Antarctic blue whales appear to be creeping back from the brink of extinction after 50 years of protection. A new study estimates the total population at 4,018, up from 418 after the last reported whale catch in 1973.
Still, their population pales in comparison to pre-whaling levels, when hundreds of thousands of blue whales swam in the Southern Ocean.
Antarctic blue whales were the primary target of commercial whaling in the 20th century. Each whale can weigh more than 400,000 pounds, making them the largest animals known to Earth and a profitable source of whale oil. Killing them was banned by the International Whaling Commission in 1964, but illegal whaling by Soviet fleets continued for another 10 years. By the time it stopped, it was almost too late for the species.
"No whale was hunted quite to the extent that Antarctic blue whales were hunted," said lead author Zoe Rand, a postdoctoral scholar in the UW School of Marine and Environmental Affairs. "We brought together historical whaling data and contemporary abundance estimates to create a full picture of just how heavily people hunted them and how they are doing now."
The results were published Nov. 5 in Endangered Species Research.
Tracking whale populations helps researchers determine whether they are recovering from whaling, but counting whales is a challenge. Although large enough to spot from space, Antarctic blue whales spend limited time on the surface. Researchers rely on a combination of photo identification and line transect surveys to estimate the total population.
Line transect surveying involves travelling in a straight line and counting animals sighted en route. In this case, a ship sails along an axis looking for whales. The number of sightings and distance from the boat can be used to calculate density, and therefore abundance.
However, these methods are imperfect and expensive. The International Whaling Commission surveyed the entire Southern Ocean in expeditions from 1978 to 1984, 1985 to 1991, and 1991 to 2004. This data was used to generate the last circumpolar abundance estimate for this population, but few line transect surveys have been completed since.
Rand and colleagues created a model that accounts for all available data sources, including historical catch data, counts from these line-transect surveys and more recently, photo identifications. The researchers estimate that there are now 4,018 Antarctic blue whales, representing 1.9% of the pre-whaling population. Whaling reduced the population by 99.8%, and researchers estimate that it is now growing at a rate of 4.5% per year.
One could argue that humans have some degree of moral responsibility to remedy the effects of whaling, but that's not the only case for rehabilitation.
Antarctic blue whales are "ecosystem engineers," Rand said. When they die, their enormous bodies sink thousands of feet to the ocean floor, where entire communities feed on their corpses. This process, called whalefall, stimulates large scale nutrient cycling and is thought to sequester carbon.
Antarctic blue whales live long lives and give birth to just one calf at a time, and only every two to three years. The extent of decimation means a long, slow road to recovery. Although their population is far from what it once was, the picture that emerged from this study is cause for cautious optimism.
"I think this is relatively realistic for the kind of population growth we would expect from them," Rand said. "I think it's probably an open question whether they will make it back to pre-whaling levels."
Antarctic blue whales are still listed as critically endangered, and it isn't clear how they will respond to environmental change. Tracking their numbers is the only way to determine whether they are rebounding or struggling under new pressures.
Rand hopes that future technological advances, such as passive acoustic monitoring, will make surveillance easier. Researchers use recordings of whale song to estimate density but haven't yet figured out how to infer the total number of animals from acoustics.
"Antarctica is a big place, and it's not just one country's ocean," Rand said. "We really need more monitoring and continued monitoring to figure out what is going to happen to this species in the future."
Other co-authors are Trevor A. Branch, professor of aquatic and fishery sciences, Paula A. Olson and Douglas Kinzey of the National Oceanic and Atmospheric Administration; Michael C. Double and Brian S. Miller of the Australian Marine Mammal Centre; Koji Matsuoka of the Institute of Cetacean Research; and Ken Findlay of the University of Pretoria in South Africa.
This study was funded by the National Oceanic and Atmospheric Association.
