Hunted nearly to the brink of extinction due to whaling in the 20th century, the Antarctic blue whale—recognized as the largest animal on the planet—dramatically declined from a population of around 200,000 to just over 300 individuals. As of 2004, estimates indicated that the population of Antarctic blue whales had fallen to below 1% of its numbers prior to the whaling era. A new study reveals that, while these whales feed across various ocean basins, they seem to function as a single population, which is crucial for future conservation efforts.
Hunted nearly to extinction during 20th century whaling, the Antarctic blue whale, the world’s largest animal, went from a population size of roughly 200,000 to little more than 300. The most recent estimate in 2004 put Antarctic blue whales at less than 1% of their pre-whaling levels.
But is this population bouncing back? Are there multiple groups of Antarctic blue whales, or is there just one? Why do these questions matter for conservation?
A research team led by Zoe Rand, a doctoral student from the University of Washington, seeks to answer these questions in a study published on November 14 in Endangered Species Research. Building on the last evaluation of Antarctic blue whales from 2004 and utilizing surprisingly detailed old whaling records, Rand and her colleagues explored whether Antarctic blue whales comprise distinct populations or if they exist as one large circumpolar group. The study included contributions from Trevor Branch, a professor of aquatic and fishery sciences at UW, and Jennifer Jackson from the British Antarctic Survey.
As Antarctic blue whales are classified as an endangered species, understanding their population dynamics is critical for their conservation. Protecting them at the population level boosts biodiversity, enabling the species to better adapt to environmental shifts and enhancing their prospects for long-term survival.
During the whaling era, biologists initiated the Discovery Marking Program, where foot-long metal rods with serial numbers were inserted into the muscles of whales. When these whales were caught, the rods were returned, along with data about the whale’s size, sex, length, and capture location. Analyzing the locations where whales were marked versus where they were caught could provide valuable insights into the movements of Antarctic blue whales, but this information was previously unused for assessing population structure.
In their new study, the researchers combined this historical data with contemporary survey data using Bayesian models to compute the annual movement rates among the three ocean basins of the Southern Ocean: Atlantic, Indian, and Pacific. The findings showed significant mixing across these ocean areas, indicating that the whales do not consistently return to the same feeding ground each year. This evidence supports the idea that Antarctic blue whales are a single, circumpolar population within the Southern Ocean.
These findings align with studies of the vocalizations of Antarctic blue whales, which have been recorded throughout the Southern Ocean. Researchers have noted that a single type of song has been identified among these whales, unlike pygmy blue whales, which produce five distinct songs corresponding to five different populations. Furthermore, genetic studies confirm that Antarctic blue whales are closely related in ways contrary to what would be expected if they were separate populations.
This study marks the first occasion that historical mark-recovery data from the Discovery Marking Program has been analyzed with modern quantitative techniques. Similar historical data exists for other hunted whale species, such as fin and sei whales, suggesting that the methods employed in this research could serve as a model for future studies on those species as well.
Despite the insights gained, there remains much to learn about the Antarctic blue whale. Although they do not seem to be geographically isolated in their feeding environments across different ocean basins, there may still be unique population structures influenced by variations in breeding habitats or migration timings. However, researchers acknowledge that very little is known about the breeding behavior of Antarctic blue whales. Combining historical whaling data with contemporary methods—like satellite tracking and photo-identification—represents the best opportunity for scientists to unravel the mysteries surrounding the largest animal on Earth.
The International Whaling Commission’s Southern Ocean Research Partnership provided funding for this research.
