One dark night in May 2009, the research vessel Lawrence M. Gould poked its bow into Wilhelmina Bay on the Western Antarctic Peninsula looking for humpback whales and the krill they eat. It was later in the Austral fall than researchers had ever ventured into the area, late enough that any whales should be departing for warmer waters. The crew was unsure whether they'd find much of interest.
Meng Zhou, a University of Massachusetts, Boston, oceanographer, peered at a screen displaying data from the ship's acoustic gear, then summoned the expedition's two lead researchers, marine scientists Douglas Nowacek and Ari Friedlaender of Duke University in Durham, North Carolina, to have a look. "All we saw was red on this huge screen, all the way down to like 400 meters," says Nowacek. "Ari and I looked at each other and looked at Meng, and we didn't really appreciate what it was. And he said 'It's krill!' "
The researchers were looking at no less than 2 million tons of the tiny shrimplike crustaceans—the biggest, densest krill swarm documented in more than 20 years. And the next morning, the sun revealed another surprise: hundreds of humpback whales in the highest concentration ever recorded. The discovery of the krill and whale "super-aggregations" sheds light on an important but overlooked foraging ground for the endangered humpbacks—one that may be threatened by the region's rapidly changing climate.
The reigning wisdom has been that humpbacks feed in the Antarctic through the summer until krill move from offshore waters to take shelter from predators below the encroaching sea ice in the Austral fall. The whales then migrate to warmer breeding grounds, where the highest humpback densities had previously been observed. But during the 6 weeks the researchers spent on the Gould documenting the interaction between humpbacks and krill in Wilhelmina Bay and nearby waters, they counted 306 humpbacks parked on the huge krill swarm, and a total of 500 throughout the unusually ice-free bay at the record-setting density of 5.1 whales per square kilometer. The researchers, who report their findings today in PLoS ONE, had to reprogram their software because it couldn't handle the number of whale sightings.
The researchers discovered a smaller but equally dense krill swarm in a nearby bay along with more humpbacks. In yet another bay, a glut of whales prevented them from even lowering their acoustic equipment into the water. And when they made a return visit to Wilhelmina Bay in May 2010, the krill and whales reappeared in similar numbers, Nowacek and Friedlaender say.
The researchers attribute their findings to the rapid warming in the Antarctic Peninsula, which now has up to 80 more ice-free days per year than it did a few decades ago. With such a rich banquet of krill, the humpbacks seem to be lingering longer into the winter—and the researchers suspect that some may even stay year round. The team detected males singing, a behavior usually restricted to breeding grounds.
That may be good news for the whales in the short term, but the long-term forecast is less rosy, Nowacek and Friedlaender say. Krill require a cover of sea ice to spawn and hide from predators, and recent Antarctic krill declines have been linked to the retreating ice. Despite its size, the huge swarm in Wilhelmina Bay and others like it are probably vulnerable, too. In the long-run, the waning ice could hurt both the crustaceans and their numerous predators.
"It's an interesting paper and it does shed light on something that we've suspected for a long time," says Geraint Tarling, a zooplankton ecologist with the British Antarctic Survey in Cambridge. Namely, they have suspected that there are enormous swarms of krill out there that draw lots of higher predators. Big swarms of the crustaceans are occasionally spotted, Tarling says, but they're rarely reported in the scientific literature. The new paper, he says, goes beyond those sightings to provide a detailed snapshot of an extreme form of krill behavior and a predator's enthusiastic response.