Blue whales are ‘long-legged’ whales, adapted for rapid long-range movements between alternate feeding areas that may be hundreds of kilometres apart, or feeding and breeding areas thousands of kilometres apart. We are starting to accumulate evidence from a variety of research methods, which supports this picture of these giant ocean wanderers.
In late 2003 and 2005, our aerial surveys found pygmy blue whales feeding nearly 500km west of the core feeding area, which lies between Cape Otway in western Bass Strait, to just west of Robe, SA. The ‘new’ feeding area was west and south of Kangaroo Island in the eastern Great Australian Bight (GAB), the western extremity of a large-scale upwelling system that includes the Bonney Upwelling (BU) and extends east to Tasmania. At this stage it seems that blue whales only use the GAB in any numbers during December.
Photo-identification of individuals has shown a re-sight in the Bonney Upwelling in 2005 of a blue whale photograpically identified in the Perth Canyon (PC), Western Australia, in 2004. This was the first evidence that the blue whales using the BU may interchange with other feeding areas. We have since found many such re-sights between the PC and the BU.
In April 2005, our satellite tagging showed direct movement of a blue whale south from the BU to the Sub-Tropical Convergence, a broad and productive oceanic mixing zone to Australia’s south where Soviet whalers illegally killed many blue whales during the 1960s. This movement has since been confirmed by other satellite tagging programs and it seems that this may be a common phenomenon as blue whales search for optimal foraging conditions. All this evidence confirms that these whales are part of a larger ‘open’ population, rather than being restricted only to the Bonney Upwelling.
Satellite tagging (by other research groups) has also shown that at least some PC and BU blue whales migrate north via the Western Australian to Indonesian waters during winter, where breeding likely occurs. There is no evidence yet that BU blue whales migrate up the east coast of Australia. Sound recordings on the east coast indicate that blue whales there have a different call to the BU blue whales, suggesting that Bass Strait may be a boundary between two populations, the ‘Indian Ocean’ population and the ‘South-west Pacific’ population.
Historical whaling and sightings data show that pygmy blue whales may spread out through an immense area of the Southern Ocean (though rarely as far south as the Antarctic ice edge). This area would be prohibitively expensive to survey in one go, so we may never know how many whales there in this population. This makes it difficult to relate apparent abundance in the BU to factors like upwelling variability, as whales may be responding more to what is happening somewhere else where we have no idea what is going on.