Blue Whale Blow (9-12m high)
- Species Name: Balaenoptera musculus brevicauda
- Distribution: Worldwide
- Population Status: Unknown
- IUCN Status: Data Deficient
Physical Features
The word “musculus” is thought to be a variant of “muscular”, however in Latin it also translates as “little mouse” – perhaps evidence that biologists do have a sense of humour! The pygmy blue whale species has the additional name of “brevicauda”, meaning “short tail”, which refers to the proportionally shorter distance from the dorsal fin to the tail flukes in comparison to Antarctic blue whales.
- Blue by name…
As the name implies, pygmy blue whales are shorter than their Antarctic cousins, obtaining lengths of approximately 24m in contrast to the “True Blues” overall lengths of as much as 30m. They also have shorter baleen plates and, as mentioned above, a proportionally shorter tail. However, these similarities are relatively subtle and make differentiation between the two species difficult in the field. Both pygmy and true blues are grey-blue in colour, particularly when viewed through the water; hence their common name. Each animal has a unique mottling pattern on their skin, allowing identification of individuals. Their dorsal fin is quite small, only around 30cm tall, and ranges in shape from being a stumpy triangle to a graceful hook.
As well as the potential for being mistaken as an Antarctic blue whale, pygmy blues may also be confused with fin whales. However, the latter animal has a typically large hooked dorsal and may also be distinguished by its asymmetric head pigmentation. Pygmy blues are also similar to sei and Bryde’s whales, however these species are much smaller and less heavily built, with less set-back dorsal fins and very different dive sequences and surfacing behaviours. Some hybrid species have been recorded: a Blue X Fin was documented in the North Atlantic (which produced fertile offspring), and a possible Blue X Humpback hybrid in the South Pacific.
Distribution
Pygmy blue whales are thought to occur worldwide, however more distribution information is known for some populations than others. Pygmy blues are found throughout the Indian Ocean at least as far south as the Crozet and Kerguelen Islands. They also occur around the entire Australian coastline; individuals sighted initially in Western Australia have been recorded in South Australia and Indonesia. Off western South America, pygmy blue whales range from the equator south to at least Chiloe, Chile.
The migratory behaviour of pygmy blue whales is variable. Some populations undertake seasonal migrations between summer feeding area and winter breeding grounds; however the exact location of these areas is unknown. It is thought that many move between the Indian Ocean and low latitudes of the Southern Ocean. Conversely, some populations are considered resident, e.g. northern Indian Ocean pygmy blues.
Life History
Looking up the nose of a whale
Little is known about the life history characteristics of pygmy blues, largelydueto a lack of distinction between the two on whaling vessels where most anatomical data was collected. Thus, it is assumed that pygmy blue whales share life history traits similar to those of other blue whale species. Both male and female blue whales reach sexual maturity between 5-15 years of age. Females appear to give birth every 2-3 year; there is a 10-11 month gestation period followed by weaning of calves after approximately 7 months. The lifespan of blue whales has yet to be reliably measured, but is thought to be up to 90 years.
Ecology and Behaviour
Pygmy blue whales are usually found alone, however they can be found in loose aggregations of up to 50 animals in areas where food is abundant. These areas of high productivity are particularly noticeable around Sri Lanka and southern and western Australia. Pygmy blue whales feed primarily on krill and forage using a technique known as “lunge-feeding”, where the animal lunges forward taking a large quantity of water and prey into its mouth. The water is then squeezed out through the baleen plates, leaving the krill trapped on the baleen for the whale to lick off. By this method, each pygmy blue whale may consume upward of 3 tonnes of krill per day. Although foraging occurs at depths of less than 100m, some pygmy blues have been recorded at depths of up to 200m, typically diving for 10-20mins.
Blue whale pod (Photo: Blue Whale Study Inc)
Although pygmy blue whales tend to be solitary, pods of 2-3 animals may be encountered migrating together. Occasionally, “racing groups” of three whales have been seen travelling at high speed, jostling and changing direction frequently. This behaviour is thought to be social and is most likely related to courtship. Other social behaviour include the use of aerial signals, such as tail- and flipper-slapping, however the animals have rarely been recorded breaching.
Blue whales make powerful, low-frequency calls; most pygmy blue whale calls fall into the 10-30 Hz range. Calls vary between oceanic regions and between subspecies, leading to the proposal of “acoustic populations” of whales. Particularly in the Indian Ocean, the detection of distinct call types has been used to suggest that at least three acoustic populations of pygmy blues whales exist. Some of these calls are thought to be used as contact calls whilst others display the characteristics of song (i.e. repeated long bouts). As in other whale species (most famously the humpback whale), it appears that only the males produce song, suggesting a reproductive function. The songs of pygmy blues tend to be more complex than those of the true blue whale, with 2-4 units in a phrase versus a single unit documented for the North Atlantic, west Pacific and Southern Ocean species.
Conservation
Due to uncertainty about its taxonomic status, it has not been possible to produce an estimate of abundance for this species. This is due to a lack of information globally. A “provisional” estimate of 10,000 pygmy blue whales existed prior to 1961; however, as the species have been extensively hunted since then, their actual population size is impossible to determine. As a result, the IUCN has listed the pygmy blue whale as “data deficient”.
Cargo Ship
Although the International Whaling Commission (IWC) has protected blue whales from whaling since 1966, illegal catches have occurred since that time. Over 8,000 pygmy blue whales were taken illegally by Soviet whalers in the 1960s and 1970s. However, at present all nations have agreed not to harvest blue whales. Unfortunately, the species still faces anthropogenic threats, in the form of pollution, ship strikes, entanglement in fishing gear, and noise-pollution from increasing low-frequency noise in the oceans.
More Information
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