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Humpback whales appear in front of ship.
Photo Credit: MISHAP Project
Humpback whales breach the surface of the water near the research vessel LAURENCE M. GOULD. Note the tag on the whale in the foreground. Scientists found high numbers of whales around Antarctica at a time when the animals were thought to migrate.

Delayed response

Humpback whales found to remain in Antarctic feeding grounds longer than expected

Large numbers of humpback whales are remaining in bays along the western Antarctic Peninsula to feast on shrimplike krill late into the Southern Hemisphere autumn season, long after their annual migrations to distant breeding grounds were believed to begin, according to a new study recently published in the journal Endangered Species Research.

The research suggests that the little-studied bays are much more important late-season feeding grounds for humpback whales than scientists previously thought. It also highlights changes that are occurring in the region in response to the increasingly delayed arrival and reduced extent of winter ice cover associated with rapid climate change.

“The old dogma is that by late autumn, the ice is heading in and the whales have headed out,” said David W. Johnston External Non-U.S. government site, research scientist at Duke University’s Nicholas School of the Environment External Non-U.S. government site and lead author of the paper, in a press release External Non-U.S. government site.

The first-ever density estimates for humpback whales, in both open and enclosed habitats along the peninsula in late autumn, detected 371 groups of humpback whales over a 654-kilometer survey area. About 70 percent of the survey took place in waters with no ice.

People on the bridge of a ship.
Photo Credit: MISHAP Project
Scientists search for whales from the bridge of the GOULD.
Whale Tail
Photo Credit: MISHAP Project
A humpback whale tail breaks the surface of the water.

At that density, Johnston said, “if you were to walk to the bridge of a ship and look around, you’d spot two whales within 500 meters of the boat. That’s higher than anyone expected.”

They conducted the study aboard the research vessel Laurence M. Gould External U.S. government site in late April through early June of 2009, as part of a National Science Foundation External U.S. government site-funded study.

Scientists have long known the waters around the western Antarctic Peninsula are important foraging grounds for humpback whales that feed on swarms of krill, but previous studies have been conducted earlier in the season or in open waters farther from land.

“Establishing the autumn density of humpback whales in the inshore regions of the western Antarctic Peninsula is crucial for understanding the role they play in this rapidly changing ecosystem,” said Ari S. Friedlaender External Non-U.S. government site, research scientist at Duke and co-author on the study.  “Our results provide a new perspective on the magnitude of predator-prey relationships in the region as the Antarctic winter sets in.”

Being the first to estimate densities in the peninsula’s narrow inshore waters was a challenge, Johnston said, because the line-transect techniques and distance sampling methods scientists traditionally use for this type of study weren’t well suited to the bays’ tight quarters, tricky currents and jutting shorelines.

“We had no idea that the whales were going to be packed up in these narrow channels and passages. We had to think on our feet a bit and use alternative sampling approaches and incorporate data from other portions of the project,” he said. For instance, data collected from tagging the whales and tracking their underwater movements turned out to be inordinately useful for estimating densities, too.

“Once we knew their dive behaviors, we could establish how likely it was that we might miss them as we were traveling along the surface of the water,” Johnston said.  “That’s not something we would have been able to do using only the traditional methods.”

Other co-authors on the study include Andrew J. Read and Douglas P. Nowacek External Non-U.S. government site, both with Duke’s Nicholas School of the Environment. Nowacek holds a joint appointment as associate professor of electrical and computer engineering at Duke’s Pratt School of Engineering External Non-U.S. government site

NSF-funded research in this article: Douglas P. Nowacek, Duke University, Award No. 0739483 External U.S. government site.

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