Despite being one of the largest animals on earth – blue whales are very rarely seen anymore and very little is known about them. They were once an abundant species in the world’s oceans but unfortunately, largely due to whaling in the 19th century, their numbers have been drastically reduced and their populations have been very slow to recover. This, together with their reclusive nature makes them difficult to study. As such, much about their population structure, distribution and migration patterns remains a mystery. 

However, a new study has recently been published in Scientific Reports which details how researchers have been able to tune in to the whales’ underwater symphony, allowing them to identify a previously undiscovered population of blue whales as each population’s song is slightly different and distinct – similarly to how humans from different regions have different accents and dialect. 

The study was conducted at UNSW Sydney and led by researcher Dr. Emmanualle Leroy, a bioacoustician and former postdoctoral researcher at the institution. The discovery was made possible because of data collected from underwater microphones (known as hydrophones) which have been placed across the world’s ocean for nearly two decades as a method of monitoring international nuclear bomb testing. The hydrophones are used by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) to detect sound waves released by nuclear bomb explosions, but the recordings also pick up on the rich soundscape of the underwater environment and can be used by marine scientists to learn more about elusive marine life.

Dr. Leroy was studying this CTBTO data when she noticed something peculiar: “At first, I noticed a lot of horizontal lines on the spectrogram…these lines at particular frequencies reflect a strong signal, so there was a lot of energy there.” 

Blue whale songs are among the loudest and lowest frequency sounds known to be produced by any animal. The fundamental frequency that calls are produced is between 8-25 Hz on average. This is barely audible to human hearing which typically ranges between 20 – 20 000 Hz. To listen to blue whale vocalizations click here!

Dr. Leroy and her team studied 18 years’ worth of soundscape recordings from the CTBTO, to determine whether the sounds picked up by the hydrophones were indeed a new population of blue whales, and not a variant song of a known blue whale population, another species of whale which dwells in the same region or artefact of the technology. At first, the call was thought to be a variation of the Madagascan pygmy blue whale or to belong to an Omura’s whale population which has recently been described in the area. However, closer examination of temporal and spectral properties of the calls revealed that they are more like a blue whale call and strongly suggest that they belong to a previously unidentified population of pygmy blue whales. In addition, the calls reflect typical blue whale migratory behavior as they were detected at five other widely distributed hydrophone arrays, ranging from Sri Lanka to northwestern Australia. The calls also display strong seasonal occurrence patterns, unlike the Omura’s whale songs which show no variation year-round, suggesting a migratory lifestyle. This population was nicknamed the Chagos population after a nearby archipelago where they were detected.

Pygmy Blue Whales (Balaenoptera musculus brevicauda)

At an average of 24 m (79ft) long, Pygmy blue whales are the smallest subspecies of blue whales that roam the ocean, compared to other commonly recognized subspecies such as B. m. musculus (or “true” blue whale) which averages 28 m (92 ft) and B. m. intermedia which averages 30 m in length. 

Pygmy blue whales are also physically distinct from other subspecies of blue whales as they have:

  • broader and shorter baleen plates
  • a shorter tail 
  • proportionately longer body in front of the dorsal fin
  • a larger head in proportion to their overall body size
  • heavier body mass compared to other blue whale subspecies of the same length
  • a different blow hole shape
  • and are, typically, a darker skin color

The morphological differences between pygmy blue whales and other subspecies are also reflected in their body movements. The shorter tail influences their diving behavior, and in pygmy blue whales, the dorsal fin and caudal peduncle submerge simultaneously, whereas in “true” blue whale species there is a delay between the dorsal fin and peduncle entering the water.

Pygmy blue whales are thought to have higher population numbers than other subspecies. Their distribution ranges from Antarctic oceans to the southern Indian Ocean and the southwestern Pacific Ocean. They have breeding habitats in the Indian and South Pacific Ocean and travel to the Antarctic in the summer months to feed. They are filter-feeders and mainly consume euphausiids (krill).

Up until recently, there were three recognized populations of pygmy blue whales, with a fourth population having been recently detected in the Arabian Sea off the Oman coast. The populations are delineated based on acoustic dissimilarities between their calls, as there are no physical differences between populations and genetic data is scarce. The populations are also associated with different regions of the ocean – one population inhabits the southwestern Indian Ocean and is identifiable by the Madagascan or type-9 song, another inhabits the southeastern Indian Ocean and produces the Australian or type-8 song, the third population is found in the northern and central Indian ocean and produces the Sri Lankan or type-7 song and the population found in the Arabian sea makes up the fourth population. All these populations, together with the Antarctic blue whale subspecies and Chagos population are sympatric in the Indian ocean.

Implications For Conservation

This finding is fantastic news for blue whale conservation as the Indian Ocean appears to hold a greater diversity of blue whale populations than originally thought, although visual sightings are still required to confirm the existence of this new population. Obtaining visual sightings is an intensive and expensive undertaking and the researchers believe that this new population’s existence won’t be confirmed anytime soon. In the meantime, they are planning to continue to study the vocalization data as they are hoping to learn more about blue whales in general, as well as each of the individual populations. Specifically, they are interested in how the whales have adapted to changing ocean temperatures over the past two decades as it may provide insight into how they will fare in the future.