Whale of a number
Super-aggregation of humpbacks found off Antarctic Peninsula
Posted May 6, 2011
Scientists reported last week in the online science journal PLoS ONE that they observed a “super-aggregation” of more than 300 humpback whales gorging on the largest swarm of Antarctic krill seen in more than 20 years in bays along the western Antarctic Peninsula.
It’s a whale of a sighting — but researchers warn the record numbers may be a short-lived phenomenon based on climate changes and human pressures under way in the region.
The Duke University -led team tracked the super-aggregation of krill and whales during a six-week expedition funded by the National Science Foundation’s Office of Polar Programs to Wilhelmina Bay and surrounding waters in May 2009. The team returned in May 2010 and recorded similar numbers. Smaller but still higher-than-normal counts were also reported in neighboring Andvord Bay.
“Such an incredibly dense aggregation of whales and krill has never been seen before in this area at this time of year,” said Douglas P. Nowacek , Repass-Rodgers University Associate Professor of Conservation Technology at Duke, in a press release.
The scientists had ventured to the region to learn more about the whales’ feeding behavior and to quantify how much krill the humpbacks and other whale species are consuming.
The researchers used acoustic recording tags to collect much of their data. The tags, about the size of a paperback novel, contain sensors and a hard drive, and attach to the whale by suction cups. The instrument tracks the animals and records their acceleration, movement and heading — information the researchers can use to help characterize feeding behavior. It can also record the whale’s vocalizations, allowing scientists to listen as whales feed and interact underwater.
Krill is the preferred meal for many of the apex, or top, predators in the region, such as seals, penguins and whales like the humpback, which can grow up to 16 meters long. Scientists have noted a decline in krill biomass in recent decades, which they say is linked to the declining duration of winter sea ice, a key habitat for juvenile krill. Changes in the climate, particularly in the winter, are affecting the sea ice.
“The lack of sea ice is good news for the whales in the short term, providing them with all-you-can-eat feasts as the krill migrate vertically toward the bay’s surface each night. But it is bad news in the long term for both species, and for everything else in the Southern Ocean that depends on krill,” said Ari S. Friedlaender , co-principal investigator on the project and research scientist at Duke, in the same news release.
NOAA scientists working in the western Antarctic Peninsula published a paper in April that blamed declining penguin populations on the drop in krill populations. The NOAA team, whose work is also partly supported by the NSF, noted that commercial krill fishing is also on the rise, putting further pressure on the delicate polar food web. [See previous article: Food shortage.]
Around the western Antarctic Peninsula, krill migrate in austral autumn from open ocean waters to phytoplankton-rich bays and fjords, where juveniles feed and the population overwinters under the protective cover of ice.
“If there are more areas with large aggregations of krill hanging out in waters where sea ice has diminished, you could see a big decrease in the standing krill stock, especially if we have a few years of back-to-back bad ice and the krill can’t replenish themselves,” Friedlaender said.
Whales can migrate long distances and might be able to find food elsewhere, but may be affected in other ways, as evidenced by snippets of unexpected sounds being transmitted by 11 whales the Duke team tagged in the study.
“We’re starting to hear songs being produced by whales in the Antarctic — sexual advertisements typically heard only in humpback breeding grounds that are located thousands of miles away from these bays,” Friedlaender said.
Humpback whales typically reproduce once every three years, “so if a female doesn’t have to go to the breeding grounds every year — if she has access to food here and isn’t being forced out by sea cover — why should she leave?” Nowacek said.
The presence of more females, coupled with access to a nightly krill feast, entices more males to stick around, too. “So this may affect the timing and location of humpback breeding and other important lifecycle events,” he said.
NSF-funded research in this story: Douglas P. Nowacek, Duke University, Award No. 0739483 ; Meng Zhou, University of Massachusetts, Award No. 0739566 ; Peter Tyack, Woods Hole Oceanographic Institution, Award No. 0739517 .