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Hundreds of humpback whales are massing in a tiny spot of ocean. Here’s why

HALIFAX, CANADA—In the fall of 1990, a few humpback whales showed up off the coast of western South Africa where they had rarely been seen before. Over the next couple years, a few more showed up, then a few more. Today, nearly 200 of the giant ocean mammals mill around a piece of ocean smaller than a U.S. football field for several months out of the year. Now, scientists think they know what’s luring what may be the largest global gathering of these cetaceans: masses of free food. Hungry humpbacks travel thousands of kilometers to feast on a rich buffet of tiny crustaceans called krill, researchers reported here this week at the biennial meeting of the Society for Marine Mammalogy.

Many animals are loath to change their behavior, particularly when it comes to food. But this—and several other studies reported here—reveals how readily humpbacks around the world come up with new hunting strategies, says Alexander Werth, a marine biologist at Hampden-Sydney College in Virginia who was not involved with the work. It also speaks to the ability of these animals to learn from each other and to develop efficient ways to eat.

Marine biologists Mduduzi Seakamela of the National Department of Environmental Affairs in Cape Town, South Africa, and Kenneth Findlay at the Cape Peninsula University of Technology, also in Cape Town, didn’t know all this for sure when they recruited graduate student David Cade to help them pin down what the whales were doing off their coastline. With the proper permits in hand, Cade—a Stanford University student who works at the Hopkins Marine Station in Pacific Grove, California—helped them outfit a dozen whales in the area with the research equivalent of a Go-Pro, a video camera that came with a motion detector and a depth finder. Together, these instruments revealed what the whales were doing underwater. What’s more, the ship where the researchers were working could monitor the density of prey in the water underneath it, so Cade could figure out what the whales were eating. 

Most of the time, the density of krill was random, and the whales’ feeding activity was slow and steady. But one afternoon, the whales all converged on one place where there was a really high density of prey—a 40-meter-thick swath of water with 107 grams of krill per cubic meter, as opposed to the average density of 66 grams per cubic meter, Cade reported at the meeting.

Once the whales had homed in on this smorgasbord, they did something no one had witnessed before. Under normal circumstances, groups of two or three whales tend to dive in synchrony across tens of square kilometers to hunt for food. During such dives, they lunge about 32 times per hour, accelerating quickly to suck in all the food and then spending about a minute between each lunge underwater. But in this dense patch of food, a free-for-all ensued. The whales were practically on top of each other, lunging about 53 times an hour and with barely half a minute’s rest.

“Basically they are just opening their mouths,” Cade said. The whales fed for a few hours that way, then split up into smaller groups and dispersed, he reported. That a simple change in the concentration of food could lead to such unusual behavior speaks to how prey density can drive the evolution of new hunting strategies, Cade suggested. It also suggests that some interesting communication is going on among the whales, as somehow ever more have learned that—for stuffing their faces—South Africa is the place to be.

Humpbacks elsewhere have gathered for other types of feasts, says Martin Biuw, a marine biologist at the Institute of Marine Research in Tromsø, Norway. Since 2010, he’s noticed groups of two dozen humpbacks temporarily in the local fjord, where herring have swum up in huge numbers to stock up for subsequent migrations south. “It could be that any time you have this mass abundance of prey,” he says, “we have supergroups and we just haven’t seen them before.”

Several other unusual feeding behaviors were discussed at the meeting: Working with Japanese researchers, Biuw documented humpbacks that hang out under fishing boats to nab dead fish. And Christie McMillan, a marine biologist at the Marine Education and Research Society in Vancouver Island, Canada, has been tracking the spread of a new feeding behavior among humpbacks living off Vancouver Island. In 2011, two were seen hanging out at the surface with their mouths open, seeming to flick in small fish. As of 2015, 16 individuals are practicing this “trap feeding,” she reported at the meeting.

 “It never ceases to amaze me how full of surprises humpbacks are,” Werth says. “They are so incredibly resilient.”