Antarctic humpbacks make a krill killing

Late-arriving sea ice boosts whales’ crustacean feast — for now

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FALL FEAST A humpback breaching in Antarctica’s Wilhelmina Bay has come to the right place for gorging on krill. A new study documents record numbers of the whales assembled there in 2009, apparently in response to a plethora of krill. A. Friedlaender

In the last two years, researchers at first hoping only to tag a few whales have happened onto hundreds of humpbacks eating themselves stupid in the bays of West Antarctica.

In the Southern Hemisphere fall of 2009, Wilhelmina Bay on the Antarctic Peninsula held at least 306 humpbacks, the densest crowd of the whales scientifically documented in one place, says biological oceanographer Douglas Nowacek of the Duke University Marine Lab in Beaufort, N.C. Another bay on the peninsula also hosted a big crowd, and a research visit to Wilhelmina Bay in 2010 found abundant humpbacks too.

These whales spent their nights feasting on the dominant Antarctic species of shrimplike krill, Euphausia superba, Nowacek and his colleagues report online April 27 in PLoS ONE. The researchers ranked the 2009 krill gathering in Wilhelmina Bay as the largest aggregation of E. superba documented in the Antarctic in 20 years. An estimated 2 million tons of the petite krill, in some places bunching 2,000 individuals into one cubic meter, were having their own feast on even tinier planktonic life concentrated in the bay by wind and water currents.

“We don’t think Wilhelmina Bay is unique,” says study coauthor Ari Friedlaender, also of Duke. Autumnal feasts in the peninsula’s bays may be an underappreciated factor in Antarctic marine life.

This krill feast “is an exciting find,” says Steve Nicol of the Australian Antarctic Division in Kingston, Tasmania, “because it is rare to have ship time and expertise to investigate these large aggregations of krill and their predators that are occasionally found.”

Friedlaender says his first hint of the spectacle came during the ship’s nighttime approach to the bay. The screen of the vessel’s echo-sounding equipment lit up “like a giant Pac-Man,” he recalls, indicating masses of organisms in the water. In the morning, he saw the domed backs of whales “in a food coma” drifting in the bay.

Climate change may be giving those whales an extra, but temporary, bonus. Krill are thought to take cover under seasonal ice. Yet the region’s ice cover now takes around 54 days longer to form than it did in 1979, says a 2008 study. That delay allowed the humpback crowd an extra 3,225 to 7,224 tons of krill in 2009, the researchers calculate. As big as such a feast sounds, they say it’s barely over a third of one percent of the total krill that surrounded them in Wilhelmina Bay that year.

The bad news, though, is that the feasting may dwindle. Researchers have reported that low ice cover one season typically foretells a krill dip the next.

“There is a tendency to look for direct causal links between climatic factors and species such as krill, and between predators and their prey,” Nicol says. “But life is rarely that simple.” His research does suggest, however, that healthy numbers of big animals actually enhance overall ecosystem productivity.

Any specific information on who’s eating how much krill helps, says conservation biologist Rodolfo Werner based in Patagonia, Argentina, with the Pew Trusts’ Antarctic Krill Conservation Project. “There is a big gap in the knowledge of feeding ecology of whales and krill,” Werner says. And quantified observations like the new whale-feast paper will be especially important this year as a much-debated conservation measure comes up for review by the international body regulating Antarctic krill fishing.

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Easing up to a drifting humpback whale in Antarctica’s Wilhelmina Bay, David Johnston’s mission is to use a long pole to thump a small recording tag onto the animal’s back so suction will hold it in place for about 24 hours. Johnston, of Duke University, affixed the devices so researchers could monitor whale dives and snoozes as well as underwater sounds.
Credit: Ari Friedlaender/Duke Univ.

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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