Traditionally, blue whales in the Southern Hemisphere have been divided into two distinct sub-species. The pygmy blue whale subspecies is characterized by its smaller size and distribution through the waters around Australia. The other subspecies, known as "true" blue whales, occupy the area around Antarctica and are generally larger. However, new genetic analysis suggests that the subspecies are blending together and that the species as a whole is moving south, towards Antarctica.
"Our genetic analyses are based on samples collected since 1990. Previous analyses are based on non-genetic, biological data from historical whaling records, which span from 1913 to 1973," explained Catherine Attard, a PhD candidate at Macquarie University who led the study. "Our genetic data unexpectedly found a higher proportion of pygmy blue whales in the Antarctic than the whaling catch data."
The "study substantially impacts the current knowledge of blue whale breeding behavior," Attard and her coauthors wrote in their report, to be published in the forthcoming issue of Molecular Ecology. Not only does it show, for the first time, hybridization between the subspecies, it also revealed a migration of the pygmy blue whale subspecies into a region off the coast of Antarctica.
Two anthropogenic impacts were identified as potential causes for the subspecies shift. Climate change, the authors write, has been observed to be responsible for a southern-ward shift of ocean predators in the Southern Hemisphere. Antarctic krill, a key prey species for blue whales, is known to have been impacted by climate change and may be experiencing a contraction of range. Changes in migratory timing and length, breeding schedules, and reproductive success may also be impacted by rising ocean temperatures.
Twentieth century whaling activity, too, may be responsible for the subspecies shift. The authors argue that deep depletion of subspecies populations into the 1960s and 1970s may have decreased regional carrying capacities, allowing for new subspecies to migrate into the Antarctic and establish themselves.
Either way, the findings suggest that estimates of contemporary Antarctic blue whale populations need to be reassessed to account for migration and hybridization. Furthermore, blue whale management plans must consider this population shift when new conservation areas are mapped and established.