One or many? Exploring the population groups of the Antarctic blue whale using historical mark-recovery data

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 recovering? Is there just one population of Antarctic blue whales, or multiple? Do these questions matter for conservation?

A team led by Zoe Rand, a University of Washington doctoral student, tackles these questions in a study, published Nov. 14 in Endangered Species Research.

Building on the last assessment of Antarctic blue whales in 2004 and using old whaling records, which were surprisingly detailed, Rand and her colleagues investigated if the Antarctic blue whales consist of different populations or are one big circumpolar population. Study co-authors are Trevor Branch, a UW professor of aquatic and fishery sciences, and Jennifer Jackson from the British Antarctic Survey.

Antarctic blue whales are listed as an endangered species, and understanding their population structure is essential for their conservation. Conservation at the population-level increases biodiversity, which helps the species adapt better to environmental changes and increases chances of long-term survival.

During the whaling years, biologists began the Discovery Marking Program. Foot-long metal rods with serial numbers were shot into the muscles of whales. When these whales were caught, the metal rod was returned, and the whale’s size, sex, length and location where it was caught was noted. Looking at where whales were marked compared to where they were caught could shed valuable insight into the movement of Antarctic blue whales, but these data have never been used before to look at population structure.

In this new study, this historical data were used alongside contemporary survey data in Bayesian models to calculate inter-annual movement rates among the three ocean basins that make up the Southern Ocean—Atlantic, Indian and Pacific—which make up the feeding grounds for Antarctic blue whales. The team found frequent mixing among the ocean basins, suggesting that whales do not return to the same basin every year. This points to Antarctic blue whales being one single circumpolar population in the Southern Ocean.

These results are consistent with studies of Antarctic blue whale songs, heard throughout the Southern Ocean. Only one song type has been recorded among the Antarctic blue whales. In comparison, pygmy blue whales have five different songs corresponding to five different populations. These results are also consistent with genetic studies, which found that Antarctic blue whales are more closely related than would be expected if they were separate populations.

This study is the first time that historical mark-recovery data from the Discovery Marking Program has been analyzed using modern quantitative methods. These data exist for many other hunted whale species, such as fin and sei whales, so the new study’s methods could provide a framework for similar analyses for those whale species too.

There is still a lot scientists don’t know about the Antarctic blue whale. Even though they do not appear to be separated geographically on their feeding waters in different ocean basins, they could still have distinct population structures based on differences in breeding habitats or the timing of migration.

However, almost nothing is known about Antarctic blue whale breeding behavior, according to the researchers. Using historical data from whaling alongside contemporary data—such as satellite tagging and photo-identification—remains scientists’ best hope for uncovering the secrets of the largest animal on Earth.