Scientists have 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.

The sightings, made in waters still largely ice-free deep into austral autumn, suggest the previously little-studied bays are important late-season foraging grounds for the endangered whales. But they also highlight how rapid climate change is affecting 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. Most studies have focused on whale foraging habitats located in waters farther offshore in austral summer.

Nowacek and his colleagues observed 306 humpback whales – or about 5.1 whales per square kilometer, the highest density ever recorded. They measured the krill biomass at about 2 million tons. Small, floating fragments of brash ice covered less than 10 percent of the bay.

Advancing winter sea ice used to cover much of the peninsula’s bays and fjords by May, protecting krill and forcing humpback whales to migrate elsewhere to find food, Nowacek says, but rapid climate change in the area over the last 50 years has significantly reduced the extent, and delayed the annual arrival, of the ice cover.

“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.

Antarctic krill are shrimplike creatures that feed primarily on phytoplankton and live in large swarms in the Southern Ocean. Penguins, seals, seabirds and many whale species rely on the protein-rich, pinky-sized crustaceans as a source of food. Commercial fisheries are allowed to harvest up to three-and-a-half tons of the krill a year as food for farm-raised salmon and for oil, rich in omega-3 acids, which is used in human dietary supplements.

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. There is a strong correlation between the amount of sea ice and the amount of krill that survive the long, harsh Antarctic winter.

Further Reading:
Duke University Marine Laboratory