knowledge about blue whales in the Indian Ocean comes from whaling data (Branch et al., 2007, 2009), and from passive acoustic monitoring (Samaran et al., 2010a, 2013; Stafford et al., 2011; Leroy et al., 2016; Dréo et al., 2018). Blue whales are particularly good candidates for this type of observation, because of their repetitive, long (more than 15 s), loud (more than 180 dB ref 1 μPa at 1 m) and low frequency (20–100 Hz) calls (Cummings and Thompson, 1971). Blue whale calls vary from one region to another and have been used to tell apart different blue whale sub-species and acoustic populations (McDonald et al., 2006). In the southern Indian Ocean, calls of Southeastern Indian Ocean (Fig. 1 a) and Southwestern Indian Ocean (Fig. 1 b) pygmy blue whale populations as well as Antarctic blue whale calls (Fig. 1 c) have clear distinct characteristics that are readily observed in the spectrogram (McCauley et al., 2000; Samaran et al., 2010a; Stafford et al., 2011).
Whaling data depicted baleen whale migration as being a consistent movement between productive feeding grounds at high latitudes in summer, and breeding grounds at lower latitudes in winter (Kellogg, 1929; Mackintosh, 1942, 1966). However, year-round captures and sightings of Antarctic blue whales in whaling grounds at high or low latitudes suggest more complex migratory behaviors (Harmer, 1931; Branch et al., 2007). In agreement with these findings, evidence of year-round acoustic presence of Antarctic blue whales off Antarctica (Širović et al., 2014; Thomisch et al., 2016) and Namibia (Thomisch et al., 2019), and observations of foraging behavior en route towards summer feeding grounds (Visser et al., 2011) supports the idea of a more complex migratory pattern. For instance, skipping migration to remain at feeding grounds may benefit individuals to maximize their growth and body mass (Shaw and Levin, 2011). In the southern Indian Ocean, Antarctic blue whale calls are detected year-round, with a lesser presence during austral summer and a shift from subantarctic locations in austral autumn to subtropical locations in the austral winter and back to subantarctic locations in spring (September to November) (Stafford et al., 2004; Samaran et al., 2010a, 2013; Leroy et al., 2016). This seasonal pattern is generally stable over the years, but some variations suggest that the migration routes are flexible (Samaran et al., 2010a, 2013; Leroy et al., 2016). In contrast, pygmy blue whales present a totally different migration pattern. Short-term (Stafford et al., 2011; Samaran et al., 2013) and broad-scale acoustic studies based on the chorus intensity (intensity within the call frequency band) depict a longitudinal differentiation of the SEIO and SWIO pygmy blue whale populations. The former are mainly present in the eastern Indian Ocean and the latter in the western Indian Ocean. Yet, they seem to share a common seasonality with an acoustic presence during all seasons except spring (Stafford et al., 2011; Samaran et al., 2013; Leroy et al., 2018b).
Despite their different distribution, SEIO and SWIO pygmy blue whales have both been recorded near the Crozet archipelago (46°24′41″ S, 51°45′22″ E) (Samaran et al., 2010a) and southwest of Amsterdam Island (37°50′00″ S, 77°31′00″ E) (Samaran et al., 2013), at the fringes of their respective habitat (see map in Fig. 2). Similar co-occurrence of pygmy blue whales (SEIO and southwestern Pacific populations) has been reported in the Bass Strait, between mainland Australia and Tasmania (McCauley et al., 2018), which marks the geographical barrier between these two populations Balcazar et al. (2015). Sympatric acoustic presence of Antarctic and pygmy blue whales has also been observed off southern Australia (Tripovich et al., 2015), near the Crozet archipelago (Samaran et al., 2010a), in the Madagascar Basin, north and south of Amsterdam Island (Samaran et al., 2013) and in the north of the Mozambique Channel (Cerchio et al., 2018).
The function of songs for blue whales is not unanimously agreed upon. The fact that only males are thought to sing (Oleson et al., 2007; Lewis et al., 2018) as well as the seasonal repetition, high source levels (McDonald et al., 2001; Širović et al., 2007; Samaran et al., 2010b) and the long durations of the calls, point to a reproductive purpose (McDonald et al., 2001; Oleson et al., 2007). Acoustic co-occurrence of different sub-species may therefore indicate potential inter-subspecies breeding areas or the use of song to isolate different populations from interbreeding (Stafford and Moore, 2005). However, the year-round detection of songs at high-latitude feeding areas (Stafford et al., 2001; Širović et al., 2004, 2009; Thomisch et al., 2016) suggests that the calls may play additional roles, such as assisting in navigation and prey detection (Clark and Ellison, 2004), or promoting the formation of pairs (Lewis et al., 2018; Oleson et al., 2007). Acoustic co-occurrence of multiple sub-species may also simply reflect differing ecological strategies and/or habitat use by each subspecies, such as off Southern Australia, which would be a migratory corridor for Antarctic blue whales and a feeding ground for the SEIO pygmy blue whales (Tripovich et al., 2015).
To progress in our understanding of these concepts, this work presents a detailed analysis of the seasonal and spatial presence of Antarctic and pygmy blue whales, based on 9 years of continuous acoustic recordings (2010–2018) at multiple sites across the southern Indian Ocean. Relative to previous analyses, we provide a longer time series and new recording locations. We also base our analysis on the systematic detection of pygmy blue whale calls, when former studies only looked at the chorus power to describe their acoustic presence. The comparison between these two passive acoustic monitoring metrics is then discussed. These data yield a clearer picture on the seasonal acoustic presence of pygmy and Antarctic blue whales and spatially how they share the IWC IOWS. This information is essential for improving the management and conservation of the world's largest animal.
Section snippets
Data acquisition
The acoustic data used in this study were recorded by the OHASISBIO (Observatoire Hydro-Acoustique de la SISmicité et de la Biodiversité; Royer (2009)) hydrophone network, located in the southwest Indian Ocean (see Fig. 2). The network was deployed in December 2009 and is still recording as of the date of publication. Between 2009 and 2016, it comprised 5 permanent mooring sites, located south of La Réunion Island (MAD), north of Crozet archipelago (NCRO), west of Kerguelen Island (WKER) and
Spatial distribution and long term trends
Fig. 3 displays the number of detected calls per year for the long-term recording sites (rows) and the three blue whale calls (columns). Since blue whale acoustic presence is highly seasonal, normalizing the number of detections per year by the number of days of recording could lead to some misinterpretation, especially regarding pygmy blue whales. For example, at site WKER the number of SWIO pygmy blue whale calls is similar in 2017 and in 2018. However the hydrophone only lasted 6 months in
Using passive acoustics to monitor blue whale populations
Passive acoustic monitoring with a wide network of hydrophones is a very effective approach to detect the presence of blue whales, as in this study. However it cannot be used to determine the abundance of whales in the vicinity of a hydrophone based on the number of detected calls. Indeed, several factors can influence the number of blue whale calls present in the data. Some of them are dependent on blue whale behavior (e.g. when, where and why they sing and how song structure changes) and
Conclusion
This study documents the presence of three blue whale acoustic populations for nearly a decade in the southern Indian Ocean. Its results are based on a long and continuous time series and on the application of a single algorithm for the detection of the different calls. These results support the spatial and seasonal distribution previously depicted in this region and the additional years and recording locations refine these previous findings, both temporally and geographically. What is
CRediT authorship contribution statement
Maëlle Torterotot: Conceptualization, Methodology, Software, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization, Funding acquisition. Flore Samaran: Conceptualization, Methodology, Investigation, Writing - review & editing, Funding acquisition. Kathleen M. Stafford: Conceptualization, Investigation, Writing - review & editing. Jean-Yves Royer: Conceptualization, Methodology, Investigation, Writing - review & editing, Project administration,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors wish to thank the captains and crews of RV Marion Dufresne for the successful deployments and recoveries of the instruments of the OHASISBIO array. Cruises were funded by the French Polar Institute and the French Oceanographic Fleet, with additional support from INSU-CNRS. M.T. was supported by a Ph.D. fellowship from the University of Brest. Her visit to K.M.S. was supported by a grant from the ISblue project, Interdisciplinary graduate school for a blue planet (ANR-17-EURE-0015),
References (63)
- et al.
Acoustic survey for marine mammal occurrence and distribution off East Antarctica ( 30-80°E ) in January-February 2006
Deep-Sea Res. Part II
(2010) - et al.
Pygmy blue and Antarctic blue whale presence, distribution and population parameters in southern Australia based on passive acoustics
Deep-Sea Res. Part II
(2018) - et al.
Seasonality of blue and fin whale calls and the influence of sea ice in the Western Antarctic Peninsula
Deep-Sea Res. Part II Top. Stud. Oceanogr.
(2004) - et al.
Antarctic-type blue whale calls recorded at low latitudes in the Indian and eastern Pacific Oceans
Deep-Sea Res. Part I Oceanogr. Res. Pap.
(2004) - et al.
Hybridization of southern Hemisphere blue whale subspecies and a sympatric area off Antarctica : impacts of whaling or climate change ?
Mol. Ecol.
(2012) - et al.
Calls reveal population structure of blue whales across the southeast Indian Ocean and the southwest Pacific Ocean
J. Mammal.
(2015) - et al.
Separating pygmy and Antarctic blue whales using long-forgotten ovarian data
Mar. Mamm. Sci.
(2009) - et al.
Past and present distribution, densities and movements of blue whales (Balaenoptera musculus in the Southern Hemisphere and northern Indian Ocean
Mamm Rev.
(2007) - et al.
Seasonal acoustic presence of blue, fin, and minke whales off the juan fernández archipelago, Chile (2007–2016)
Mar. Biodivers.
(2020) - et al.
Occasional acoustic presence of Antarctic blue whales on a feeding ground in southern Chile
Mar. Mamm. Sci.
(2018)
Acoustic monitoring of blue whales (Balaenoptera musculus) and other baleen whales in the Mozambique Channel off the northwest coast of Madagascar (SC/67B/SH/14). Technical report
Int. Whaling Comm. Scientific Committee
Potential use of low-frequency sounds by baleen whales for probing the environment: evidence from models and empirical measurements
Echolocation Bats Dolphins
Balaenoptera Musculus, (Errata Version Published in 2019). Technical Report
Underwater sounds from the blue whale, Balaenoptera musculus
J. Acoust. Soc. Am.
Long-term autonomous hydrophones for large-scale hydroacoustic monitoring of the oceans
Migratory movements of pygmy blue whales (Balaenoptera musculus brevicauda) between Australia and Indonesia as revealed by satellite telemetry
PLoS One
Baleen whale distribution and seasonal occurrence revealed by an ocean bottom seismometer network in the Western Indian Ocean
Environmental evidence for a pygmy blue whale aggregation area in the Subtropical Convergence Zone south of Australia
Mar. Mamm. Sci.
Steady inter and intra-annual decrease in the vocalization frequency of Antarctic blue whales
J. Acoust. Soc. Am.
Vocal characteristics of pygmy blue whales and their change over time
J. Acoust. Soc. Am.
A blue whale (Balaenoptera musculus) feeding ground in a southern australian coastal upwelling zone
J. Cetacean Res. Manag.
Sparse representation-based classification of mysticete calls
J. Acoust. Soc. Am.
Southern whaling
Proc. Linn. Soc. Lond.
The pygmy blue whale, balaenoptera musculus brevicauda, a new subspecies from the antarctic
Whales Dolphins Porpoises
Chairman's report of the thirty-first annual meeting
Rep. Int. Whal. Comm.
Song variation of the south eastern Indian Ocean pygmy blue whale population in the perth canyon , western Australia
PloS One
A review of food of balaenopterid whales
Sci. Rep. Whales Res. Inst
What Is Known of the Migrations of Some of the Whalebone Whales
Patterns of genetic variation in Southern Hemisphere blue whales and the use of assignment test to detect mixing on the feeding grounds
J. Cetacean Res. Manag.
Long-term and seasonal changes of large whale call frequency in the Southern Indian Ocean
J. Geophys. Res.: Oceans
Seasonal and diel vocalization patterns of Antarctic blue whale (Balaenoptera musculus intermedia) in the Southern Indian Ocean: a multi-year and multi-site study
PLoS One
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