Pygmy blue whales were only described as recently as 1966. The name Balaenoptera musculus brevicauda (Ichihara 1966) comes from measurements of "pygmy" blue whales from the Subantarctic Indian Ocean but may also apply to other populations of smaller blue whales. Brevicauda means "short tail", referring to the proportionally shorter distance from the dorsal fin the flukes in this subspecies as compared to Antarctic blue whales.
Size, shape and distinctive characteristics
Most of the descriptions of pygmy blue whales are in relation to Antarctic blue whales. As the name implies, pygmy blue whales are shorter overall than Antarctic blue whales with maximum lengths of 24 m. They also have shorter baleen plates and a proportionately shorter tail. This subspecies is also thought to be more "tadpole" shaped than the more streamlined Antarctic blue whale. They can be very difficult to distinguish at sea from Antarctic blue whales as they share characteristics of mottled gray-blue skin, small dorsal fins and broad, flat rostrum.
Pygmy blue whales are found throughout the Indian Ocean at least as far south as Crozet and Kerguelen Islands; around Australia and possibly up into Indonesia.
Blue whales off Chile might also be pygmy blue whales and these animals range from the equator south to at least Chiloe, Chile.
Ecology and Behaviour
Pygmy blue whales tend to be solitary although numerous animals are found in areas a of high productivity particularly in the northern Indian Ocean, off Sri Lanka and southern and western Australia. Blue whales are lunge feeders that rely on large concentrations of krill.
The migratory behavior of pygmy blue whales is variable with some populations considered resident (i.e. northern Indian Ocean) while others may undertake seasonal migrations. There is no known wintering area for pygmy blue whales but it is likely that many move between the Indian Ocean and low latitudes of the Southern Ocean.
Blue whales produce very loud, long, low frequency sounds that are regionally distinctive leading to the proposal of "acoustic populations" of blue whales. Some of their sounds are thought to be used as contact calls while others, when repeated in long bouts, are considered song. To date, evidence supports only males producing song and therefore it has been proposed to serve similar functions as humpback whale song. Pygmy blue whale songs tend to have more complex sounds with two to four units in a phrase versus a single unit documented for the North Atlantic, west Pacific and Southern Ocean. Particularly in the Indian Ocean, the detection of distinct call types has been used to suggest that at least three acoustic populations of pygmy blue whales exist.
Because very little is known about pygmy vs other blue whales and distinctions between the two were often not made on whaling vessels, we have to assume that pygmy blue whales share life history characteristics with other blue whales. Both male and female blue whales are thought to reach sexual maturity between 5-15 years of age. Females give birth every 2-3 years and gestation is 10-11 months followed by weaning of calves after 7 months.
Like other blue whales, pygmy blue whales are thought to feed primarily on krill species including Nyctiphanes australis, Euphausia recurva and E superba.
The abundance of pygmy blue whales is unknown both due to the unresolved status of this subspecies and due to a lack of data globally. A "provisional" estimate of 10,000 animals prior to 1961. As animals were hunted extensively since then, the present status is impossible to determine.
Just over 1200 animals was proposed for the Madagascar Plateau based on a single-survey estimate of 424 likely pygmy blue animals from 1996.
The status of pygmy blue whales is listed as "data deficient" due to a lack of any estimates of abundance or clear delineation as to what constitutes a "pygmy" blue whale (see Gilpatrick et al. 2008). At present the IUCN limits this term to Indian Ocean blue whales.
Blue whales have been protected from whaling since 1966 by the International Whaling Commission. Despite this protection, illegal catches, many of which were likely pygmy blue whales, continued in the Indian Ocean and North Pacific until 1972. At present there is no direct threat from whaling but other anthropogenic threats exist. These include noise pollution (masking) from increasing low-frequency noise in the oceans; pollution and ship strikes.
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