Minke whales have been discovered feasting furiously under Antarctic sea ice in a hunting strategy not seen before, scientists report.
Tracked using digital tags, the whales gulped mouthfuls of krill nearly every 30 seconds-the fastest "lunge" rate ever observed in filter-feeding baleen whales, according to the new findings.
The study, published in the August 13 issue of the Journal of Experimental Biology, marks the first record of Antarctic minke whales feeding under surface ice.
"In fact, it is also the first time that minke whale foraging behavior has been documented anywhere in the world using high-resolution multi-sensor digital tags," team member Jeremy Goldbogen, a marine biologist at Stanford University's Hopkins Marine Station in Pacific Grove, California, said in an email.
"Researchers have been trying to tag minke whales for years, but what these whales do underwater remains elusive due to their small size, which makes them very agile and quick" and, as a result, difficult to tag, Goldbogen said.
The team managed to attach two of the $25,000 devices using carbon-fiber poles and suction cups to a pair of whales in Wilhelmina Bay on the Antarctic Peninsula.
Gulping Krill Quickly
Depth, acceleration, and body orientation data from the subsequently recovered tags revealed a unique hunting tactic: The minkes skimmed the underside of the ice, rapidly gulping krill (a shrimplike crustacean that whales and other Antarctic marine predators thrive on).
This gulping action of baleen whales is known as lunge feeding-they accelerate to high speed and engulf large volumes of prey-laden water, which is then filtered by their plates of baleen-and the minkes were each doing it as many as 24 times per dive.
Blue whales, by contrast, lunge up to four times on each dive, and humpback whales up to 12 times.
The team suspects the astonishing rate at which the minke whales fed is linked to their relatively small size (though up to 33 feet [10 meters] in length, they're still the smallest baleen whale).
Goldbogen noted that larger baleen whales are able to engulf more water relative to body size than smaller species are.
"We have different strategies influenced by body size," he said. "At one end of the spectrum, blue whales take fewer big gulps of water and prey, whereas minke whales take many small gulps."
The minke's near-surface, multi-gulp strategy may be particularly well suited to vacuuming up smaller gatherings of prey, such as patches of krill under sea ice.
Rus Hoelzel, a minke whale expert who wasn't involved in the study, said the lunge rates reported by the team sounded "extraordinarily fast, but perhaps not impossible."
Based at Durham University's School of Biological and Biomedical Sciences in the U.K., Hoelzel said his own studies of foraging minke whales focused on fry feeders around the San Juan Islands in Washington state.
There, the lunge rate-observed as the whales scooped up fish corralled at the surface-"wasn't anywhere near as fast as once every 30 seconds."
But, he said, Antarctic waters present the whales with different foraging situations.
"Krill tend to concentrate near the edge under the sea ice," Hoelzel said. "If you've got basically a wall of food, you take a bite as frequently as you can."
Even so, Hoelzel noted that the new study tracked only two diving whales, and he would like to see the digital tag data validated, "perhaps using 'crittercam' technology."
"It can be kind of subtle determining the signal for a lunge" when a whale is underwater, "and then of course the lunge doesn't necessarily mean feeding," he said.
As for minke whales going ice-hunting, Hoelzel isn't surprised.
"It seems like a good place for minkes to forage," he said. "My guess is that other whales probably will as well."
The discovery that sea ice is a previously unrecognized feeding niche for Antarctic minke whales was made in an area where ice cover has decreased dramatically in the past 30 years, the study team warned.
"Because sea ice plays a role in the krill life cycle, changes to sea ice due to climate change could affect the abundance and distribution of an important food resource of one of the most common, yet poorly understood whales in the Southern Ocean," Goldbogen said.