Super Aggregation of Whales and Krill in Antarctic Bays

A Duke University-led team of researchers has observed a “super-aggregation” of humpback whales feasting on the largest swarm of Antarctic krill seen in more than 20 years, in Wilhelmina Bay, along the Western Antarctic Peninsula.

Such a massive collection of whales and krill in one location are the result of the effects of rapid climate change in the region.

“Such an incredibly dense aggregation of whales and krill has never been seen before in this area at this time of year,” says Douglas P. Nowacek, Repass-Rodgers University Associate Professor of Conservation Technology at Duke.

The researchers visited Wilhelmina Bay in May of 2009 and then again in May of 2010. In 2009 they observed 306 humpback whales and measured the krill biomass at about 2 million tons. 2010 saw similar numbers, while in neighbouring Andvord Bay, smaller but still higher than normal numbers were recorded.

Normally, at this time of year the krill make the migration from open waters to the phytoplankton-rich bays and fjords which allow them to feed and reproduce, under the safety of ice cover, helping them replenish without being eaten by penguins, seals and whales. However, upon arriving to a bay that is just as rich in phytoplankton but no longer has the cover of ice is a new, and potentially threatening issue.

And not just for the krill either. The entire ecosystem which lives on the krill could be harmed.

“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,” says Ari S. Friedlaender, co-principal investigator on the project and research scientist at Duke.

“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 themserlves,” Friedlaender says.

Scientists have already noted a drop in krill abundance over the last 50 years, which corresponds with a reduced extent and delayed arrival of the ice cover. Allowing krill stocks to drop could have further consequences. With smaller numbers of krill, penguins and seals — who often have small foraging ranges and can’t eat any prey other than krill or hunt without the presence of sea ice to save them swimming for hours — could be seriously affected.

And the whales are already beginning to exhibit odd behaviour, according to the Duke team.

“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 says.

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 says. 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.”

The Duke study, was recently published in the journal PLoS ONE, once again highlights how man’s abuse of its environment is having deleterious effects across a whole range of ecosystems.

Source: Duke University
Images by Ari Friedlaender, taken in Wilhelmina Bay, Antarctica

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