Distribution of blue whale populations in the Southern Indian Ocean based on a decade of acoustic monitoring

Abstract

Globally, the Indian Ocean appears to have the greatest blue whale (Balaenoptera musculus ssp) acoustic diversity, with at least four acoustic populations from three defined sub-species. To understand how these different populations use this region as habitat, we first need to characterize their spatial and seasonal distributions. Here, we build on previous passive acoustic monitoring studies and analyze a passive acoustic dataset spanning large temporal (9 years) and spatial (3–9 sites covering more than 12 million km² of potential acoustic habitat in the southwest Indian Ocean) scales. A novel detection algorithm was employed to investigate the long-term presence of Antarctic blue whale and SEIO and SWIO pygmy blue whale calls. We found that Antarctic and pygmy blue whales have completely different spatial and seasonal distribution in the southern Indian Ocean. Antarctic blue whales are heard almost year-round on the whole array, with great inter-annual variability. The two pygmy blue whales share a highly stable seasonal acoustic presence, but their geographical distributions overlap at only a few central Indian Ocean sites. However, Antarctic and pygmy blue whale acoustic co-occurrence is common, especially in sub-tropical waters. These temporal and spatial distributions strengthen our understanding of seasonal occurrence and habitat use of distinct populations of blue whales in the southern Indian Ocean. A better comprehension of the ecology of Indian Ocean blue whales will require interdisciplinary studies to examine the drivers of the variability seen from passive acoustic studies.

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.