Blue Whale Population Research Articles

Ocean-Wide Conservation Genomics of Blue Whales Suggest New Northern Hemisphere Subspecies.

The blue whale is an endangered and globally distributed species of baleen whale with multiple described subspecies, including the morphologically and genetically distinct pygmy blue whale. North Atlantic and North Pacific populations, however, are currently regarded as a single subspecies despite being separated by continental land masses and acoustic call differences. To determine the degree of isolation among the Northern Hemisphere populations, 14 North Pacific and 6 Western Australian blue whale nuclear and mitochondrial genomes were sequenced and analysed together with 11 publicly available North Atlantic blue whale genomes. Population genomic analyses revealed distinctly differentiated clusters and limited genetic exchange among all three populations, indicating a high degree of isolation between the Northern Hemisphere populations. Nevertheless, the genomic and mitogenomic distances between all blue whale populations, including the Western Australian pygmy blue whale, are low when compared to other inter-subspecies distances in cetaceans. Given that the Western Australian pygmy blue whale is an already recognised subspecies and further supported by previously reported acoustic differences, a proposal is made to treat the two Northern Hemisphere populations as separate subspecies, namely Balaenoptera musculus musculus (North Atlantic blue whale) and Balaenoptera musculus sulfureus (North Pacific blue whale). Furthermore, a first molecular viability assessment of all three populations not only found a generally high genomic diversity among blue whales but also a lack of alleles at low frequency, non-neutral evolution and increased effects of inbreeding. This suggests a substantial anthropogenic impact on the genotypes of blue whales and calls for careful monitoring in future conservation plans.

Capture–recapture estimates of Antarctic blue whale abundance and population growth rate

AbstractPhoto‐identifications of Antarctic blue whales (Balaenoptera musculus intermedia) collected from 2003/2004 to 2018/2019 were used in a capture–recapture analysis to estimate abundance and population growth rate for the circumpolar Antarctic. Two capture–recapture models, POPAN and Pradel, were applied to these data. Estimates of annual abundance and their variances from the left and right side photo data were inferred using multimodel averaging, weighted by corrected Akaike information criterion (AICc), of 26 model configurations with prespecified survival rates from .75 to 1.0. These estimates based on the left and right side databases were then combined using inverse‐variance averaging into single estimates for each year. The POPAN superpopulation estimate (total number of individuals present during the sampling period) was 3,506 whales, 95% confidence interval (CI) [2,107, 5,832]. The estimated abundance from the final year of the study in 2018/2019 was 1,817 whales, 95% CI [714, 4,624]. The abundance estimates in this study may be biased downwards due to capture heterogeneity as a result of unequal spatial sampling. Population growth rate estimates were 11% from Pradel and 10% from POPAN. These rates were within the confidence interval estimated by a previous study for Antarctic blue whale population growth rate.

Insights into Blue Whale (Balaenoptera musculus L.) Population Movements in the Galapagos Archipelago and Southeast Pacific.

The Galapagos Marine Reserve is vital for cetaceans, serving as both a stopover and residency site. However, blue whales, occasionally sighted here, exhibit poorly understood migratory behavior within the Galapagos and the broader Eastern Tropical Pacific. This study, the first to satellite tag blue whales in the Galapagos (16 tagged between 2021 and 2023), explored their behavior in relation to environmental variables like chlorophyll-a concentration, sea surface temperature (SST), and productivity. Key findings show a strong correlation between foraging behavior, high chlorophyll-a levels, productivity, and lower SSTs, indicating a preference for food-rich areas. Additionally, there is a notable association with geomorphic features like ridges, which potentially enhance food abundance. Most tagged whales stayed near the Galapagos archipelago, with higher concentrations observed around Isabela Island, which is increasingly frequented by tourist vessels, posing heightened ship strike risks. Some whales ventured into Ecuador's exclusive economic zone, while one migrated southward to Peru. The strong 2023 El Niño-Southern Oscillation event led to SST and primary production changes, likely impacting whale resource availability. Our study provides crucial insights into blue whale habitat utilization, informing adaptive management strategies to mitigate ship strike risks and address altered migration routes due to climate-driven environmental shifts.

Comparatively poorer body condition of south-east Indian Ocean pygmy blue whales on their southern migration

The south-east Indian Ocean population of pygmy blue whales (Balaenoptera musculus brevicauda) forages in habitats between temperate foraging grounds off southern Australia and breeding grounds in Indonesian waters. With the opportunity to forage during migration, limited seasonal variation in body condition would be expected. However, increasing numbers of individuals in poor body condition with prominent spinal columns and other features of emaciation have been sighted during their southern migration. This study aimed to measure the morphology and body condition (relative body volume to body length) of pygmy blue whales during their northbound and southbound migration. Unmanned aerial vehicles were used to obtain images of 42 pygmy blue whales (one calf, five juveniles and 36 adults, including one lactating female) in 2021 and 2022. The body lengths ranged from 15.32 to 24.32 m, and the mean relative head length, length to blowhole, width of eyes, fluke width and length of peduncle were 21.8, 18.1, 12.8, 22.9 and 21.3% respectively. There was a significant decrease in body condition of 15.3% between north and southern migrating adult whales. Given the importance of body condition for reproductive success, understanding if this loss is due to natural seasonal variation, or habitat or prey disturbance from environmental environmental or anthropogenic sources, has implications for the future management of the species.

A High-Quality Blue Whale Genome, Segmental Duplications, and Historical Demography.

The blue whale, Balaenoptera musculus, is the largest animal known to have ever existed, making it an important case study in longevity and resistance to cancer. To further this and other blue whale-related research, we report a reference-quality, long-read-based genome assembly of this fascinating species. We assembled the genome from PacBio long reads and utilized Illumina/10×, optical maps, and Hi-C data for scaffolding, polishing, and manual curation. We also provided long read RNA-seq data to facilitate the annotation of the assembly by NCBI and Ensembl. Additionally, we annotated both haplotypes using TOGA and measured the genome size by flow cytometry. We then compared the blue whale genome with other cetaceans and artiodactyls, including vaquita (Phocoena sinus), the world's smallest cetacean, to investigate blue whale's unique biological traits. We found a dramatic amplification of several genes in the blue whale genome resulting from a recent burst in segmental duplications, though the possible connection between this amplification and giant body size requires further study. We also discovered sites in the insulin-like growth factor-1 gene correlated with body size in cetaceans. Finally, using our assembly to examine the heterozygosity and historical demography of Pacific and Atlantic blue whale populations, we found that the genomes of both populations are highly heterozygous and that their genetic isolation dates to the last interglacial period. Taken together, these results indicate how a high-quality, annotated blue whale genome will serve as an important resource for biology, evolution, and conservation research.

Diversity of mitochondrial DNA in 3 species of great whales before and after modern whaling.

The 20th century commercial whaling industry severely reduced populations of great whales throughout the Southern Hemisphere. The effect of this exploitation on genetic diversity and population structure remains largely undescribed. Here, we compare pre- and post-whaling diversity of mitochondrial DNA (mtDNA) control region sequences for 3 great whales in the South Atlantic, such as the blue, humpback, and fin whale. Pre-whaling diversity is described from mtDNA extracted from bones collected near abandoned whaling stations, primarily from the South Atlantic island of South Georgia. These bones are known to represent the first stage of 20th century whaling and thus pre-whaling diversity of these populations. Post-whaling diversity is described from previously published studies reporting large-scale sampling of living whales in the Southern Hemisphere. Despite relatively high levels of surviving genetic diversity in the post-whaling populations, we found evidence of a probable loss of mtDNA lineages in all 3 species. This is evidenced by the detection of a large number of haplotypes found in the pre-whaling samples that are not present in the post-whaling samples. A rarefaction analysis further supports a loss of haplotypes in the South Atlantic humpback and Antarctic blue whale populations. The bones from former whaling stations in the South Atlantic represent a remarkable molecular archive for further investigation of the decline and ongoing recovery in the great whales of the Southern Hemisphere.

Year-round presence of northern and southern hemisphere Blue Whales (Balaenoptera musculus) at the Galapagos Archipelago.

Information about blue whales (Balaenoptera musculus) in the Eastern Tropical Pacific is scarce with only few whales sighted within the last decades. Molecular, photo ID and acoustic studies suggest a connectivity of Chilean and Antarctic blue whales to potential breeding areas off the coast of Peru and in the Galapagos, whereas blue whales of the Costa Rica Dome relate to the Northern Hemisphere population feeding off the coast of California and breeding areas are yet unknown.
 With opportunistic sighting records in the Galapagos, we document group size and distribution throughout the year during a 17-year period from 2001 to 2018. Sightings slightly increased during the study period with 50% of the whales seen as singletons. Whales were seen year-round but mostly during the Austral winter (82%) and calf sightings occurred from June to September. With 18% of the sightings occurring during the Austral summer, we propose there is geographical overlap between equatorial Galapagos and Northern hemisphere blue whales. One blue whale was identified in the Galapagos in February 2009, resighted in the Costa Rica Dome in January 2014, and sighted again in the Galapagos in May 2016, when whales from the Southern Hemisphere could already be present. The fact that blue whale sightings were recorded throughout the year and the identified individual strongly suggest a non-migratory blue whale population and that in the Galapagos overlapping of both North and South East Pacific blue whale populations can occur.

Biogeographic characterisation of blue whale song worldwide: using song to identify populations

Blue whale songs provide a measure for characterising worldwide blue whale population structure. These songs are divided into nine regional types, which maintain a stable character. Five of the nine song types have been recorded over time spans greater than 30 years showing no significant change in character. The nine song types can be divided into those containing only simple tonal components (high latitude North Pacific, North Atlantic and Southern Ocean song types), those comprised of complex pulsed units in addition to the tonal components (Pacific Ocean margin song types from California, Chile and New Zealand), and those which have the greatest complexity of all and the longest cycling times (Indian Ocean song types from Sri Lanka, Fremantle and Diego Garcia). We suggest that temporally stable differences in song provide another characteristic for comparison with genetic and morphological data when defining blue whale populations. Furthermore, as Mellinger and Barlow (2003) recommend, when there is a lack of other data or lack of clarity in other data sets, evidence of distinct differences in songs between areas should be used as a provisional hypothesis about population structure when making management decisions. Worldwide study is needed to better understand the various populations and subspecies within species like the blue whale that have large geographic distributions and have both migrating and resident populations.

Patterns of genetic variation in Southern Hemisphere blue whales and the use of assignment test to detect mixing on the feeding grounds

A total of 111 samples from Southern Hemisphere blue whales were sequenced for 420 base pairs of the mitochondrial control region and all but one of those were genotyped over seven microsatellite loci. Comparisons were made between samples from three broad geographic regions: the southeast Pacific Ocean; Indian Ocean; and around the Antarctic continent. Each of these strata was found to be highly differentiated from the others, in both mitochondrial and nuclear data. The genetic differentiation between the geographic ranges of the nominal subspecies (i.e. true blue whales in Antarctica vs. pygmy blues in Pacific and Indian Oceans) was not markedly greater than between the populations of pygmy blue whales. Assignment tests using the microsatellite data provide some insight into detection of feeding-season mixing, although existing methods have some limitations.

Geographic variation in external morphology of North Pacific and Southern Hemisphere blue whales (Balaenoptera musculus)

Geographic variations in size and proportions of blue whales (Balaenoptera musculus) were investigated using length data acquired from whaling records and aerial photogrammetric surveys. Results showed that blue whales found in the eastern Pacific off Central and North America are significantly shorter (by approximately 2m) than blue whales found at higher latitudes in the North Pacific. Results support the occurrence of a morphologically distinct eastern North Pacific (ENP) blue whale population which migrates in spring/summer from warm waters off Central America and Mexico to temperate feeding grounds along the west coast of North America. Southern Hemisphere blue whales sampled with vertical aerial photogrammetry off northern Peru and the Galapagos Islands were similar in size to the ENP blue whales. However, the population affinity of these southern blue whales remains uncertain. No length data were available for blue whales formerly captured off southern Japan and Korea. Nonetheless, a history of diminishing fishery catches and a lack of any recent sightings suggests that these whales were members of a geographic population that is now severely depleted or extinct. Based on comparisons of total length, length of rostrum and length of tail region, ENP blue whales were found to be morphologically similar to the ‘pygmy’ blue whale (B.m. brevicauda) described from the Kerguelen Island region of the southern Indian Ocean. ‘Antarctic’ blue whales (B.m. intermedia) from the Southern Ocean were found to be statistically significantly larger than their conspecifics at high latitudes in the North Pacific. These results support the hypotheses that blue whales that migrate from warm seas to cold feeding grounds in high latitudes are larger than those whose distributions are limited to low and mid-latitudes. Differences in morphology may reflect selective pressure on populations to adapt physiologically to energy demands associated with different migration, environmental and ecological regimes. As some of the results come from populations located far apart in different oceans, questions remain concerning the continuity of populations within and among ocean basins. Consequently, research using fishery data and approaches such as photogrammetry, telemetry, acoustics and molecular genetic analysis should be continued to better understand the worldwide blue whale population structure.

Estimation of abundance and population dynamics of the Antarctic blue whale in the Antarctic Ocean south of 60°S, from 70°E to 170°W

AbstractThe Antarctic blue whale population size has decreased due to commercial whaling, and it is listed as endangered by the International Union for Conservation of Nature. Its population status has received widespread attention and the abundance of Antarctic blue whales has been estimated through surveys conducted by international and national programs. This study estimated the abundance of Antarctic blue whales south of 60°S from 70°E to 170°W using data collected during Japanese national surveys. Furthermore, using this information and existing abundance estimates from the IDCR‐SOWER surveys conducted by the International Whaling Commission, we estimated and forecasted the population trajectory of the Antarctic blue whale since the beginning of its exploitation using Bayesian state‐space surplus production models. The abundance estimate for 2009 was 476, 95% CI [242, 972], corresponding to 0.8%, 95% CI [0.3, 1.9] of the prewhaling levels, and the intrinsic growth rate in this population was low at 0.042, 95% CI [0.009, 0.093]. This analysis suggests that the population size in this region is lower than that during prewhaling. The input of new abundance estimates in the future is expected to accurately estimate population dynamics compared with the current assessment.

Seasonal distribution, movements and taxonomic status of blue whales (Balaenoptera musculus) in the northern Indian Ocean

There is a distinct population of blue whales, Balaenoptera musculus, in the northern Indian Ocean. The taxonomic status of these animals has long been uncertain, with debate over whether this population represents a distinct subspecies, and if so which name should apply. They have most frequently been assigned to B. musculus brevicauda, but are currently considered to be B. m. indica. The movements of these blue whales within the northern Indian Ocean are poorly understood. This paper reviews catches (n = 1,288), sightings (n = 448, with a minimum of 783 animals), strandings (n = 64) and acoustic detections (n = 6 locations); uses ocean colour data to estimate seasonality of primary productivity in different areas of the northern Indian Ocean; and develops a migration hypothesis. It is suggested that most of these whales feed in the Arabian Sea off the coasts of Somalia and the Arabian peninsula during the period of intense upwelling associated with the southwest monsoon (from about May to October). At the same time some blue whales also feed in the area of upwelling off the southwest coast of India and west coast of Sri Lanka. When the southwest monsoon dies down in about October–November these upwellings cease. The blue whales then disperse more widely to eke out the leaner months of the northeast monsoon (during about December to March) in other localised areas with seasonally high productivity. These include the east coast of Sri Lanka, the waters west of the Maldives, the vicinity of the Indus Canyon (at least historically), and some parts of the southern Indian Ocean. The data are consistent with the hypothesis that at least some of the blue whales that feed off the east coast of Sri Lanka in the northeast monsoon also feed in the Arabian Sea during the southwest monsoon. These whales appear to migrate eastwards past the north of Maldives and south of Sri Lanka in about December–January, returning westwards in about April–May

Review of the Sri Lanka blue whale (Balaenoptera musculus) with observations on its distribution in the shipping lane

A population of blue whales is resident off the southern coast of Sri Lanka and has been observed year-round by the crew of a whalewatching vessel, Raja & the Whales, a few miles south of Mirissa Harbour. Over the course of three years (1 December 2009 to 30 November 2012), a total of 485 blue whale sightings were reported with an average of 4.56 individuals observed per sighting. This number does not represent the total number of individuals seen because repeat sightings were highly probable. Calves were observed during the northeast monsoon. Sightings were confined to an area of about 200 n.miles that is bisected by a heavily trafficked shipping lane. Much of this area is characterised by submarine canyons and sloping bathymetry, which contributes to monsoonal seasons of high productivity and upwelling suitable for feeding whales. While the numbers of injuries and fatalities due to ship strikes are not known, four dead blue whales were observed along the southern coast over the course of five months (1 January to 31 May 2014). It is of great urgency to understand the identity and size of this population, reduce ship strikes and address all issues threatening this population in order to arrive at possible mitigation measures for its protection.

Long‐term acoustic monitoring of nonstereotyped blue whale calls in the southern Indian Ocean

AbstractMonitoring the presence of blue whale (Balaenoptera musculus ssp.) stereotyped calls has been a widely used method to assess the different populations' distribution worldwide. All blue whale populations also produce nonstereotyped vocalizations, or D‐calls. Here, we monitored the presence of D‐calls in long‐term records from a large hydrophone array located in the open southern Indian Ocean, using an automated detection method and manual validation of the detections. D‐calls were detected at all sites of the array, which extends from 24°S to 56°S, but the majority of them were detected at the two southernmost sites. We observed a latitudinal shift in their seasonal occurrence, with more D‐calls in the north during austral autumn and winter and more in the south during austral spring. The geographical occurrence of D‐calls compared to that of songs indicates that blue whale acoustic behavior switches from a song‐intensive and sparse‐D‐call emission in the north to song‐moderate and more intensive D‐call emissions in the south. These findings support the hypothesis that both call types are used for different purposes, as D‐calls are mainly detected around foraging grounds and songs in wintering grounds. Monitoring both call types might therefore be a relevant acoustic indicator of blue whale behavior.

Surfacing rates, swim speeds, and patterns of movement of Antarctic blue whales

During three surveys in the austral summers of 2013, 2015, 2019, data on Antarctic blue whale blow rates, dive times, swim speeds, and broadscale movements were collected using video photogrammetric tracking and intra-voyage photo-identification. A total of 24.4 hours of video observations were suitable for blow interval or movement analysis. Similar to other blue whale populations, Antarctic blue whale dive behaviour comprised a sequence of short dives followed by a long dive with a mean dive time for short dives of 17.6 s, and a mean long dive time of 189.3 s. Two separate methods were used to estimate the average blow rate for Antarctic blue whales, giving estimates of 59.7 and 63.2 blows per hour. The overall mean swim speed over the course of all suitable video track segments was 1.59 ms-1, but there were significant differences between years in the mean of the overall movement rate for each track; average movement rates were lower in 2015 compared to 2013 and 2019 (0.90 ms-1, 1.84 and 1.55 ms-1respectively), with higher rates of turning in 2015. In 2019, there was faster overall movement through the study area in a consistent direction. The total number of photo-identified blue whales re-sighted intra-season in 2013 was nine (out of 50 identified individuals); in 2015 it was seven (out of 46); in 2019 two (out of 25). Whales remained for several days with little overall movement within the 2015 study area, whereas they were moving through the study area in 2019, which would explain the low number of intra-season resights. The predominant heading in 2019 was towards the area of Antarctic blue whale concentrations at the entrance to the Ross Sea observed in previous years. The photo-identification data also show a high proportion of resighted whales with coherent movements. This suggests that Antarctic blue whales might travel together, at least over periods of several days or sometimes up to a month. The differences between behaviours in 2015 and 2019 in particular may be related to differences in the characteristics of krill swarms between the study areas.

Temporal occurrence of three blue whale populations in New Zealand waters from passive acoustic monitoring

AbstractDescribing spatial and temporal occurrence patterns of wild animal populations is important for understanding their evolutionary trajectories, population connectivity, and ecological niche specialization, with relevance for effective management. Throughout the world, blue whales produce stereotyped songs that enable identification of separate acoustic populations. We harnessed continuous acoustic recordings from five hydrophones deployed in the South Taranaki Bight (STB) region of Aotearoa New Zealand from January 2016 to February 2018. We examined hourly presence of songs from three different blue whale populations to investigate their contrasting ecological use of New Zealand waters. The New Zealand song was detected year-round with a seasonal cycle in intensity (peak February–July), demonstrating the importance of the region to the New Zealand population as both a foraging ground and potential breeding area. The Antarctic song was present in two distinct peaks each year (June–July; September–October) and predominantly at the offshore recording locations, suggesting northbound and southbound migration between feeding and wintering grounds. The Australian song was only detected during a 10-day period in January 2017, implying a rare vagrant occurrence. We therefore infer that the STB region is the primary niche of the New Zealand population, a migratory corridor for the Antarctic population, and outside the typical range of the Australian population.

Presence of Southeast Pacific blue whales (Balaenoptera musculus) off South Georgia in the South Atlantic Ocean

AbstractThe Southeast Pacific (SEP) or Chilean blue whale population is largely distributed between Northern Chilean Patagonia and the Eastern Tropical Pacific. Off South Georgia (SG), the majority of blue whales caught were Antarctic blue whales, but recent genetic and acoustic records indicate the possible presence of SEP individuals. To assess the presence of SEP blue whales off SG, we carried out new analyses of acoustic data previously reported as containing pygmy blue whale song and analyzed the length frequencies from 20th century catch data. The month of acoustic data reported to contain the presumed pygmy blue whale song was examined (August 2006), and 13 days were found to have songs which visually and quantitatively matched SEP2 songs from Northern Chilean Patagonia. The fundamental frequency of SEP2 song off SG, however, was not in line with the predicted frequency shift trend of the SEP2 songs in the SEP. A mixture analysis of lengths in historic whaling catches indicated that 3.3% of the catches from SG could be SEP blue whales, although this declined to 0.6%, 95% CI [0.0, 2.6] when fit to nonrounded lengths. Our results suggest that any SEP blue whales off SG are likely rare vagrants, at the edge of an endangered population.

Seasonal Trends and Diel Patterns of Downsweep and SEP Calls in Chilean Blue Whales

To learn more about the occurrence and behaviour of a recently discovered population of blue whales, passive acoustic data were collected between January 2012 and April 2013 in the Chiloense ecoregion of southern Chile. Automatic detectors and manual auditing were used to detect blue whale songs (SEP calls) and D calls, which were then analysed to gain insights into temporal calling patterns. We found that D call rates were extremely low during winter (June–August) but gradually increased in spring and summer, decreasing again later during fall. SEP calls were absent for most winter and spring months (July–November) but increased in summer and fall, peaking between March and April. Thus, our results support previous studies documenting the austral summer residency of blue whales in this region, while suggesting that some individuals stay longer, highlighting the importance of this area as a blue whale habitat. We also investigated the daily occurrence of each call type and found that D calls occurred more frequently during dusk and night hours compared to dawn and day periods, whereas SEP calls did not show any significant diel patterns. Overall, these findings help to understand the occurrence and behaviour of endangered Chilean blue whales, enhancing our ability to develop conservation strategies in this important Southern Hemisphere habitat.

Detecting, classifying, and counting blue whale calls with Siamese neural networks

The goal of this project is to use acoustic signatures to detect, classify, and count the calls of four acoustic populations of blue whales so that, ultimately, the conservation status of each population can be better assessed. We used manual annotations from 350 h of audio recordings from the underwater hydrophones in the Indian Ocean to build a deep learning model to detect, classify, and count the calls from four acoustic song types. The method we used was Siamese neural networks (SNN), a class of neural network architectures that are used to find the similarity of the inputs by comparing their feature vectors, finding that they outperformed the more widely used convolutional neural networks (CNN). Specifically, the SNN outperform a CNN with 2% accuracy improvement in population classification and 1.7%-6.4% accuracy improvement in call count estimation for each blue whale population. In addition, even though we treat the call count estimation problem as a classification task and encode the number of calls in each spectrogram as a categorical variable, SNN surprisingly learned the ordinal relationship among them. SNN are robust and are shown here to be an effective way to automatically mine large acoustic datasets for blue whale calls.

Multiple pygmy blue whale acoustic populations in the Indian Ocean: whale song identifies a possible new population

Blue whales were brought to the edge of extinction by commercial whaling in the twentieth century and their recovery rate in the Southern Hemisphere has been slow; they remain endangered. Blue whales, although the largest animals on Earth, are difficult to study in the Southern Hemisphere, thus their population structure, distribution and migration remain poorly known. Fortunately, blue whales produce powerful and stereotyped songs, which prove an effective clue for monitoring their different ‘acoustic populations.’ The DGD-Chagos song has been previously reported in the central Indian Ocean. A comparison of this song with the pygmy blue and Omura’s whale songs shows that the Chagos song are likely produced by a distinct previously unknown pygmy blue whale population. These songs are a large part of the underwater soundscape in the tropical Indian Ocean and have been so for nearly two decades. Seasonal differences in song detections among our six recording sites suggest that the Chagos whales migrate from the eastern to western central Indian Ocean, around the Chagos Archipelago, then further east, up to the north of Western Australia, and possibly further north, as far as Sri Lanka. The Indian Ocean holds a greater diversity of blue whale populations than thought previously.